ducatipaso.org

Try this on for size. i get blowback through the horizontall cylinder carburettor, when bike has warmed up. changed the coils, reguiator,battery and spark plugs.

This sounds a bit strange, minimad. If you're having trouble on just one cylinder, why change the regulator and battery? If the trouble is spark, how come it only happens when the motor's warm? How about checking out the front carburetor? Does it blow back under acceleration, or at idle? Maybe the front carb is rich, so it runs OK only when cold.

Idie or acceleration it dos't change,and ouly when warmed (about 20mins after startup). changed jet size to bigger one, still didn't change.

How well are your carbs synchronized, are you using reccomended plugs, and does the front plug look rich or lean?

yes and yes. it's carbon fouling both spark plugs, with both bigger jets and small jets. it's like the motor start to run on one cylinder, and starts to fire at uneven times. can't reuse spark plugs again, even after cleanup. (motor won't start)

So you're having trouble with both cylinders?

minimad wrote:Try this on for size. i get blowback through the horizontall cylinder carburettor, when bike has warmed up. changed the coils, reguiator,battery and spark plugs.

It might be time to look at valve clearances to make sure they are not too tight. When the engine warms it might be causing the intake valve to stay open.

Testing the ignition pickups might reveal something that only fails when hot too.

Good Luck,

Jon

skin: Can't say, when flames start to come out of back off carb, i hit the kill switch.
jcslocum:had the ignition pickup tested.( ok).haven't checked the valve clearances.
Thanks

Sounds like ignition timing. My only experience with that was to set it statically when I had the motor out. It wasn't hard to do. You should be able to get the driveside engine cover off without taking the motor out.

Does anyone else have any advice?

Just about the backfiring or spitting back through the carb, check the rubber mount from the carb to the manifold. Make sure there are no cracks or splits in the rubber. They run lean as they suck air. It was common with older Dukes.

Thanks to all your help. happy riding

I would agree with jomo on this one look for airleaks