Tuning out a sputter

[82atc250rkart]

Hey guys I'm hoping yous can suggest a reasonably easy explanation for this sputter I'm getting. I've gone as Lean as I'm comfortable going with the main it's at 150 and needle is on middle clip. It idles fine and gets on the pipe well but I have an annoying sputter off coasting/idle if I stab the throttle it sputters then takes off fine. After reading plenty the sputter is indicative of rich running but I dont really want to go leaner in idle jet as it's running fine in that department on 1.5 turns on air screw. It's had a top end rebuild and 2 tanks through it. Before I had trouble with last top end this sputtering wasn't existant. I have gone down to standard size barrel now so jetting may be too fat in idle circuit? I have come down in the main circuit from a 155. Attached is pic of plug after a good run.

[yaegerb]

Need a lot more info.
Bike, carb, airbox, all jets, etc.

[Poco Loco]

Hows the reeds on your trike? Good and flat with a nice seal?

[Kamnahrt]

Based on the plug, you are within the safe range so I wouldn't worry about burning it down. For the sputter, have you tried moving your needle up or down 1 notch to see if it hurts/helps. I would try and move it down a clip position to lean it up some on the transition.

[350for350]

If I remember right, this is an air cooled 250R engine on a racing go kart.

[Rob Canadian]

If I remember right, this is an air cooled 250R engine on a racing go kart.

Correct. Grass runner. And a story behind it also.

[82atc250rkart]

Hey guys only just got back to the forum thanks for your interest. Yes 82 atc250r in a grass kart. It had a trick top end before and I had alot of trouble with it seizing. It ended up being a round piston that couldn't expand correctly causing it to seize on the Gudgeon sides of piston even with 6 thou clearance. It now has a standard top end so I'm guessing I should go back to a more standard jetting. Just got my idle jet out and it's a 68 the standard jet listing seems to be 50 so I think I've found my problem. I think I'll go with a 58 to find a good middle ground to start with as the sputter is minimal and can usually bypass it with a quick jab of the clutch to get it up In the revs. However I would like to just slam on the throttle and it pulls away like it used to.

[82atc250rkart]

I have tried moving the needle to no avail. It goes really well through the revs once past the stutter however as soon as you've coasted with closed throttle (in gear clutch out) and the try to re accelerate the stutter returns. I have tried winding air screw out a bit more to lean it up but I guess it doesn't have enough effect.

[82atc250rkart]

Reeds looked mint nice flat no gaps seals clean and intact. It's had all new gaskets and been pressure tested all checked out mint

[yaegerb]

Hey guys only just got back to the forum thanks for your interest. Yes 82 atc250r in a grass kart. It had a trick top end before and I had alot of trouble with it seizing. It ended up being a round piston that couldn't expand correctly causing it to seize on the Gudgeon sides of piston even with 6 thou clearance. It now has a standard top end so I'm guessing I should go back to a more standard jetting. Just got my idle jet out and it's a 68 the standard jet listing seems to be 50 so I think I've found my problem. I think I'll go with a 58 to find a good middle ground to start with as the sputter is minimal and can usually bypass it with a quick jab of the clutch to get it up In the revs. However I would like to just slam on the throttle and it pulls away like it used to.

what model carb is on it?

[barnett468]

Yes 82 atc250r in a grass kart. It had a trick top end before and I had alot of trouble with it seizing. It ended up being a round piston that couldn't expand correctly causing it to seize on the Gudgeon sides of piston even with 6 thou clearance.

What?


PREVIOUS KAWASAKI INTERNATIONAL R & D PROJECT ENGINEER AND ATV DEPARTMENT SUPERVISOR AND THE ONLY PERSON ON 3WW WITHOUT AN EDIT BUTTON OR A STAR :)

[82atc250rkart]

what model carb is on it?

30mm keihin round slide

[82atc250rkart]

What?


PREVIOUS KAWASAKI INTERNATIONAL R & D PROJECT ENGINEER AND ATV DEPARTMENT SUPERVISOR AND THE ONLY PERSON ON 3WW WITHOUT AN EDIT BUTTON OR A STAR :)

I'm not sure which part didn't make sense but I'm going to assume it's the round piston part? Should be oval slightly to allow for correct expansion when hot. It seems whoever built motor lathed this piston to fit amongst other things and it would lock up no matter how rich ya ran it or nicely ya rode it. With new stock top end I've had no such troubles just this stupid stutter