who has had the carbs bored on their sled? what brand and model? did you notice much improvement? i had the carbs bored on my t cat and it seemed to me to work well. just curious about other results.

Boring carbs on most sleds is a great way to gain top end power, and usually doesn't cost allot fo money to do it. Flat slides tripples get pricey though.

You are basically opening up the ability for the engine to injest more air. Now, I have seen on a SRX where they took off the stock 33m carbs, and put on a set of 38mm carbs, and the sled lost allot of bottom end. Yea, it gained top end, but 100 foot ETs went in the basement. In 660ft drags, it was actually slower, but was going more MPH.

On my Mach I bored my own carbs from 38mm to 39.5, but I never dynoed it stock.

This kind of mod has to be done with some common sense. You must be prepared to do some carb tuning to get the gains, and be able to spend a little of mony on fuel. They tend to get worse MPG. And since one is taking the time to enhance the intake flow of the carb, a good set of reed cages is a necessity to get the full potential of the mod.

I purchased some 40mm MIKs on Ebay for my son's 1989 F Plus LT. When they arrived I dismantled them to clean and set them up for this machine. I was surprised to find that one measured only 39mm at the engine side and the finish on the inside of both carbs was less than desirable. I jigged them up on my lathe and opened up the area from the slide forward to 42mm. I then cleaned up the inlet bell and fabricated a velocity stack to fit inside which projects into the stock airbox. The results were as 800MXZ said. The only difference is I started with a grossly undercarbed motor (34mm carbs). With a few more inexpensive updates I will realize the full potential of the larger induction system.

Bored carbs are a great low cost upgrade, if your engine needs more air flow. The velocity of the air passing through the carbs must be maintained to facilitate the correct fuel air mixture consistently.

Over boring can cause problems with off idle throttle response and can actually lower 100 foot times while top end may be better. Unless you are a racer, I really don't see an advantage to larger carbs unless you've done major mods or have a flow starved engine as thumbdoctor mentioned.

hi there. i have a set of 44mm that i had bored out to 45.5mm and tper bored then i had the face of the carb cut. the results where very imperive. now they had no iddle circuit. but it realy did make a huge diference in the 660' and 1000' . on the dyno we did see an increase of 12h.p. over 44mm. and that was @ 9000 rpm. we had also put the same carbs on a f1 sled for the ovals that had 44mm bored to 45.5mm but not had the face cut and taper bored. and we saw an increase of 6 or 8 h.p. if i remember right. so for some carbs for some sleds make a big difference.

I agree that bored carbs in someapplications can be a huge benefit. Now on the ones you had taper bored, did you take them out to 45.5MM at the slide then tapered larger from there? What do yo mean when you say you cut the face? I've heard of cutting the engine side of the carb when you have a solid flange to work with in order to match up the flow of the carb to the intake tract of the cylinder. But we really don't have that option with most sled carbs. Would the sleds you used the modded carbs on be trailable?

I had 42mm's on my MX but when the throttle cable broke i had to go back to the 36's. I couldnt find a cable long enough so that the slide in the carb would be down all the way. With the 36's i notice more accelartion. The 42s give it a bog.

ICE Breaker

Which type of "MX" do you have.... ?

basicaly you just bore the carb out to 45.5 straight thru. then you go on the large side of the carb where the air box would usualy go. and if you look there is like a 1- 1.5' lip more or less. well that is cut off there. then open the face up to match. now for trail... well not realy but for pure acceleration yes. drags and speed runs.

My buddy bored his carbs from 40 to 42 mm. Anyone have a ball park figure how much hes going to have to jet up? According to our math they should flow about 10% more so is about 10% what we would jet up?
Ryan

good question ryan.I would like to know that one myself.I am wondering about the loss of velocity and how it effects the flow of fuel into the airflow.it could be that at 10 percent it is still not picking up the right amount of fuel because of the velocity loss ??? .

