:) Hello

One of my friends have BIG BIG trouble with burndowns.....

Some background :

Sled : Ski-Doo MX-Z "583"

Engine :
* Rotax 583 ( Only 3500 Km / 2175 Miles )
* Case ported
* Twin Crank Shop Trail Pipes into a single Canister , VERY Quiet !
* 2 * VM-44 Mikuni ( Bored out Stock VM-40 carb )
* Jugs/Cylinders is Stock !!!
* RV "500"


Stock jetting: ( VM-40 )
Needle jet : 224 AA-0
Jet needle : _ECY-1-
Main jet : PTO--> 280 , MAG--> 260
RV : "502"

Present jetting: ( VM-44 )
Nj : 224 AA-2
Jn : _ECY-1-
Mj : PTO--> 310 , MAG--> 330
RV : "500"


The Problem :

The engine runs FAT at WOT and also ran FAT at idle ( when engine is warm )
BUT on the midrange the EGT's goes sky high !!
He have had 5 Burndowns !!! , everytime on the midrange
( He is a little mentaly down..... :cussing: :cussing: )

He have tried to go up on the main jets, No difference !
He also have tried to move the E-clip around, No difference !


I will try to help him out, and I think YOU fellow sledders have the info He need ! :)

My thoughts are that the Needle ( _ECY-1 ) is wrong , and Ski-Doo has at least a bunch of own needles to the VM-40/44 , BUT no one have the numbers about the "Tappering"
( hope you understand what I mean... )

Or ??

Please HELP !!

SWRules

Without taking too much time to go thru all the needles to check them out thoroughly, my first response from a gut feeling is get rid of the overbored carbs!

The idle is not controlled by the needle to any great extent. The WOT is controlled by the main. If you are fat at the top you need to jet down. You can try a needle that is very thin in the mid-range, but I think what is happening is that the bore is so big that you can't get enough velocity thru the venturi to draw the fuel thru the needle jet at mid range speeds.

I think that you need to try a set of stock 40's on it.

I'll try to put more thought into it later when I have more time.

are your needles right for the needle jets you have? those work in sets.

What you need to try is BB-0 or BB-5 n.j , they will richen up the mid range. Then jet from there , drop the mains & pilots jet,s untill you get it right on.

:) Hello

He has allready tried to do it that way.....

( More like the old Formula-Z way with VM-40's )

Needles : 7DL7
Needle Jet : BB-0 even with BB-5

But things just got worse!
Hey , It almost dripped gas out of the exhaust outlet........... :(

Please help out on this one !!


SWRules

Get rid of the carbs !! They are way to big for that motor. Should be more like 38's. Second if your fat on top jet down on your mains, If you are having problems matching up the jet needle with the needle jet try drilling out the needle jet .002 this should fix your midrange HOTSPOT if you insist on running the bored carbs. P.S. you are loosing a ton of low end and midrange power with those carbs, that motor cannot produce enough signal to pull that much cfm at an efficient rate.

dont drill on your jets. do you know needle jets go in .005 mm increments? you will never accurately drill one. i agree that your problem is in the needle or needle jet.

Lasse you could try lowering the fuel level a touch, if you have fuel running out of your carbs.

:) Hello

Here are the Ski-Doo Needles for the VM 40 & 44
that I have found !!
Maybe more..... ?

The Swedish dealers does not give out the "Tappering" numbers about the
Ski-Doo needles !!
( I'm talking about the numbers that you can find in a Mikuni chart ! )


Ski-doo Part #
_DPI 1 ---> 404 157 700
_ECY 1 ---> 404 157 400
_EDY 1 ---> 404 156 700
_DHY 6 ---> 404 161 840
_DFY 1 ---> 404 161 847
_EJ 5 ---> 404 133 400
_DL 7 ---> 404 147 800
_EG 6 ---> 404 147 200
_EM 7 ---> 404 151 000

SWRules

you might try a flatter slide cutaway or there is something wrong with the overbore job, what did crank shop say if you asked them???????????
Is he using the stock air box?????????
What is the midrange exh. temp just before burndown?????????

Have you tried the other carbs yet? I think you really need to do that before you chase the needles any further.

Keith

I hope this is some help to you and your friend Lasse. Although I did not find your particular needle numbers, this will help you understand taper specifics.

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:(

Re : zenq62 / Thumbdoctor / idooski / others...........


