Jet Sizes

[mike1979]

I am trying to tune my carb but I have a couple questions.
I am working on a 350X with a Mikuni carb with a twist throttle. It has a cobra pipe on it.

Question one - How many jet sizes up should I go with this free flowing exhaust?

Question two - How much difference does EACH size make. I have a 145 in it right now and I believe stock is 138. Does this sound like way to much? I am getting some steady smoke out of the exhaust. I am running a Uni air filter with no air box lid.

I am tempted to return everything to stock but the funds are, as usual, pretty tight. I am thinkg of selling the mikuni and twist throttle in order to purchase a stock carb and thumb throttle. I have a 250sx carb to get rid of too.

[Mr_RPM]

i dont think the smoke is a related problem, your probably burning oil. if 138 is stock and you have a pipe, UNI and no lid and everything else is stock then a 145 sounds good to me. but do a spark plug reading. if dark, black, wet with oil gas then your rich and you need to run a smaller jet. if white then go bigger. normally the dark grey is a good reading. but if your burning oil it will make it hard to do a plug reading, burning oil normally makes it look like your running rich.
normally when your re jetting going up and down in size increments of 5 is the way to go. unless your way off. going up and down by 5 (140 to 145) doesn't effect it greatly

[TeamGeek6]

there shouldnt be ANY smoke out the exhaust, the X is a 4 stroke.
Thats WAY rich, there should be no need to inrcrease jets unless theres a big change in altitude.
Exhaust doesnt cause changes in AFR, thats an old wives tale. Changing exhaust does change how the spark plug fires in the cylinder and that has an indirect change on jetting, but its tiny.
Always jet according to spark plug color, not "what parts on it."
Heres the Bible on spark plugs:

http://www.dragstuff.com/techarticles/reading-spark-plugs.html
http://www.dragstuff.com/techarticles/plug-pictures.html

look at the spark plug photos. NONE of them are brown or black like they say here to do. Brown/black is FOULED.

[Rawlins350x]

I just purchased some bigger jets myself. I put a new uni filter on and a supertrapp exhaust. I guess bigger is the way to go until you find the sweet spot? I am going out to the garage in a little bit to try it out.

[Mr_RPM]

his exhaust smoking is probably because the machine is 25 years old. time for new piston, piston rings and valves.

[DixiePlowboy]

An air cooled 4-stroke can smoke two different colors for two different reasons.

Blue smoke is oil burning, and black smoke is too much fuel(rich jetting or "flood smoke")

[Mr_RPM]

i dont imagine jetting up from a 138 to a 145 even with stock exhaust and intake would cause it to run so rich that it has flood smoke. maybe, but thats just my opinion. mike, what color is the smoke?

[CRAZY70MAN]

Take your time and get ready to buy a whole bunch of jets to get it dialed in. Both sow and main jets. I removed my 38mm Mikuni and K&N after 10 jet combo's and little success. Stock airbox and carb now with full DG pipe and it is just right for me now. My 38mm jet #'s were totally different so I cannot tell you any info there though I dropped from 50 to 20 in slow jet and 3-4 sizes down in main and notta???? Those open filters I am told are very tricky to get fuel flow just right. Good luck, wish you success.....