I was pondering that too Barry. More volume but less speed. Question is how much more volume and how much less speed? And therefore how much is it going to help/hurt. He's got a lathe so it was free but hes not touching mine until I see how his work! 670HO's came with 44's so it doesnt seem like 42 would be too much. I'd rather have better throttle response than 2 miles an hour on the topend but I guess we will see how it works out this winter.
Ryan

if you are running a stock sled or even trail ported for that matter i would just stay stock. especialy if you are running 44mm. you havent added any h.p. so why would you need more fuel? for all around stay with the stock carbs.but if you go with 42mm i wouldnt think there would much of a change other then say go up say 3-4 sizes on the main jet. you could even pull your neddles up a couple of notchs. no if you had 38 or 40 mm then i would say yeah either go with 40mm or 44mm and if your racing then go with 45.5mm

Originally posted by RNM2399@Jun 4 2003, 07:25 PM
Question is how much more volume and how much less speed? And therefore how much is it going to help/hurt.
Neither will help, both will hurt. More volume will demand more fuel to maintain temps, less velocity will decrease venturi over the tube (needle jet), which will pull less out.

The only SAFE way to jet when making fuel mixture changes is to jump up big, then slowly come down after testing while closely watching plug and wash readings.

If you have an EGT gauge, establish a baseline temp that is confirmed with plugs and wash before you bore, then use that for readings.

Never use synthetics while adjusting fuel mixture by using plug and wash readings.

10% is a pretty good number for jetting for two mm of boreing. the kicker is that you need to jet up 10% of the cubic feet per minute the origional carbs were flowing, as well as the same percentage of air, which means a air box mod. i threw on a bm power breather and went up four jet sizes on my t-cat, that had 3 mm of boreing done.

just a ball park, as corey said jet high to start. :D

Yeah I think we will start out way rich and then lean it out.

Corey: So what you are saying is that its not going to help at all? 670HO's ran real well with 44's.
Ryan

Ryan, bored carbs will probably help the top end and full load pulling, but since velocity changes almost exponentially with changing venturi size, it's hard to say exactly what is going to happen with out the use of a flowbench or just plain old experience. You have to keep in mind that the resonance that Doo built into the airbox is not going to be right anymore either which will change the performance characteristics. A lot can happen by just boring out a little mat'l.

Thanks Keith,
Makes me glad those arent my carbs! If it works I will do mine though.
Ryan

i have a 02 800 that i had crank shop do a 900 big bore in and now it is drag only and my buddies say to bore the 40's out for better ET, but i'm scared to screw up what is other wise a beast now. what do you think?

I'd say that with a big bore and assuming you're twin piped, go for it!! You can probably utilize the extra flow.

As a rule of thumb we jet up 10 sizes for bored carbes. If oyu started with a 260 main then go to a 360 main etc...

GLH

I went from 40s to tapered 42s with good luck after i got the nj and the jn problem fixed on a 01 mxz 800 moded. So far so good but haven't had a ton of test time yet.

doo900, if it is a drag only sled now then bore the carbs!! boreing the carbs on a sled, in my opinion, is for one reason drags. like twin pipes on a normally single piped twin, the daily riding is a pain due to tuning issues. it's gonna happen but, don't anyone change this topic start a new post about twin pipes. :D for drags, with a big bore on stock bore carbs, let it breath more!

Larger (bored out) carburetors are more sensitive to atmospheric, temperature and altitude changes. Most OEM snowmobiles use smaller than optimum sized carbs to maintain tuning stability (less jetting changes over broader operating conditions) so consider where and when you will be running your machine to see if it is really financially viable. It's great to have a fast combo, but a real bear to waste good sledding conditions retuning. :(

well i bored the carbs last night , we went 2mm over to a total of 41.9 mm, but i'm not sure how high to put the needle jet . the stock hieght is .027 but i can't go any lower than .035 because the jet bottoms out cause of the bore now. although i am still .024 lower than stock. i'm not really sure but i think it will be rich this way. this is gonna be fun, i take it out to test and tune thursday to see how it goes.

Let us know how things turn out on Thursday. I am curious on how it works out. I want to bore my 800's carbs, and like as much feed back as I can get.


