Practically Classics Forum

Hi Folks

Any suggestions where I should start looking for a cure to backfiring from the exhaust (actually more like popping) on the overrun?

Engine is 1725 twin Strombergs in a Sunbeam ALpine, runs well, plugs are nice colour.

Thanks !

I dont know this for definate but would putting a smaller jet in the carb cure it as less fuel would be going through and getting wasted?

It's normally caused by modern low octane fuels- my Vitesse does it. If I use unleaded only with the timing retarded to suit it sounds like a bowl of rice crispies on over-run- snap,crackle,and pop. It improves with octane booster and the timing as close to factory as it'll let me, but it'll still spit and pop on the over-run. I did once bung in some of that super fuel they do at a price on the pumps and with a bit of fiddling with the timing vernier on the distributor it more or less cured it- trouble is it's quite a bit more per litre so I just put up with it, it sounds quite nice!

My experience is roughly in line with Mr Rusty with a 66 MGB - really smooth running and accel is superb but it does burble in the exhaust on overrun although I would not call it a backfire. Actually I think the sound is great
I live with it as the engine is so sweet and fidlling about may detract from performance - I remember the old RR Merlin used to pop back spectacularly on Throttle back accompanied with flames from the exhaust
Also, if you have ever witnessed a dragster in full cry just watch when the power is cut, fireworks from the exhaust
The cause - a camshaft giving more valve overlap that slow revs can deal with, low octane fuel, engine carboned up, throttle butterfly sticking slightly open, SU piston sluggish, Stromberg diaphram split, valves not seating, or exhaust leak at the manfold, take your pick.

Thanks, very interesting!

I have just changed the fuel additive, and also it has just started not returning to idle after being revved (throttle sticks slightly open) .... so two things as suggested for me to investigate!!

Also... I seem to be losing oil from the carburettor dampers (ie it needs frequent topping up, but no sign of where it is going) so I guess it's time to have the carb's off!!

Thanks for your replies

Too many beans.

Leaky manifolds. Inlet or exhaust gaskets will cause popping on the over run.

Or an air leak.

A subject that I have recently had cause to consider. One of two things are needed, it occurs to me

1) Exhaust valve leakage
2) Unburnt fuel in the exhaust tract (due to incomplete combustion)

Might be worth concentrating on those first.

To add a "What if",anyone got any idea how a combination of a misfire, occassional backfire/popping and No 1 cylinder not working as hard as it should (the exhaust manifold is still cool enough to touch on that outlet) on an XK engine can be down to carb adjustment/balance?

I'm hopeful it is, but not sure that it can be:- anyone feel up to re-assuring me?

Hello well as above really, bad valve/valve seats, air leak into exhaust, gasket, manifold,joint, weak mixture and something that gets missed quite often Tappet clearances closed up. So before anything else I would check the valve clearances and do a compression check just to see if you have the start of a valve problem.
All the best.
Cheers.
Geoff.