* The "Crank Shop" said just to get 3-5 sizer bigger main jets than OEM and to change so the bigger main is on the MAG side.
That was all !!!

* The air box is gutted

* The overbore job is NOT anything wrong with !

* The EGT number at burndown ----> Nobody knows :D

* He is not willing to try anything before he has a pretty good grip on whats wrong !
( It's expensive with burndowns :( )

* Thanx Thumbdoctor , that was exactly the ones i was refering to,
BUT I already have these Mikuni taper specifics....
But I don't have the Ski-Doo needles taper specifics !!!

Does anybody have them ?

HELP !!!

You have the Ski-Doo part # above in this subject.....


SWRules

does it go down during the run or after you let off? do the rpms peak then fall off? is there sign of detonation on the crown, alum on the plug? did you go up on the pilots? is the airjet plug in place on the carb?

:)

Re : hillpounder & others


* It has never "go down" on a WOT run !

* It's more when he is cruising down the trail at "Midrange"
and sometimes when he is slovly letting of the trottle.

* I haven't seen any sign of detonations on top of the piston crown.

* The Spark plug shows very very very little ( or no ) sign of Aluminium on it, and not every time !!

* The carbs does not have the Air Jet "plug" at all !!!
Is this Wrong ?
( Part # 404 131 700 )

What's the function of the Air Jet "plug" ?


SWRules

Originally posted by Lasse@Oct 16 2003, 04:31 PM
:(

Re : zenq62 / Thumbdoctor / idooski / others...........


* The "Crank Shop" said just to get 3-5 sizer bigger main jets than OEM and to change so the bigger main is on the MAG side.
That was all !!!

* The air box i gutted

* The overbore job is NOT anything wrong with !

* The EGT number at burndown ----> Nobody knows :D

* He is not willing to try anything before he has a pretty good grip on whats wrong !
( It's expensive with burndowns :( )

* Thanx Thumbdoctor , that was exactly the ones i was refering to,
BUT I already have these Mikuni taper specifics....
But I don't have the Ski-Doo needles taper specifics !!!

Does anybody have them ?

HELP !!!

You have the Ski-Doo part # above in this subject.....


SWRules
If the airbox is gutted, then you will need to jet even larger.

If there is nothing wrong with the carb bore job, then I have to go back to my original thinking and say to get rid of the bored carbs. They are too big for you to create the needed velocity to pull fuel while running in the midrange with that little engine.

You have created way too much area with the overbore on the carbs.

Read up in the carb section of A. Graham Bells book Two-Stroke Performance Tuning. Once read, I think it will all come to light.

Too much air, not enough velocity. Guaranteed!

Lasse,

I did some more digging for you and found some Jet needle specs in the Bombardier Yellow Book. At least the specs indicate some info on the rich or lean end of the scale your dealing with.

Lasse,

I spoke with Sudco Mikuni about how they equip their Snowmobile racing series Miks. Here is their starting point:

VM44-7 - 540 MJ - 35 PJ - AA5 NJ - 7DH2 JN - 1.5 TV - .07 Main Air Jet

That is a basic starting point and they nearly always reccomend installing a power jet kit on 40mm or larger due to velocity issues.

Lasse, 2 things you say hint to the problem. 1. partial throttle burndown, 2. plugs are missing in the air jet passage.

The air passage should be plugged if the passage is vented to the venturi, you'll see a hole above it in the bore . If it does not have this hole an airjet should be installed, size of the airjet can be tuned around with the mix screw so isn't super critical. If you don't fix this you will continue to have problems.

:) Hello.

My friend have just picked up 2 new ones VM-40 ! :D

But :

What's the function of the "Air jet Channel" ?
What's the point to put an "Air jet Plug" in it ?

Even this new ones was completly"empty" in the threaded "Air jet channel" !!
Why ?

I mean , Why are the carbs equiped whitout "it" if they should have "it" ??

Please explain the function of this "Channel"

SWRules

my open mod 583 has 44`on it and here`s what i`ve got for a setup.

540 mains, and needle set on 2nd position, can`t remember what pilot jet it is., air screw set 2 1/2 turns out. DO NOT RUN CLAMP ON AIR FILTERS!!!!!! if you want gut the air box and run that RUN AN AIR BOX. Running clamp ons will burn down a crankshop motor in no time! I have a gutted air box. another thing too check is the silencer, see if all the fiber glass packing is blowed out, if so this can cause a lean burn down. my egt`s ran around 650 at idle, 1050 on bottom, 1140 on mid and 1120 on top. doo not let it exceed 1250-1300 if soo shut it down! and break it in with doo mineral oil, then switch too ams oil or other syenthetic personally I run ams oil series 2000 in both the open mod 583 and the 440 hope this helps i will call crankshop and see what they say.