Thanks,
Brian


SWRules

Another option is to "oval bore" the carbs. This modification will maintain the velocity over the needle jet during low to medium throttle openings and give more peak flow at the top end. This machining should be left to the specialty machine shops. :)

well i did'nt see a big improvement , i still got to tune a little more it was REAL rich in the mid range at 4000-6500 rpm , i dropped my needles all the way and it seems better now, i also went up to 620 mains but i am going to 600's, she pulled a 6.211 @106.58 in 660 , my best up till then was a 6.236 @105, i'll keep messing with it till she's junk and have to buy some electrons haha

WOW! Picking up .035 and 1.5 mph is a big improvement if boring the carbs is all you did. Keep us informed.


Brian






SWRules

RNM2399: Actually, I was referring to fuel flow, not performance. I thought the question you asked was whether either would help as far as fuel flow goes. FYI: My carbs are bored.

tifa_5_2000: GLH is referring to a sport wide standard. Most professional draggers will tell you they jump HIGH when making modifications to be extra safe, then drop down as needed. Your using the 10% rule which is great on paper, but here in the real world, noone really knows how much extra air we will be pumping.

This is a perfect example of how math on paper does not necessarily equal the same in real world. You are doing the bore increase math and hoping like hell that you did infact increase air flow only by 10%.

In my experience, I have had many sleds with bored carbs and the end results was never increasing jetting by 10% when I went up 10% in bore size.

Just a thought. This is why pros tell you to jump big, TO BE SAFE, then come back own as you test. They are trying to save some people an engine.

I could not have said it better myself Corey :) . Most of us dont have the extra money to be rebuilding our engines every time we make a modification. Better safe than sorry, tune from there.



Brian


SWRules

Originally posted by mxzwfo@Jul 11 2003, 09:38 AM
I could not have said it better myself Corey :) . Most of us dont have the extra money to be rebuilding our engines every time we make a modification. Better safe than sorry, tune from there.



Brian


SWRules
Exactly Brian.

And everyone should think of the negatives and positives here. What really is the harm if you go too big? A few fouled plugs? A boggy sled? You can jet down as needed here.

But just think of the negatives if you go too small. Not every sledder tunes like a pro. Some just don't know that you can't go pin the throttle for a mile-long run right after making some modifications.

Some don't realize that safe jetting baselines MUST be established BEFORE you go off racing and pinning. The HIGH jump in jetting after making modifications was created as a SAFE way for sledders to tune and modify, while staying safe. It was meant to be a SAFE rule of thumb.

Saying this rule is ridiculous is doing a serious injustice to those out there that are not experts, like me...

I really think that you guys are over simplifying it just a bit.

When you bore your carbs, you are trying to increase flow. The cost is loss of air velocity at sppeds other than WOT. When air velocity drops, the pressure in the venturi increases which means that less fuel will be drawn thru the needle jet.

You'll have to go up in main jet size to correct for the increase in air being drawn in. Going up 10 sizes is in NO WAY out of line. 10% bigger venturi in no way translates into a 10% bigger jet. There are way too many other variables involved. Air velocity for a given venturi size is increased with direct relationship to your engine speed. Don't quote me on this, but it seems that I remember that the pressure inside the venturi decreases by about 4 times proportionally as velocity increases. So if your velocity is lower by, say 20%, at half throttle due to a larger venturi you will be drawing less fuel thru the needle jet due to higher pressures within the venturi.

You correct for lean condition at WOT by increasing the main. As mentiones above, the signal for fuel is going to be lessened at half throttle due to lower velocity and higher venturi pressure. To correct for that, you will need to lift the needle. And if you still are lean in the mid-range, you'll have to change needle jets or needles or both..(good luck to all that run the TM-40's on the seriesIII that have fixed needle jets) Or you can just live with the fact that your sled runs very good at WOT and just runs so-so at other speeds.

I guess the point I'm trying to make is that boring carbs out is not just as simple as boring and changing mains. (no matter how big you go on jetting) For almost any mod you make, there are trade offs...one thing always effects another.


One way to minimize the effects to throttle settings other than WOT is to oval bore as Thumdoctor mentioned. Now, don't make the mistake of thinking that 'oval boring' means that your venturi is actually oval shaped. It is still round. It just means that the bore is not bored concentric to the current bore. Here is a link that explains oval boring quite well.Oval boring. (http://www.pilot-odyssey.com/~hoser/web/OvalBoring.htm) Now with this type of bore job done on your carbs, you still maintain higher velocities throughout the RPM range so you don't have as many of the problems associated with carb boring.

