Need some opinions on a problem my brother began having yesterday. He's got a stock Sport 400. We rode last week and everything was great. He got it out yesterday and it won't idle and doesn't have any guts at all through low or midrange. It'll just cough and sputter until he gets the rpms way up and then it seems to run great. Once he manages to get it up to speed it smooths out and has great top end. He pulled the carb off and gave it the once over, it appeared clean and the gas is new. Any ideas?

 

http://forums.atvconnection.com/thre...rigpost=258965

This thread seems to address the same problems.
Sooner or later all two stroke motors come down with this affliction. The vibration of the motor causes the inlet needle valve to wear and the float level of gasoline goes too high and the engine runs rich.

This would probably be a good time to do a complete rebuild on the carburetor while it is apart. The parts do not really cost that much.

 

Hey, that sounds like the ticket. We'll give it a try tonight and see what happens.

Thanks!

 

I had similar symtoms on my 400 4x4. I checked over everything. It turned out to be from mud that got in the exhaust and was restricting flow. I cleaned out the mud and the machine ran perfect.

 

Sounds Like it is running rich alright. Has anyone played with the idel mixture screw on the side of the carb thinking it was the idle RPM screw. Check this. Screw it in all the way (rich), then back it off about 1 1/2 turns and then try the low and mid again. If it is still running like a 1 leg dog, keep backing the screw off in 1/4 turns until it starts to come right. Don't lean it up to much though as you will end with problems!

 

Hey 00TB250~

What year is the machine? On my 96' Scrambler, there was a recall for the stock silencer. It was apparently too restrictive and would get plugged up real quick. My machine seemed to run the same way you're describing. I bought an aftermarket silencer, and the machine came to life! Also, the oil injection on my Scrambler was set to run extremely rich from the factory and that also caused it to run like a 1 legged dog. I was constantly fouling plugs and performance sucked to say the least. I think I had to replace a check valve in the oil injection system, and now it is running as strong as ever! First thing I'd do is check the carbeurator, spray it down with carb cleaner and wipe it clean. Adjust the needle, and go from there. Good luck!

 

Has it gotten warmer there in the last week?
If so how much?
Remember 2 strokes need to be jetted at different altitudes and temperatures.

 

We tried everything from moving the needle down to adjust the air screw and cleaning the carb. Could be a jet issue with warmer weather but this is the same jet he ran all last summer. It's weird, it ran fine one day and then this the next. Didn't really look at the exhaust, it's a stock 99 except for a DG oval silencer. I think he's going to go ahead and take it to the dealer and let them look at, 8 hours of work and no luck is enough - time to spring some cash I guess so he can get back to riding. Thanks for everyones suggestions.

 

I had a similar problem with my stock piston on a 99 Scrambler 400. Turned out the top of the piston was scuffed from what has been referred to on this forum as a "cold" seizure. Apparently if you do not allow the bike to fully warm up and then ride it cautiously the first 5 minutes or so you can seize the piston. Hopefully that is not the case with your freinds but it might be a place to start. A compression check by your dealership should be able to determine if the piston is the problem.

 

We've been wondering if something like that could be the problem. Generally you wouldn't think of it as a piston issue since it runs fine on top end but those things can be difficult to diagnose. I'm sure the dealer will find it if thats it.

It'll be nice if it is the piston in a way since he's already decided to drop in a wiseco, port and mill the head if thats the case.