If this is a late build 1996 or early 97 model, it could be simply the vent line from the fuel tank is pinched off too much from a ty-rap.

There were dozens and dozens of burndowns from midrange driving at 2/3 to 3/4 throttle with the 1997 sled.

From the factory, whoever was installing the plastic tyraps on the tank ventline down the frame was putting them on too tight, thus choking off the tube.

There was a bulletin that went out about this problem and for dealers to warn the customer and solve this problem.

I seen one tank that actually caved in on the first couple of tanks of fuel.
I seen one sled that running on the trail, melted and deformed the belly pan so much that the hood would not seal in the lip...
Popped the hood and you could literally see the metal on the silencer was grey with white speckles. Unbelievable hot...
Dont have a clue why the sled did not burn down.

Dealer fixed vent line and changed belly pan and my pal never had that problem again.

Just have a peek at the vent line for laughs... :crazy:

this may be a stupid question but what does it mean when the engine "burns down" is this when it seizes or when it blows up or something?? :confused:

blurr, yes. when the engine seizes.

My experience with UFO's from Thunder Products is that they will pull more fuel thru and effectively richen the mixture mostly in the midrange. When you install UFO's they instruct you to lean out the midrange 2 clip spots on the needle and downsize the pilot one size. I found that to be right on with my 38's. Maybe installing the UFO's and leaving your needle setting where it is will richen up the midrange sufficiently then jet down on the mains and pilots to clean up the top end and idle. Good luck.

Get rid of the bored out carbs I aggre with SPEED IS EVERYTHING. put stock carbs back on. Your sleds displacement cannot create enough velocity of air through the venturi and thus will not pick up enough fuel causing a lean out. Only alternative to use those carbs is to jet way over what your using now.

:) Hello !

Does anybody have any more help to give to my friend about his problems ??

Help needed !!

THANX !

SWRules

:(

Well.......

My friend did like you guys told him to !

The new Recipie !

2*VM-40 Carburetors.
AA-5 Needles.
0,5 Turn Out on the Air screw.
Plugged Air Jet inlet.

The result......

:cussing: :cussing: Burndown !!! !!! !!!

He was just driving in the Mid range at about 50 Km/h for about 2 Km when it blow !!!

My friend blew his fuses :tongue: right away !

and thats the way it still is in the garage.


PLEASE HELP HIM OUT !!!

What's wrong ??


SWRules

engine timing ok?, cracked carb boot(check all the way around them) rotary valve cover o-ring ok,? bad crank seal?(is it one cyl all the time or randomly both?) i know you changed needles but havent heard anything about needle jets just because you went to a thinner profile on needles doesnt mean it will still pass enough fuel if the jet itself is lean. instead of wasting money on pistons spend a little on new needle jets,go up a couple sizes and work down there easy to change and its easier than a meltdown. restriction in fuel line/pump, anything that can starve it for fuel? i would definately look at the needle jet/needle combination again i had the crank shop do my trail porting and running the needles they told me to and i still had to fatten it up inthe midrange. also did you have it rebored when you started having trouble? cause if you did it could be to tight of abore job,is it four corner seizing both sides of the exhaust port and both sides of the intake?

might want to stick a dial indicator on it and verify the TDC's are 180 apart? even a little bit can advance the timing on one hole quite a bit driving into deto

How about taking pictures of the burnt piston and cylinders and posting them for us. Like Machz1 said, is it always the same piston that goes down, or is it both equally? Has the motor been pressure tested? Make sure there are no air leaks. Put in a richer needle jet, but maybe first go back to what the Crankshop recommended for jetting. Are their recommendations based on stock card calibrations? If so, find out the stock calibrations, should be in the owners manual, and even go up on the mains a few sizes and tune from there. Get it to idle first, then set the main jet at WOT, then set the mid-range. Raise the needle as high as it will go and adjust until plug colour and piston was are good.

To me, it seems like there may just be issues else where. Like a massive air leak, crank seals, etc... or even a problem with the exhaust. Is it the same cylinder each time?

Honestly, the proper way to jet tune a new build of an engine, with no knowledge of a baseline jetting, is to start off getting the idle right, then starting at the bottom going to wide open.