Just hate to see someone bore their carbs to gain slightly on top and have to struggle with a dog in the trails. JMHO.

tifa_5_2000, read this Jetting Base Line (http://www.snowtechmagazine.com/articles/baseline/baseline.html) Then read Kevin Cameron's article "The Mysterious Fuel Mixture Fishhook". SNOW*TECH Oct/Nov 2000. This will reinforce what Cory & Keith have said. If you can't find the copy of SNOW*TECH, PM me and I'll scan / send it to you.

Originally posted by tifa_5_2000@Jul 11 2003, 11:01 PM
swwebmaster: so what your saying is the 10% method IS jetting safe. what is the diameter of a 300 main jet and a 400 main jet?
No, that is NOT what I am saying. Your are using percentages and that is dangerous expecially for novice tuners.

Percentages complicate things bigtime and can confuse tuners in to thinking they are jetting safe. Idooski's point is right on the mark.

By using percentages, you confuse users into thinking that if they increase air by a certain amount, they only need to increase fuel by that amount. It is by no means that simple. Here's an example.

If I increase air through my carb by let' say 10%, if the base air speed is in someway increased, I made not need a jetting change at all. On the other hand, I can increase air through the carb by boring, which slows the speed of air, requiring a much larger increase in jetting. Keith has it right as it can be a compound issue. More air and less air speed equals bigger fuel need as it will draw much less fuel up the tube.

I am not sure what you are getting at in 10 size differences in jets. Whatever it is, it is not a 10% difference in any case. For example, a 400 hex main has a flow rating of 350 grams/minute. A 500 hex main has a rating of 450 g/m. In any event, this is not a 10% bias and even if it was, it still wouldn't indicate whether or not you actually achieved a 10% air volume increase.

Since it is nearly impossible for the average tuner to ever know exactly what volume change he or she has, AND what exactly their particular flow (read: speed) has done, AND how either will relate to venturi effect, it goes without saying that perecentages can be dangerous.

It would be nice if fuel/air/speed/venturi calculations were that simple, but they're not. Keith's point is that there are many other factors that you are not considering.

Which takes me to my point. Why not just do it the safe way? Jet up big, then come down as required.

Originally posted by tifa_5_2000@Jul 11 2003, 10:58 PM
MY sled. 01 xc800sp, has been ported, cc'ed, piped slp single, high flow box, and bored 42.5mm carbs and i will be jetting from stock 460 to 500 per doug flannery of flannery race engines. even though i agree w/the safe method of rich jetting. i will be sure to post the results when it snows. and about the 10 size jump, just for bored carbs i say NOWAY, unless 10 sizes actually equals about 10%.
So what you are saying here is that ANY time you modify your engine, jet up 10% no matter what modifications you have done.

That's basically what you are doing. You have a modified engine with absolutely NO idea what percentage of air increase you have and yet you are jetting up 10%.

It would be nice to have a seasoned pro that does these modifications for a living telling all of us what to jet to. But since not all of us have this luxury, it is better for us to live by a little safer rule of thumb.

All I want to know is how the heck you calculated that you got a 10% gain in volume of air to your engine with the mods you have described. I can't wait for that forumla.

To me, it sounds like you advocate guessing at what you might have increased air by but precisely increasing fuel by 10%. It doesn't make any sense. I recommend jetting up BIG after modifications simply because we are guessing what implications our air delivery will have.

We guess at those changes in air and we guess at the change needed for fuel. BUT, the idea is to guess BIG on the fuel side.

You can always come back down.

Wouldn't you be messing with all ranges. low-pilot jet,mid- needle,and high- main ?Basics still apply. Seems like low and mid would be more critical or harder to dial in. Hate to see anybody trash the motor just warming up.

