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Biggie9367

Another yz426 backfiring

12 posts in this topic

Ok so my bike is backfiring on accel and decel. Also, it is hard to start, no idle adjustment and fuel mixture screw quarter turn out to start. When I bought bike it would only run with choke on. Changed jets with no luck so cleaned and bought another parts carb to create one good carb. Put all stock mains and pilots in after rebuild. Put carb on but now only starts with choke on, and if I keep it revved it will run with the choke off. Did the mixture screw check with only a noticeable bog down about 3 turns out. Took valve cover off to confirm timing chain and left can gear was off a tooth. Adjusted but now not sure if I should try and start or adjusts jets too while apart? Any suggestions?

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Sounds like valves to me. All the symptoms - backfiring and hard to start.  Also, if you had the timing off you could have bent a valve and possibly have a big fix on your hands. I only know that because of my RMZ 450 I have now lol

Edited by kNewc

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Did the mixture screw check with only a noticeable bog down about 3 turns out.

 

That indicates your pilot jet is too small.  You could try it again now that you have the timing back where it belongs, but 3 turns out means either too small a jet or you still have something obstructing the pilot circuit.

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The screw that's attached by a cable to a black plastic knob is the idle speed. 

 

The small screw in the linkage below the throttle cable wheel is for accelerator pump timing.

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Almost never needs adjusting.  The pump is supposed to be timed so that the squirt is as early as possible WITHOUT hitting the slide as it opens.

 

 

 

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Did you check for cracks on the carburetor manifold?

If air is leaking you wont be able to tune the carb

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No cracks but also no hoses, as when I rebuilt it, the guy forgot to give them back. May be a problem but from others I've talked to it should have little bearing on the backfiring

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Just curious if you figured out the problem? Reason for asking is I rebuilt my carb and took the top off to get at the slide and I took a screw out which slid the whole assembly over and I was not able to set the idle. When I finally figured it out it started first kick.

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So still no change, adjusted timing one mark. Have not had chance to check cable clearance yet but did notice the pilot jet was what looked to be clogged and cleaned it out. Will throw in tonight and see if anything has helped

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