Could also be a engine build issue. Can you answer these questions?

Cylinder Squish

Cold Cylinder Compression

Compression Ratio

Required Octane/Octane Using

and don't forget TIMING


I am wondering how this engine can be burnt down and re-built over and over, is he buying or plating cylinders?

Are the pistons burnt int he middle of the piston, edges, or sides?

As DynoJoe said, there can also be a fuel delivery problem too. The engine could also be running so lean on bottom and top end that it seems "Fat", but just doenst get enough fuel to ruin itself.

:D :D

Some news on this one......

It's randomly Left & Right piston who seizures.

The pistons have had very small signs of four corner seizure !

95 or 98 Octane is being used !

I have chacked everything for him and made some small changes...

Here is the new recipie



* Blocked "Air Jet" in the carb inlet.
* BB-0 Needles
* 45 Pilot jet
* 1,5 out on the Air screw
* Blocked cooling return line between the head and the "plastic chamber" ( I don't know the name in english !! :blush: )
* I have chamfered the piston skirts to aid lubrication


This time it seems to work :)

He has now put on 350 Km on it , No truble at all so far !!

The EGT is around 1000-1100 F. on WOT :ohmy:

Your high egt reedings and and burn down are definetly a sine of a mid- range lean condition and the right situation is to go to a ;ritcher neadle jet along with amatching jet neadle i have a 670 mach I with 44mm factory carbs and this is the neadle jet and from it hope this helps AA7(224) and the jet neadle 7EG06-3 these are stock ski doo so they should match the 44MM carbs

:) Hello

One of my friends have BIG BIG trouble with burndowns.....

Some background :

Sled : Ski-Doo MX-Z "583"

Engine :
* Rotax 583 ( Only 3500 Km / 2175 Miles )
* Case ported
* Twin Crank Shop Trail Pipes into a single Canister , VERY Quiet !
* 2 * VM-44 Mikuni ( Bored out Stock VM-40 carb )
* Jugs/Cylinders is Stock !!!
* RV "500"
Stock jetting: ( VM-40 )
Needle jet : 224 AA-0
Jet needle : _ECY-1-
Main jet : PTO--> 280 , MAG--> 260
RV : "502"

Present jetting: ( VM-44 )
Nj : 224 AA-2
Jn : _ECY-1-
Mj : PTO--> 310 , MAG--> 330
RV : "500"
The Problem :

The engine runs FAT at WOT and also ran FAT at idle ( when engine is warm )
BUT on the midrange the EGT's goes sky high !!
He have had 5 Burndowns !!! , everytime on the midrange
( He is a little mentaly down..... :cussing: :cussing: )

He have tried to go up on the main jets, No difference !
He also have tried to move the E-clip around, No difference !
I will try to help him out, and I think YOU fellow sledders have the info He need ! :)

My thoughts are that the Needle ( _ECY-1 ) is wrong , and Ski-Doo has at least a bunch of own needles to the VM-40/44 , BUT no one have the numbers about the "Tappering"
( hope you understand what I mean... )

Or ??

Please HELP !!

SWRules
[/b]


The main jet is only for 3/4 to wide open throttle. So if you're fat when the throttle is wide you should be able to reduce main jet size. Your problem is in mid-range, so I would leave the needle alone for now, because that is where it ran good stock, and change out the tubes, what the needle goes through. They should be labeled with a Q - #. If I remember right, the smaller the number the leaner or narrower the tube. So my advise would be to order a 1-2 size larger tube to put in and try that.

My old 650 Wildcat was running very rich in the mid-range, but great wide open and idling. I went from a Q-2 tube to a Q-0 tube and that fixed the problem.

Good Luck!

hes probably burning it up in the mid range,when he bored those carbs he should have used a smaller cut away to compensate for the slow down in air velocity,with the bigger bore in the carbs the air flow thru them slows down sucking up less fuel creating a leaner condition thru the whole fuel flow,and if those carbs werent done by someone who knows exactly what they were doing ,they will be ruined

Mid range melt down!! Oh have I seen this before. The NJ is too lean, end of story. The rotary valve engine requires richer NJ when modified. And dont forget that the advancing Rotary Disc opens one Carb from the bottom up and from the top down on the other. Jetting will be different from one side to the other. Mains will be 2 sizes larger, neadle one or two clips higher and pilots bleeds richer. Buy larger Neadle Jets.