Originally posted by tunedbyear@Jul 12 2003, 12:23 PM
Wouldn't you be messing with all ranges. low-pilot jet,mid- needle,and high- main ?Basics still apply. Seems like low and mid would be more critical or harder to dial in. Hate to see anybody trash the motor just warming up.
You are exactly right. A bored carb is going to have a more immediate and subsantial effect on low and mid range due to the lower velocity and increased pressures in the venturi. At the very least, the needle should be raised and the fuel screw turned out or the air screw turned in depending on what carbs you're running. I would even increase the pilot jet size before I start it for the first time. A fouled plug is cheap!

I'm not convinced that these TM-40's we are running on the Doo's will even accomadate an overbore. Mine quit flowing at a 580 size main jet. I took the main out and ran it with no difference. With the equipment I was trying to run, these carbs simply could not deliver enough fuel to keep up. Just something to think about.

Well guys, I think we got the point across to anyone else that reads this that you should overcompensate for changes in power configuration to the rich side as far as jetting is concerned.

There's always going to be someone that wants to test the squeekmonster. :doh:

Keith

QUOTE*
There's always going to be someone that wants to test the squeekmonster.


"better a lion for a day than a lamb your whole life"

Don't worry buddy. Been there done that. Bought new cylinders and pistons. I decided it was time to start listening to those who know and are willing to offer advise and to start learning more about what I was doing.

We're just trying to show a safe and proper procedure for starting her up after doing a carb bore. If you just want to do it your way, that's entirely up to you. But, don't try to prove us wrong on this. You can't.

if air is up 10% fuel is up 10%.

I hate to be the bearer of bad news but this is an incorrect statement. Pure an simple. It is just not as simple as this.

NO, not at all, im saying if you increase air, you increase fuel the same % and youll be very close.

You're still missing my point. The question is HOW do you know how much air you've increased in the first place? Guess? And BTW, your statement about increasing fuel the same amount as increasing air and you'll be very close is not correct. I believe we have demonstrated this point already.

better a lion for a day than a lamb your whole life"

A amateurish, expensive and completely unnecessary statement. Don't get me wrong, I am NOT trying to single you out. I just think that you are making a mistake.

I am not sure how your post changed from discussion to challenge, but I don't understand your point. Why are you comparing what I am saying to what Flannery said?

He is giving you information to your ONE particular application. We all are talking about across the board rule of thumb.

Your math is wrong in any event, but even if it was right (which it's not), your at 13% change. Correct according tp you right? What about your airbox? Doesn't that mod account for anything?

See, my point is that you really have no idea at all how much you've increased air to your engine. You have no idea just how much you have slowed the velocity. Yet you know for sure that your 10% jump in jetting is sufficient.

Eventually, it will catch up with you. That is all I am saying.

BTW, 2 years ago, I ran 310 mains in my TCAT when it was a modified engine in the grass at 80 Deg. Last year, after just a bump to 1100, I now run 440 mains to maintain a relatively safe EGT in grass.

I am just glad I didn't use that 10% rule then.

Ok, here is my .02 When you bore let's say from 38mm to 40mm, you are going up 6% in bore size. But when you figure area, you are going up 10% (1133 mm/sq. vs 1256 mm/sq.) To me, this seems likes you will loose allot of signal. With a loss of signal (vacume on the jet) there is a point at which you will have to jet up to maintain proper air/fuel mixture.

Anyone remember UFOs for round slides? They increased air velocity (allong with signal) to the point you had to jet down with them. A substancial drop in jets in matter of fact, just by changing the shape of the underside of the slide area. Now by increasing the area of our venturi by 10%, to me you will loose signal and thus need to go up in mains by more than 10%. 10% translates into what, 3 jets?

Also a thought here is taking a stock sled with righ jetting and boring the carbs. If the sled was 5 jets rich to start with, you might only have to jump one or two jets.

In my experience, we built a 2000 SRX700 last year for a guy. The sled was spanking when it left the marina, but the guy out of conversation listened to another tuner and took off the 34.5mm (stock bored SRX carbs) and put on a set of 38mm Flats. This totally killed the sled under 100ft. The intake velocity went down so much, that it lost all the responsivenes and torque, it gained 4 hp on the dyno, but lost allot on the track.

NOTE: The larger the diamater of the carb, the less effect (in percent) that boring will have on it. 2mm is much more drastic on a 30mm carb, than on a 44mm carb.

