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E: Engine Premium fuel causes backfire ?????


Howard

Well Known and Active Website User
VOC Member
I'm having random kickbacks (backfires) when starting.

The engine starts easily, runs as well as ever, ticks over at about 800 rpm, but very occasionally it kicks back on start up for no apparent reason.

Having tried everything in the book and more, it occurred to me this morning that the latest bout (and I've no idea if this is true of the others) was just after I'd decided to give the old girl a treat and put premium fuel in instead of bog standard with Ethanol.

Has anyone else noticed this? I'm wondering if it's due to the premium fuel burning quicker ........... or whatever.

Howard
 

erik

Well Known and Active Website User
VOC Member
Which ignition System is fitted to your motorcycle? Erik
 

Howard

Well Known and Active Website User
VOC Member
Battery under 12 months old. Not used for 2 days and showing 12.9 volts today. I've fitted a new Pazon ignition box, but not a new trigger plate yet .... I think that's just a switch and doesn't affect timing, but I'm going to swap it anyway.
 

Howard

Well Known and Active Website User
VOC Member
As you were. It's not a backfiring problem, it's mechanical. Looks like too much movement causing the gearbox shaft to lock up occasionally, same result in the starter gears as a backfire. New bushes and bearings should help.

Howard
 

Kiwi_Tim

Website User
VOC Member
the higher the octane the slower the burn, that's why you can get away with high compression on 100 octane without knocking. Try running a motor on methanol and it will be very smooth because the fuel burn is not short and sharp.
 

davidd

Well Known and Active Website User
VOC Member
the higher the octane the slower the burn, that's why you can get away with high compression on 100 octane without knocking. Try running a motor on methanol and it will be very smooth because the fuel burn is not short and sharp.
I know that fuel is a subject that I will never know well, but I think that the "slow burn" idea is more true with street fuel than race fuel, so it is probably fine to say it is slower burning. However, octane is a measure of how easily fuel flashes or ignites, not flame propagation speed. Flame propagation is relatively constant. Having said that, street fuel can be blended with slower burning blends to literally slow down the flame propagation. This is not done in race fuel.

I would have followed Chankly and checked for an air leak. I know this will cause backfires on over run, but I don't know about starting. I do know that engines that are lower in compression than the octane fuel is designed for tend to produce less power. I would change back if you had no detonation or pre ignition with the lower octane.

David
 

vibrac

Well Known and Active Website User
VOC Member
I would not let the battery off so lightly voltage is not everything I have just fitted a Pazon to my just built Trigrumph unit 500 the battery was 12.6v but she did not run clean put on a new battery same voltage and she runs smooth
Somebody said once it's amps that do the work.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
I would not let the battery off so lightly voltage is not everything I have just fitted a Pazon to my just built Trigrumph unit 500 the battery was 12.6v but she did not run clean put on a new battery same voltage and she runs smooth
Somebody said once it's amps that do the work.
Howard now seems to have found the cause of his backfires to be mechanical but if you do have coil ignition starting problems it is worth trying to check voltage under working conditions with just the battery supply, especially if you have an electric start. I had a problem on an electric start bike (not a Vincent) and the voltage at the coils was OK static but during cranking was low. Better earth connections to the coils improved things. Some electronic systems can also default to no retard when the supply voltage drops below a threshold. Early Boyer units suffered from this. I can't comment on newer units as I have no experience with them or other makes.
 

greg brillus

Well Known and Active Website User
VOC Member
Yes some of the newer ones play up with low voltage...........Some of them are very sensitive to electronic tachometers as well, the makers instructions should be read several times if you are unsure.
 

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