02 426 backfiring, stalling and making no power.

Put everything back together this morning fired it up and I've still got the backfire.

Edited by Jtt_R

Sounds like timing. You may want to start by seeing if the piston is actually at TDC when the marks on the flywheel say it is.

They are, I had to have it a TDC to do the valve clearance. Now I can't get it to start at all..I tried to bring the idle up a little and totally screwed everything up. Does anybody know how many turns out the fuel screw should be and also how many turns out for the idle screw? I'm about to lose my mind on this one.

After some serious pondering I'm starting to think my issue is electrical. The bike was idling fine and when you pulled the choke you could defenitly tell it was high idle. Then in the middle of my ride it dies out. Now: it doesn't idle as smooth and the with the choke out it is not a smooth high idle like before so... if I'm having an electrical issue and I'm missing it would cause it to run rough. AND if I'm missing then I'm having unburnt fuel into the exhaust which when the next cycle comes around it combusts cause the flame and backfire.

With this information does anyone have any ideas or suggestions?

Edited by Jtt_R

Jtt, how does the bike run when it's cold? Mine seems to run great when it's cold, but after a minute or so of idling and warming up, the backfiring starts.

This tells me that it is probably a fueling issue. I think maybe it's running really rich which is why it runs ok cold but not when it's warm.

I checked my coil and other electronics (with the exception of the CDI box because there is no check for it) and everything was in spec.

I just ordered a 450 carb today so I'll let you know how it goes.

Do your electrical checks according to the manual. Test for spark, too.

The manual is where I got the specs. It is definitely getting good spark as it runs well when it's cold.

I used my brothers 07 which is running fine to trouble shoot everything. When it first started I put in a new plug then one at a time tried his CDI box, no change, and then coil pack, no change. I've read that it may be the stator. Some say it just failed randomly and the same problems started occuring. They also said that it can change with the heat, meaning the stator can read fine at idle but under load and with the engine warmed up it fails. Unfortunately for me there is no cheap way to test this. It's basically buying a new unit and putting it in.

I'll let you know when I come up with $250 and can buy one. In the mean time I'll keep my eye on the thread and let you guys know if I figure anything else out. Thanks for everyones help.

You can't try the stator from your brothers bike? I wish I had a donor bike to try parts from!

I used my brothers 07 which is running fine to trouble shoot everything. When it first started I put in a new plug then one at a time tried his CDI box, no change, and then coil pack, no change.

How exactly did you use the CDI or coil from an '07 on your '02? Just curious.

How exactly did you use the CDI or coil from an '07 on your '02? Just curious.

My bike is an 07..there are two different bikes in this thread with the same symptoms.

You can't try the stator from your brothers bike? I wish I had a donor bike to try parts from!

Pulling his stator would require an oil change, pulling the side cover, new gaskets...just too much work to do on his bike just to see. I ended up finding a used one off an 08 that came with the 08 cover for $100. Should be here on Tuesday so I'll let you guys know. I'm leaning heavily on this being the problem as the more I've troubleshot everything the more it points to this. Once the bike heats up it stalls and won't refire until it cools off.

Pulling his stator would require an oil change, pulling the side cover, new gaskets...just too much work to do on his bike just to see. I ended up finding a used one off an 08 that came with the 08 cover for $100. Should be here on Tuesday so I'll let you guys know. I'm leaning heavily on this being the problem as the more I've troubleshot everything the more it points to this. Once the bike heats up it stalls and won't refire until it cools off.

sounds like you might be onthe right track there,hope it works out for you.

Jtt, please do let us know. I should have my "new to me" carb tomorrow. I should have it in tomorrow evening. I'll let you know if it changes anything. I'm hoping it does, but not getting too optimistic.

Jtt, how does the bike run when it's cold? Mine seems to run great when it's cold, but after a minute or so of idling and warming up, the backfiring starts.

This tells me that it is probably a fueling issue. I think maybe it's running really rich which is why it runs ok cold but not when it's warm.

I checked my coil and other electronics (with the exception of the CDI box because there is no check for it) and everything was in spec.

I just ordered a 450 carb today so I'll let you know how it goes.

The bike will start when it's cold fairly easily but it still doesn't run right. That's why i think mine is electrical. The flames and popping i think is due to it not firing completely so there is excess fuel in the exhaust. so when i give it a little throttle and it gets hot the fuel in the exhaust fires.

I've never known backfiring in the exhaust from too rich a mixture unless actually caused by a major air leak somewhere.

Stator windings certainly can fail in a temperature related way. I've had this a couple of times. The insulation on the windings can break down and expansion cause shorting somehwere or a tiny break in the windings that only opens up with expansion on heating up.

The first time I had it I had travelled 300 miles to do a speed hill climb on my Rotax-Morini. Got halfway up the hill at Doune and it died. Coasted back down and when it cooled off it started again. We diagnosed that by putting a voltmeter across the connections of the stator ignition winding, Started it up and it gave X (I forget exactly) volts. Ran it for half a minute and the voltage suddenly dropped to zero and it stopped. Pretty easy to do without dismantling. Otherwise as Grayracer said compare ignition component resistances and output voltages at kickover against the figures given in the manual.

I tried pushing to the line then kicking it up and launching straight off - still got only 3/4s of the way up the course :angry: No Bosch dealer for miles to get a new 'ignition charging coil' so I didn't even record a practice time. At least that winding was a separate part you could replace fairly quickly so from then on I carried a rewound spare. When I went short circuit racing with that motor it also seemed that as the windings started to go bad the ignition timing would go off and take the piston with it - but that's another tuned to the point of imminent destruction two-stroke story.

My Gilera GFR 125 had a similar problem just a couple of years ago. Died on a run. Recovered by breakdown truck to home. Put a new plug in and it startd so I went for a ride round the block, died again after about 5 minutes - but pushing distance from home. Checked the stator resistance and it was half what it should be. Rewound and all was well again.

I've had coils show a completely different resistance value after every time I rapped on them with a screwdriver handle.

I'm disappointed to see that the Yamaha manual gives no clue as to what the output voltage for the 'source coils' should be at kickstart rpm as I reckon that could actually be a better diagnostic than winding resistance given the possibility (usually remote except on ancient stuff to be fair) of a rotor low on magnetism. At least "some" volts AC I guess. For 'fun' trying to sort ignition systems you can't beat the weird Ducati Elettrotecnica system on old Morinis.

I got the carb from the 450 today. I threw it in and the bike fired right up. It had me fooled again but started showing the same symptoms once it got warm. This means that it CAN'T be a fueling issue.

I'm really thinking this could be the stator now I guess. I will try to test it soon and see what happens.

Have you checked your Flywheel and the woodruff key, Ive heard these can shear that key and the timing your checking could not be accurate.

That's a valid question in the case where the bike just won't start, but this one starts and runs good when it feels like it. Unless the flywheel is still moving on the crank, which would be unusual but not impossible, a sheared key doesn't explain it. Besides, if the key had sheared, he would have found something wrong when he checked the timing at least once. The flywheel moving by itself would have made it lie about where TDC is, and that would make cam timing appear to be off when it was not. Nothing like that came up.

It's starting to look more and more like an intermittent electrical problem, like a stator coil or spark coil with internal wiring damage.

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