Originally posted by tifa_5_2000@Jul 12 2003, 09:36 PM

800mxz could you please figure that percentage out for my carbs 40mm to 42.5mm, in surface area? (which is also volume, right?)

thanks, mike
40 = 1256 mm/sq
42.5 = 1418 mm/sq

It's an 11.4% increase.

I am sorry, I don't understand your above question.

Originally posted by tifa_5_2000@Jul 12 2003, 10:09 PM

40 = 1256 mm/sq
42.5 = 1418 mm/sq

It's an 11.4% increase.

you need to divide 162 into 1256 not 1418 to find increase percentage.

T/F if the motor was right to start with and you bore the carbs, does your ideal fuel to air ratio change?
believe it or not thats the same question.
you need to divide 162 into 1256 not 1418 to find increase percentage.


No. You need to divide 1256 into 162, I started with rounded numbers. Give or take, it's around ~12%.

T/F if the motor was right to start with and you bore the carbs, does your ideal fuel to air ratio change?
believe it or not thats the same question.

It doesn't HAVE to but it could easily. Again though, I don't see your point.

Look, if you are saying that if you increase your bore size OR bore area (you originally stated bore size) by a fixed number such as 10%, all you need to do is increase your jet size by 10% and your back to ideal, we might as well agree to disagree.

It's ok. We can disagree. :)

Ok I will really simplify it. Bire your carbs and don't do anything to the jets and let me know how it turns out.

An incrase in square mm is not an actual increase in volume. What is not being talked about here is SIGNAL (vacume), truse me, I have a 800 Rotax without needle jets, and that little lip does ALLOT for signal (Signal is the amount of "draw or vacume pulling up on the fuel in the main jet).

An engine will only draw so much air. Unless you change porting and pipes, it is going to be fairly limited. Im not saying if you bore it will not help, but you will eventually hit a wall where it is effective. Now, this air pump has one hole to the atmosphere that allows air in, and that is the intake. Now, increase the size of the hole to the atmosphere, but draw the same amount of air thru it. Your air is going to be moving slower (less signal). Since the air is moving slower and with less signal, less fuel is going to be drawn up the jet. If you bore the carbs out and change nothing else, it will actually use less GPH (gallons per hour) of fuel, thus being a leaner mixture.

Originally posted by 800MXZ@Jul 12 2003, 10:42 PM
Ok I will really simplify it. Bire your carbs and don't do anything to the jets and let me know how it turns out.

An incrase in square mm is not an actual increase in volume. What is not being talked about here is SIGNAL (vacume), truse me, I have a 800 Rotax without needle jets, and that little lip does ALLOT for signal (Signal is the amount of "draw or vacume pulling up on the fuel in the main jet).

An engine will only draw so much air. Unless you change porting and pipes, it is going to be fairly limited. Im not saying if you bore it will not help, but you will eventually hit a wall where it is effective. Now, this air pump has one hole to the atmosphere that allows air in, and that is the intake. Now, increase the size of the hole to the atmosphere, but draw the same amount of air thru it. Your air is going to be moving slower (less signal). Since the air is moving slower and with less signal, less fuel is going to be drawn up the jet. If you bore the carbs out and change nothing else, it will actually use less GPH (gallons per hour) of fuel, thus being a leaner mixture.
We DID talk about venturi effect (signal) above, but it got bumped from our discussion. Re-read my posts. I stated that it was not just increasing bore size that you needed to worry about. (A few stated it actually).

My EXACT reason for saying using percentages is bad news. It doesn't take into account issues like venturi.

Well if it was me, and all I was doing was boring the carbs, i would jet up of course. This is a fairly broad question to answer, but I would go up 10 sizes and come back down to good piston wash. I am going to assume about 5 to 6 jets. But that is just me. All I know is brass is allot cheaper than aluminum.

