Need help

I have an 01 426. Has run fine for 40+ hrs. On my last ride it started to hesitate when I hammered the throttle. It got so bad it stalled several times. It has also started to backfire real bad and won't idle. When I set the idle and then give it gas the idle will not come back down for 20+ seconds and then may die. Here is what I have done: completely disassembled and cleaned carb 3 times, changed plug, tested for vacuum leak(by spraying cleaner around carb and boots), re-adjusted float, changed needle, main and fuel screw to all kinds of variations. Nothing helps. I plan to check the timing tomorrow. I have reached the end of what I know. Any suggestions, do I check the TPS or CDI and if so, how. I appreciate your help!!!!

If you aren't experiencing a lean carb condition, the next thing I would check would be valve lash. These things have a tendency to "tighten" over time. Are you sure your pilot jet is clean?

I will clean the pilot again, but have already. Can you explain valve lash? Being a former two stroke guy I don't completely understand.

Thanks for the input.

Pull the header pipe off and have a look at the exhaust valve. If it has a lot of carbon build up on it, it mat not be seating correctly. You can get enough carbon on them to cause them to hang open. Thanks's to BK for this info. Not sure if this is your problem, but it's worth a look.

Also, clean all the eletrical connection with contact cleaner. I know when I took mine off to clean had a ton of mud in the TPS connector.

Good Luck



01 YZ426F #85 Vet C

[This message has been edited by forloop (edited 04-02-2001).]

On my 400, I found the linkage for the accelerator pump was tweaked just enough to hinder operation of the accelerator pump. Next time you yank off the carb, I would inspect that very closely. Is your throttle returning with a good snap or is it a little sluggish? I also have found that is a necessity for accel pump operation. Does the bike bog all RPM's when you gas it or just at lower RPM's? How did your air filter look? I assume you checked that already. What was the most recent work you did to the bike before the problem began? Sometimes you create your own problems and won't know it immediately. It sounds like an accel pump/carb issue, but don't let that blind you in looking at other smaller things that may have cropped up. It sounds like your on the right track with your troubleshooting. An air leak would explain the poor idling. Good luck and keep us posted.


99 YZ400

Thanks PK. The throttle is slow to snap back. I just learned from someone elses prior post that the accelerator pump shaft "snaps" into the actuator arm. Mine was not snapped in and I did not snap it in when I put it back together. I am hoping this solves it. Will try tonight. My filter was checked. I had not done any changes at all to the bike. It was stock except for a pipe and minor jetting changes. That was all done over 30+ hours ago. That's what stumps me.

The accelerator pump rod should snap in place and be held there by the linkage. If it is not pumping and running lean some backfiring will result.

You may also need to richen the pilot screw by turning it out, try 1/4 turn increments. This is more common with the fast idle/ hanging at higher rpms.

What clip position is the needle set at? It should be in clip #3 or #4. The middle clip (#4) will have less hesitation and give more quick response but #3 will run cleaner. If you go leaner on the clip the pilot screw may need to be richened to avoid hesitating off the bottom.


Really good advice and insight from the other guys. Now, to answer the question about valve lash, this is a measure taken when the engine is cold. The lash is the clearance measured with a feeler gauge between the cam lobe and valve shim buckets with the engine at TDC of the compression stroke. This measurement is taken with the engine cold to allow for thermal expansion at normal operating temperatures. If the lash is below the minimum spec on any valve(s), then you won't get a good seal during engine operation. If an intake valve is too tight, compression leaks backwards through the carb. You can just imagine what that must be like. Conversely, an overly tight exhaust valve...or valves...can wreak havoc on engine performance and reliability. It's very important to keep the cylinder head in good working order since this is a 5-valve, relatively high compression and high revving engine.

Thanks for all the advice. Last night I tried everything that was posted with the exception of checking the valves. It might have got slightly better, but that is probably wishfull thinking. I also noticed as I was testing it that if I hold the rmp's steady the bike will miss/stumble every 10-20 seconds. I guess I will have to check the valve clearance next. On a positive note, I now can remove the carb in under 8 minutes.

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