426 Backfires when I let off then idles poorly

I 've read other threads suggesting exhaust leak(it's tight) or pilot screw adjusting,the only thing this problem only shows up after hours of riding,so I thought it might a overheating problem but there is no boiling or loss of coolant .Like I said for the first few hours it runs like a dream so I 'm a little Thanks for any suggestions.

Check your valve clearances. I have seen a valve that was starting to get tight cause similar problems. If they are on the edge of tollerance then they may get tighter as the engine warms up. Hope this helps.


I just had the 450 cam put in by a shop so I all the clearances were checked,also the bike should be warm within the first few minutes of riding no?Any way I will keep that in mind Thanx.

How about an intake leak? I would still check the clearances just to rule them out. Other than that you may have an electrical problem that only crops up once things get good and hot. Possibly a plug that is failing at higher temps, or a connection that gets loose when hot. Does it run any differently under load when this happens, or is it just at idle? Any way you could take a temp reading when this starts happening? Maybe you are having some sort of vapor lock or something similar going on that could be heat related. A temp may be very helpful in diagnosing this problem. Sorry I couldn't be more help.


The thing runs great under load,just when I slow down does it act up,I bought a new pilot screw ,the kind that you can turn with your fingers and I will re check all exhaust fittings,so I will start there and hopefully that will do it.

Remember that just because the nuts and bolts are tight doesn't mean it isn't leaking there. There may be a better way, but I use a lighter to find exhaust leaks. You will see the flame blow when you find one. Like I said, there may be a better way but I have done this in the past with success. Hope this helps, good luck.


I use a lighter to find exhaust leaks. ... there may be a better way but I have done this in the past with success.
Apart from the obvious issue with gasoline and all, it's tough to use that method in the wind, which means you're indoors with a running engine.

There's another way. Use a 2-3 foot piece of hose anywhere between 1/2" and 1" in diameter with one end cut at an angle to make it easier to get into tight spots. Put the other end in your ear and probe for leaks with the cut end. When you find one, you'll hear it.

Thanks Gray. I am indoors with the engine running, usually in my garage. I figured that it wouldn't be running long enough to matter anyway, never thought of using a hoseoscope. Good info.


too lean?

Maybe the fuel in the tank heats up over time on a long ride? Just a guess.

Maybe the fuel in the tank heats up over time on a long ride? Just a guess.

I think mine is doing some of that. I am gonna try some heat tape on the bottom of the tank and insulate/re-route the fuel line. Mine only does that over about 90 degrees.

If your looking for a vacuum leak get some ether(starting fluid) and spray around the carb when you found it the motor will knock rev up just don't put it in by the air filter:bonk::crazy: that will till you if you are sucking air someplace

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