Originally posted by tifa_5_2000@Jul 12 2003, 10:48 PM
this all started
But you have to admit it's a pretty sweet discussion. :p

Originally posted by tifa_5_2000@Jul 13 2003, 12:04 AM

T/F if the motor was right to start with and you bore the carbs, does your ideal fuel to air ratio change?
believe it or not thats the same question.
It doesn't HAVE to but it could easily. Again though, I don't see your point

ideal fuel to air mixture cannot change, its always 14.7 to 1.

i ask this question to figure out why 10% doesnt equal 10%.

someone else help ???
If at any time the efficency of the motor changes, which it just may by changing air/fuel, your mixture requirements may change with it.

Don't confuse a theoretical stoichiometric ratio of air and fuel (14.7 to 1) with the required ratio to maintain safe EGTs. They are very different. When you ask about an ideal fuel mixture, everyone including myself doesn't think stoichiometric ratio, we think about a ratio at which the engine performs at a sustainable maximum horsepower.

Two-stroke stoichiometric ratios can never really be acheived. The amount of fuel delivered to the cylinder never equals the stoichiometric ratio because complete mixing of all of the fuel and air in the cylinder can never be totally achieved; it is limited by the amount of air present and the effectiveness of the mixing process. In two-stroke engines, the actual mixture is ALWAYS richer than the stoichiometric ratio.

We need to stop talking about theories and ratios not acheivable by average tuners. In real world applications, the engine's ideal mixture (my definition of ideal) varies depending on mods done to the engine and resulting efficiency of the engine. It also changes as the power-to-weight ratio changes which happens anytime more horsepower is acheived.

Originally posted by tifa_5_2000@Jul 13 2003, 03:33 PM

14.7 to 1 is probably pretty hot or is it? i think its ideal.
on egts wont a rich condition show up as hot just like a lean cond, due to fuel burning in the pipe?

stoi·chi·om·e·try n.
1 Calculation of the quantities of reactants and products in a chemical reaction.
2 The quantitative relationship between reactants and products in a chemical reaction.



agree :) just boring your carbs wont always make a difference unless the engine was under carbed to start with...it can even hurt your performance(due to velocity).

there is a direct relation of air to fuel. if the egts were safe to start with i think its a safe bet to say youll never have to increase jet size more than the bore size percentage increase. so 7.223% bore increase will never need 12.665% fuel increase, actually i think the max fuel increase would not exceed 7.223%(bore size inc).

fact: 10 jet sizes is actually about 25% fuel increase.
14.7 to 1 is probably pretty hot or is it? i think its ideal.

It is. But again it is theoretical and not achievable on a two-stroke motor. So since that ratio can't be acheived in the first place, why would you want to start using in it your math? Since a tuner doesn't know where he/she is at in the first place, this ratio means nothing to them. EVERYONE jets to wash, plugs, and EGTs. 14.7 to 1 means nothing to the vast majority of snowmobilers yet you are using it as a guideline. A number that you can't reach in the first place.

on egts wont a rich condition show up as hot just like a lean cond, due to fuel burning in the pipe?

No. A rich condition shows up in reduced EGT temps.

stoi·chi·om·e·try* * n.

I know the definition. :)

if the egts were safe to start with i think its a safe bet to say youll never have to increase jet size more than the bore size percentage increase.

Better hope that the engine wasn't seriously air restricted beforehand.

so 7.223% bore increase will never need 12.665% fuel increase, actually i think the max fuel increase would not exceed 7.223%(bore size inc).


Sure about that? Cause your actually increasing area much more than 7.2%. You're closer to the 12% than you think.

The important thing you are forgetting is that the two are not directly proportional. Again, you are ONLY looking at bore size increase and forgetting about volume (area) and the effect on the venturi, the difference in engine efficiency, the change in power-to-weight ratio, etc.

Don't any of these mean anything in YOUR thinking?

Originally posted by tifa_5_2000@Jul 11 2003, 11:29 PM
TD do you know hex jet diameters?
tifa_5_2000, I checked my Sudco Mikuni tuning manual and called them, they don't know of any cross reference chart between Mik hex jet numbers and actual drill sizes. Give me a range you wish to work with and I'll measure them. Keep in mind, drilling jets is not a great practice if that's what your planning. :blahblah:

FYI, I always believed Mikuni large hex jet numbers reflected CC fuel flow per minute. 450 = 450cc per minute. After talking with the folks a sudco in the UK, I've learned the numeric value is actually grams per minute until you reach a 500 size which the attached article says otherwise. Read this (http://www.snowtechmagazine.com/articles/dontuse/dontuse.html) and ask yourselves, "has this happened to me?" :nervous:

am i really that off the wall or is everyone scared of you? why cant someone else post? there were a couple comments earlier from others, make them back their claims up!

