alright guys..lets get down to it..jetting..
this is not related to a pacific sled/motor..
what I`m trying to figure out..is base line jetting vs outside temps..it seams all I ever read..is when a guy ask about a perticular sled, and someone else who owns one, or is fimiliar with it, responds telling him what jets to run for a givin temp (not refering to this site persay, but in general)
anyway..
just for EXAMPLE...lets say a guy picks up a set of pipes..and the spec sheet calls for him to jet to say 145`s, and he`ll be good down to 10F..well...what if he`s gonna be riding in temps that run around 0F, or even -10F for example..whats the basis...for jetting changes vs temp changes??
would he go one size bigger? two sizes bigger?? what would ya go by, to get an idea about how many sizes you should change?? 1 size per 10 degree drop below what they said you were safe too?? two sizes?? three???

like if a guy was then gonna grass drag..and he currently had his sled jetted perfect for when its say 20F outside..what would he jet to when planning on grass draging when it was 80F outside??
what I`m trying to figure out..is whats the range of a given jet size, in other words, lets say your jetting was perfect for 20F, how much warmer would it have to be, or how much colder would it have to get, before you would need to change your jet size, to keep a good air/fuel ratio, and still be safe, yet have good performance?? not sure if that made sence, but cant really think of any other ways to explain what I`m wondering..
anyway, thanks in advance for any and all thoughts!

AL

this chart works on mikuni carbs with a hex type jet. it is for an aviation type motor but the math is the same for a sled. if you have a mikuni round jet or a kehin carb see the chart below posted by hillpounder. KEEP IN MIND THIS IS JUST AN ESTIMATE AND SHOULD NOT BE SUBSTITUTED FOR SOUND TUNING PRACTICES LIKE PLUG READING,PISTON WASH, AND EXHAUST GAS TEMPURATURE. it is usefull if you are planning a trip from say vancouver to revelstoke, and want to know what jets to bring along. note kimpex have the cheapest jets around at about $4 ea.

Thanks a lot. I was looking for something like that too.

i`ll give ya my take. most sleds from the manufacturer are jetted to run wot for at least , at least 2500' long at -10 f at sea level. the distance is important becuase temps are always building and let them build long enough the pistons will melt. so... you can take away 1 jet size for every +15 'f over -10f. you can take away 1 jet size for every 2000' in altitude. if your drag racing 500' you can lean out 2 jet sizes...lean is mean and lean is hot down two if you only want to make power for 500` or so. something up with the previous info as it states that all rotax motors are delivered with (if i recall) + 15c jetting. thats like 60'f more or less. i think that info might be taken from a aviation site, but whatever. i didnt study the chart, the info i give i`ve been dooing for years.

Thanks Gwerty..
apreciate the info guys..

That chart and formula works on a mikuni hex jet only. If you have a mikuni round jet or keihen carb jet it won't work out right, you will need to use a different chart.

Edit to include charts. 1st chart is Keihin/round mik, 2nd chart is mikuni hex. thanks to Barry Holtzman

Explain?? what exactly would be the math for the regular round, screw driver jets??
Thanks..
AL

thanks for the heads up. i was not aware of the difference in calibration between the different types of jets. which type is calibrated by size and which type is calibrated by area again. also what sleds use the round type?

Thanks for posting the chart..
now, just to see if I`m getting this right..
I see that chart is starting out even so to speak, at -40, and going in the direction of jetting down, so what about going the other way?
for example, if my jetting is dead on at say 25F, and i wanted to figure out what to start at for say 5F.. (mikuni round jets), what would be the numbers for jetting up? would the first drop of 20F be my jetting size X 101?, then a drop of 40F be my current jetting size X 102?

if you are going from+25-f down to+5-f at sea level then you will have to fatten up the jetting (more fuel,larger jet) according to hillpounders chart #1 mikuni round jet is about 2% FATTER FOR A 20 F DEGREE CHANGE IN TEMP. EXAMPLE 320 X 1.02 =326.4, ALWAYS ROUND UP SO IM GUESSING 328 OR 330 JET SIZE

cool got it... thanks everyone for your input!!

AL

I use a Mikuni Pocket Tuner. It's a slide rule type calibration chart it's sorta dummy proof. When I got it I didn't know squat about jetting and it helped. I got mine from dennis Kirk, but I think most Sled catalogs have one (or google it)...only a few bucks It covers both round and hex.

I use a Mikuni Pocket Tuner. It's a slide rule type calibration chart it's sorta dummy proof. When I got it I didn't know squat about jetting and it helped. I got mine from dennis Kirk, but I think most Sled catalogs have one (or google it)...only a few bucks It covers both round and hex.[/b]
they fit in the tool box great.

"Carb jet's" info on carb tuner and book


I have just finished with a jetting issue myself during installation of a Holtzman ATACC (automatic temprature and altitude carburator compensator)" the installation instructions actually quote referring to Olav Aaen's, Aaen's Carb Tuning Handbook, Olav's phone #(262)552-8981 he is very knowelagable and easy to talk with he lives in MN.

2 stroke motors are just big air pumps...air in and out play the major factor on power made....cold air is more dense....the denser the more get's crammed in the more power avail(and leaner the mix)this leaner mix needs more fuel.

each jet's opening determines the amount of fuel that will flow (other factor is the needle which lifts out of the jet and jet nozzle)

on a mildely tuned stock machine the temperature range is broader...

the more performance tweaked from a machine the more sensitive to temp it will become
each machines jet "sensitivity" is based on a number of factors....

the porting,port timing profile,carb size (air speed),compression and exhaust header/expansion chamber (pipes)layout are the determining factors