3000 rpm missfire with flame

pev · Jan 17, 2016

My 1973 850 Mk2 starts ok and mostly runs well apart from an occasional missfire on the right hand cylinder at about 3000rpm. Revving it on the stand there is a sharp crack noise and occasionally a yellow flame emerges from the silencer on the rhs. To clarify, at around those revs, the crack happens every few seconds and the flame appears every 3rd or 4th crack. The plug colour looks about right and there is no pre inginition, or "running on" after switch off. The idle seems fine and it is ok at higher revs. I showed to someone and he suggested a valve problem; I'm suspecting the carb but it is occasional rather than continuous - I thought I'd tap into the collective wisdom here before tearing things apart at random.
cheers
Pev

concours · Jan 17, 2016

Ignition.... still got points?
Could be a broken wire to the points

pev · Jan 17, 2016

Thanks for the response - it's got electronic ignition, no idea what brand.

concours · Jan 17, 2016

pev said:
Thanks for the response - it's got electronic ignition, no idea what brand.

The original wires may still be in use, the ones that run down the frame to enter the timing chest can flex & break. We'll need more info.. see what kind of E.I. you have, Boyers have a pattern failure ( replacing-boyer-leads-t17499.html ) that can behave just like you describe.

pev · Jan 17, 2016

Thanks Concours - I'll investigate further and report back. May take a day to get access.

sportsroof · Jan 17, 2016

Far as I know, there are a few Noccers in Bristol, have you tried to get any of them to help you?
Cheers, Martin

Rohan · Jan 17, 2016

Could be as easy as a dud spark plug too - tried swapping them out for new ones ?

If one doesn't fire as often as it should, unburnt fuel builds up in the exhaust, and then every so often ignites when a good firing cycle does happen.

aceaceca · Jan 17, 2016

When my Boyer had one of its bad wire connection days, it would explode out the muffler. Scared the willies out of me as it happened all of a sudden. If you have a Boyer it could be a loose wire connector just inside the points cover. While there to fix that the other failure is where the wires connect to the circuit board. Drill them and put small brass machine screws to hold them on. In the old days a friend of mine would take his moms 1956 Oldsmobile and run it up, turn the key off and let a good charge of unburned fuel build up in the exhaust system and then turn the key back on. Big boom. He used to scare little old ladies and men doing that right beside their car.

pev · Jan 18, 2016

Thanks again for the responses guys. I am not a club person so I guess I'm on my own with this one, which will hopefully be OK as the symptom is very consistent and replicable. Also I can do most things apart from welding and fabrication, given a bit of a steer and advice.

New plugs are on the way by post (I'm actually in the country rather than in Bristol) and I will go through the ignition system. I will keep the board updated, especially if I find the solution!

freefly103 · Jan 19, 2016

Suggest you check all earths, particularly ignition.

Craig · Jan 19, 2016

Yup, and also run dedicated ground wire from head back to battery ... several years ago some one , on here told me to do this mod ... my life has been easier ever since I did this simple chore ... I would wager this miss is electrical in origin ...
Craig

pev · Jan 20, 2016

Thanks again, I'm beginning to believe it's electrical as well - it makes by far the most sense. An occasional miss which leads to unburnt fuel in the exhaust system which eventually ignites fits the symptoms perfectly. I'm away for a week now but will update when I nail it.

cash · Jan 20, 2016

pev,
what coil set up is on the bike?
Dave

3000 rpm missfire with flame

Similar threads