Backfiring.

[liljegren]

O.K., so I'm just a dumb boatbuilder, but what specifically causes an engine to backfire? Thanks in advance to all my tutors out there...Cheers, Matt.

[pete376403]

Backfiring as in loud bangs in the exhaust? With closed throttle, on overrun it is usually an air leak in the exhaust - the air plus unburned fuel is ignited by hot exhaust pipe component.

Backfiring as in banging back through the carb is usually ignition timing way out, or less likely cam timing out.

[Spuds1234]

Backfiring as in banging back through the carb is usually ignition timing way out, or less likely cam timing out.

Wow I didnt know you could get backfiring out the back of a carb.

Does it sound any different to normal exhaust backfiring under deceleration?

[pete376403]

You've never seen a 500 single with a big cam and carb (eg Norton Manx, Matchless G50 or similar) having the throttle snapped open before the motor is ready (warmed up enough) to deal with it? Big bangs indeed.

Even just trying to kick start a street motor without retarding the ignition enough (in the days of magnetos with manual advance) would get you a nice backfire in the carb (and a good hit to your ankle)

Don't they teach ANYTHING anymore?

[F5 Dave]

O.K., so I'm just a dumb boatbuilder, but what specifically causes an engine to backfire? Thanks in advance to all my tutors out there...Cheers, Matt.

So do we assume you are talking about your boat?

Or Perhaps the VFR in your bike list?

Or some other 'engine'?

[liljegren]

Not stupid enough to own a boat! The backfire is on my Gs 550 postie bike. Cleaned the carbs, starts fine, but gets rougher as it warms up. Trying to get it warmed up tonight to check ign timing, but dies repeatedly as it gets hotter. Tank breather clear, fuel filter new, coughs and dies like fuel starve, backfires here and there back thru the carbs.
Maybe I should stick to fixing boats!!

[bogan]

Not stupid enough to own a boat! The backfire is on my Gs 550 postie bike. Cleaned the carbs, starts fine, but gets rougher as it warms up. Trying to get it warmed up tonight to check ign timing, but dies repeatedly as it gets hotter. Tank breather clear, fuel filter new, coughs and dies like fuel starve, backfires here and there back thru the carbs.
Maybe I should stick to fixing boats!!

check inlet valve tappet clearance, and valve timing, if it's neither of those it'll be ignition timing i reckon.

[marsheng]

And now for the real reason !!

When the mixture is lean, it burns more slowly. A backfire is when the exhaust valve opens before the mixture has completed burning.

This is normally occurs when you close the throttle from a high rpm and the mixture becomes very lean or when cold, the fuel air mixture is still running lean (Choke off too early) while the engine warms to operating temperature.

[F5 Dave]

Ahh, some more information, now we're getting somewhere.
Sorry for the sarcasm but you see my point.

Tell us about your airbox & filter. . . I think that will expose a bit about the lean condition.

Also from previous experience I know there is sod all room between the tank outlet & the carbs (mine had CVs). Any attempt at fuel filter caused a kink in the hose. However they now make filters with a right angle, check if bike shop carries them. But also check tank. If there was any silt in the carbs (gets through any filter) the tank needs sealing. All Suzis of that era do it & sealing is the only solution. If done before check the tank vent in cap isn't blocked.

[liljegren]

Thanks folks. Spoke to previous owner today, and he suggested I use 91, or half 91, half avgas instead of 98(Gull). This may get a cleaner burn, cleaner plugs, better running, and hence no backfire. Anyone agree? Once I get it running better, then I can finalise timing etc.

[marsheng]

Petrol grades do not make any difference to the engine performance. All the grades really do is allow higher compressions to be used. There is differences in the composition of each octane rating but they all burn just about the same. Changing octane will not fix you bike unless you have a high compression engine > 11: 1 and you are running on 91 octane.

PS in The USA, the chemical mixtures of the fuels change from season to season so you are never exactly sure what you are getting.

[T.W.R]

Petrol grades do not make any difference to the engine performance. All the grades really do is allow higher compressions to be used. There is differences in the composition of each octane rating but they all burn just about the same. Changing octane will not fix you bike unless you have a high compression engine > 11: 1 and you are running on 91 octane.

Explain Detonation then ? or more precisely one of the 4 possible reasons listed in basic engineering being "use of fuel with too low an octane"

Basic fault diagnosis.

Back Fire

1) Too lean air/fuel mix
2) Over heated engine
3) Incomplete closure of intake valve

After Fire

1) Ignition failure due to defective spark plug
2) Too rich air/fuel mix
3) Retarded ignition timing
4) Incomplete closure of the exhaust valve

[pete376403]

. There is differences in the composition of each octane rating but they all burn just about the same.

Not exactly. Higher octane fuels burn slower. Extreme example - Honda had to run on lower (than race fuel) octane in the 5 and 6 cylinder GP bikes back in the 60s cos 100 octane burned too slow - at high revs the fuel had not completed its burn by the time the exhaust valves opened

[F5 Dave]

And potentially the calorific value is lower as there is less base stock & more aromatics.

[liljegren]

Fresh fuel, new plugs, charged battery (total loss system), and lo and behold, running much more smoothly, specially at the bottom end. Backfire seems to have been cured, so maybe a combo of dirty plugs, and not being fully warmed up? Anyway, now off to Puke for some dydnamic tests. Thanks for all your posts and advice, I've learned a few things! Cheers, Matt.