Uhmm, looking through this post, I believe there are 2 or 3 that agree with me; none that I can tell have come forward for your way of thinking.

Did you ever stop and think that maybe they're not afraid, maybe they just think you're wrong? :p

I didn't really want to post a comment like this but for some reason you keep taking those little digs. Me personally, I think you're "off the wall". :D

Originally posted by tifa_5_2000@Jul 14 2003, 09:08 AM
clutchman was kind of.

oh well, you win.
It's not a question of winning or losing, it's a discussion, and a good one at that. But I agree, no sense of beating a dead horse.

Well, heres my two sense after having done many mods including tapered bored carbs (tm40s). I am old school in that i always jet big then i jet down until my egt's read what i want them to, it doesn't have to be this complicated unless you just have alot of free time. I must say some of you have exstensive math knowledge when it comes to volumes, ratio's, and percentages. I can tell you from years of experience with jet engines that all the formulas in the world sometimes dont help but being cautious along with experience does. JET BIG and spend the money you save on more mods. HAPPY SLEDDING !!

Speed...you do know that there are several posts missing from this thread, right? There was someone we were trying to convince that jetting big was the proper thing to do rather than do as he was going to do, which was jet up 10% because he opened the carbs up 10%. We're with you on this one. Jet big, then worry about how fat you are rather than when you're going to burn down.

Keith

Thanks keith, i was just backing you guys up on this one. There is one thing that puzzles me, and that is when we try to help those with less experience on this website some still insist on making the same mistakes we did many years ago, keep trying maybe they will listen.

i apologize to all of you and snowmobile world and corey and keith for deleting my posts. it was stupid. most of the stuff is in swwebmasters quotes. sorry, mike

Thanks Mike. Apology accepted.

Glad you decided to stick around. Everyone makes mistakes. The wise learn from them.

Keep the rubber side down...

Keith

SWRules

hey guys i read all of your posts. It got pretty overwhelming there for a bit. ;) I will be the first to admit that i am a novice, compared to you guys. I have alot to learn. the mathematical figures have never crossed my mind when I am tuning my carbs. Gear ratios yeah! any ways I am wondering about the whole oval bore thing. To me this seems the most logical. Has anyone went this route. I have a extra set of TM40 and I'm going to try it. I plan on doing them myself at work. What i am not sure about is how much material I should take out. 2,3,4mm. If anyone could help I would appreciate it. Thanks Andy :p OH yeah they will be strapped on a stock 700mxz. soon to be ported have templates, and these will be strictly for the drags. SWRules

2mm is all you want to take out of the 40's from what i saw after i did mine , these carbs worked pretty good for a bore job in fact i ended up with my 580 mains , i found that the 40's the way they are built wont flow over 580 jets anyways , no signal over the jet, no math done just test and tune on the track, i ended up putting on 44 rounds so i got a set all done if your interested in them.i got the 900 down to a 6.08 @110 mph in 660',

a well known fact: it's cheaper being to rich than to be to lean!!!

Thanks for the input doo900. I have a extra set of 40mm iwas planning on 2.5mm over. are your carbs a straight bore or oval. I'm going with oval. I have got some great input from idooski. I'm going to make the bore oval than taper the bore open to the size of the carb boots on motor side I wil post the reults. I will have times with the stock flat slide than times with round slides. shouldnt be to long now just waiting on a pipe. Doubt they will be any where close to your times but i'll do my best and have fun doing it. SWRules

:) Hello

....this is truly a good & interesting Topic !



I think all of you Jetting Pros here, have a ingenious/delicate problem here :

http://www.snowmobileworld.com/forum/index...showtopic=25270 (http://www.snowmobileworld.com/forum/index.php?showtopic=25270)

Please HELP !!!

SWRules