Backfiring Spada.

[ducatilover]

i have been making lovely big bangs all week. i have replaced the plugs, set the pilot screws to specification and also had them running richer to see if that helped, checked the leads, striped the intake and made sure there are no intake or exhaust leaks, have just done the clearances [valves] and they are at .15 intake and .20 exhaust.
perhaps the rear cylinder wants to be a bit richer than the front?

im assuming that perhaps the carbs are not in sync? i have thrown a vacuum gauge at it, both were fluctuating between 30cm/Hg to around 3cm/Hg every time it fired, which is why i checked for intake leaks and checked the plugs [they were fucked so i replaced them]

last time i did the clearances the honda shop in palmy said .17 intake AND exhaust. they were wrong, so its done around 15000km on the wrong clearances.

any suggestions as to why the bike is backfiring?
any help would be great, im out of ideas

[Sully60]

When are the backfires occuring, on or off the throttle?

[ducatilover]

off the throttle. loud fuckers too

[Sully60]

off the throttle. loud fuckers too

Check the exhaust again, all of it.

[ducatilover]

Check the exhaust again, all of it.

i have taken it off, checked it and i remembered to put it back on. there are no leaks. none on the intake side either.

[xwhatsit]

i have taken it off, checked it and i remembered to put it back on. there are no leaks. none on the intake side either.

Might be a silly response, but you might've checked the exhaust but how about its seal with the exhaust ports? Nice new gaskets in there?

[Sully60]

Pass then. (going on the internet diganostic that everything you've checked is as you've said)
Backfires are usually caused by unburned fuel air mixture in the exhaust, when the exhaust is sealed there's not enough fresh air to allow it to combust in a way that causes back fires.

Do those carbs have an air shut off valve, small diaphragm on the side of each carb, it's possible that when these don't function fuel is allowed past the butterfly when closed.

[ducatilover]

Might be a silly response, but you might've checked the exhaust but how about its seal with the exhaust ports? Nice new gaskets in there?

the seal is immaculate, no air leaking out there, that was one of the first things i checked.

[ducatilover]

Pass then. (going on the internet diganostic that everything you've checked is as you've said)
Backfires are usually caused by unburned fuel air mixture in the exhaust, when the exhaust is sealed there's not enough fresh air to allow it to combust in a way that causes back fires.

Do those carbs have an air shut off valve, small diaphragm on the side of each carb, it's possible that when these don't function fuel is allowed past the butterfly when closed.

i thought it may have been the air shut off valve, but they seem to be working fine. im going to have to find another vacuum gauge and play with the pilot screws i think