'03 Bayou 250 ignition miss?

[Jeff_S]

I am 100% sure the carb cannot get any cleaner.

I knew I should have looked at the flywheel key. I also need to figure out the knocking noise this thing makes. I searched some old bayou posts and read that some people had a worn primary clutch cause knocking sounds. Do you think I should look into that? I haven't found anything on it yet that would cause the sounds it is making.

[Jeff_S]

Finally got the flywheel puller, pulled the flywheel and the key is perfect shape, no marks or shearing.

So... Kinda lost on where to go with it now, as it seems I have checked nearly everything. I still haven't put air in the cyl. to see if I get air out of the valves. Guess I'll try that.

[RCW]

With a knocking sound, it can be just about anything from a loose cam chain, worn cam chain guides, worn valve guides, a wrist pin failing, broken rings, and the list goes on.

At some point if all the routine diagnostics fail to turn up a problem, it will be necessary to tear it down and perform visual inspections.

[Jeff_S]

With a knocking sound, it can be just about anything from a loose cam chain, worn cam chain guides, worn valve guides, a wrist pin failing, broken rings, and the list goes on.

At some point if all the routine diagnostics fail to turn up a problem, it will be necessary to tear it down and perform visual inspections.

Yeah, I think I may be at that point unfortunately. I just hate to take apart a running machine with such an odd misfire. Always seemed like an ignition problem, but at this point I really don't know.

[RCW]

A misfire is one thing, and a knock is something that requires physical inspection and correction before major damage is done.

[Jeff_S]

Well, I finally got some time to put air in the cyl, and see if the valves are leaking.

I cranked it to TDC, put in 50, then 75 psi and I don't get any noticeable air out of exhaust or intake.

Guess I'm gonna start taking it apart.

[Jeff_S]

Cylinder and head are off now, no real noticeable problems in here either. Piston does have some scuffing on the intake side, but really doesn't appear to be too bad.

On another note, bro in law just bought an '02 Bayou, so I have a good running atv to swap parts and try on mine now. I checked compression on his and his is only 10 psi higher than mine. Mine 145psi, his 155psi. His runs really well, so I hope to be able swap some stuff and figure this problem out.

I guess I will probably just rebuild it now that I have all the seals broke open and it is all apart. Planning on new cam chain, sprockets, guides. Also, plan to get it bored and get a new o/s piston, rings. Valves appear to seal and both hold liquid when I filled up the cc with water. So, thinking about leaving these. Rockers appear fine. Still need to pull cam and check. Plan to pull the valve springs to check them as well.

Anything else I am forgetting?

[Jeff_S]

Well, I am getting this thing back together and I finally found a couple parts with some wear. I pulled the cam, and I thought 1 of the lobes looked bad, so I pulled both rockers and the intake rocker slipper pad is worn down pretty bad. The cam has wear on it too and has a lip on 1 side of the lobe. I hope this is the problem, although it's probably another $150 into this thing.

[hbradsher]

My son's bayou 250 is a 2003 and is doing just like yours. Did you find the problem?

[Jeff_S]

I never found any one thing really wrong with it. The worn rocker arm, I believe, was causing my knock and possibly making it backfire too. However, after rebuilding most of the topend it STILL backfires at idle sometimes. Makes me crazy. It does run much smoother and quieter than before. More importantly, the backfiring/missing is 90% gone. I am thinking mine just needs some jetting fine tuning to hopefully make it perfect. I think the carbs on these are extra hokey and inconsistent, making jetting and troubleshooting hell. I have jetted probably 20 mx bikes and never had problems like this.

Good luck!