Sunday, July 27, 2008

Honda XR 600 Sticky Throttle: Problem Solved

If you have a used Honda XR 600 it's likely that you have experienced this problem: At high revs the engine suction is so large that the throttle sticks open, specially when the engine is warm, keeping the bike at high revs (something very scary if you are trying to slow down approaching a corner for example).

It doesn't happen to a brand new bike probably because the wear in the carburetor slide produces this after a few years of use.

There are many websites that talk about this problem, and people have tried to solve it by lubing the throttle cable, the rail where the slide goes up and down, and even drilling a hole in the slide to relieve the negative pressure caused by the engine suction (this last one doesn't seem like a very good idea, as it's equivalent to keeping a partially open throttle). The problem it that the lubing doesn't last and the problem comes back

My bike's throttle used to get stuck on downhills after the engine was warm, and lubing the cable or changing it didn't help. To get the revs down I had to use the kill switch, blipping the throttle only got the revs higher and higher as the suction increased and you had to finally use the kill switch to turn it off anyway. Of course the throttle returns perfectly with the engine off, for there is no engine suction then.

The solution

I took the carb apart two times; one to clean it (which did absolutely no good) and the second one to analyze the problem some more.
Pushing the slide forwards with a finger from the air filter side to simulate engine suction, revealed that the slide stuck at the lower 45° corners it has when they encountered the round opening at the engine side which is very sharp.

So all I did was eliminate the lower corners with a file and that was almost it ... (the top of the illustration shows the slide as seen from the front of the bike).

...then I rounded the corner of the slide that meets the front opening of the carburator so that there would be less chance of the slide getting stuck in the sharp border towards the engine, as shown on the lower part of the diagram (from the point of view of your right knee).

I reasembled everything and took the bike for a ride and it seemed better, then a few days later I rode about 10 miles without a single incident. Today I went to do some hill trails riding with a friend for 3 hours, and didn't have any problems even after hours of riding with a hot engine and a lot of downhill (50 percent of the road was downhill...).

So what you do to correct the wear the engine causes on the throttle slide rails?
You wear the carb slide some more on the right places, so that it doesn't get stuck. Works like a charm.



Bryan said...

This sticky throttle prob has driven me nuts since I got my bike in 1997! It's been a great bike but I'm about to get rid of it if I can't fix this prob.. The only thing that's different about mine is that I've never had my throttle stick on- all my problems are at the beginning from idle to low throttle. It's a bad lurch that makes the bike no fun to putt around cuz it's so hard to ride it smooth... Thanks for your ideas on your posting, if this doesn't work I'm getting rid of it.. ~Bryan~

Bryan said...

Of anyone knows of an aftermarket pipe for an xr600 that's at least as quiet as stock or if anyone wants to trade my supertrapp straight across for a stock exhaust pipe let me know... Thanks Bryan @ 949-212-6950