Backfiring Through Inlet Manifold - Ignition Advanced or Retarded

[101RRS]

Simple question. Solely from an ignition perspective, a backfire out the inlet manifold is that ignition too advanced or retarded?

I was trying to fire up a RV8 4.6 that has EDIS for ignition on starter fluid. With EDIS in default mode and while the engine is trying to start it is backfiring big time through the inlet manifold. It does not have petrol on it as yet but does have an old style LPG system but before I try to get the engine to run on the LPG I want to make sure it is not going to backfire on me.

I feel that the engine is advanced more than the default 10 degrees and will need to adjust the timing but just seeking advice whether a backfire through the inlet manifold has timing too advanced or retarded. I only need to set the timing roughly at this stage and once engine is actually running I will get it timed to run at about 8 BTDC.

Thanks

Garry

[aRRon]

Simple question. Solely from an ignition perspective, a backfire out the inlet manifold is that ignition too advanced or retarded?

I was trying to fire up a RV8 4.6 that has EDIS for ignition on starter fluid. With EDIS in default mode and while the engine is trying to start it is backfiring big time through the inlet manifold. It does not have petrol on it as yet but does have an old style LPG system but before I try to get the engine to run on the LPG I want to make sure it is not going to backfire on me.

I feel that the engine is advanced more than the default 10 degrees and will need to adjust the timing but just seeking advice whether a backfire through the inlet manifold has timing too advanced or retarded. I only need to set the timing roughly at this stage and once engine is actually running I will get it timed to run at about 8 BTDC.

Thanks

Garry

Hi Garry,

There can be a number of reasons for backfire but I was taught a rule of thumb that backfire through inlet is ign too advanced and popping in exhaust is too retarded all other things being equal.
Sometimes when they're too advanced you get a funny "nyah, nyah nyah" cranking sound as the engine is firing back at the starter. Yeah I know trying to put noises into text is weird.
Maybe there's also starter fluid lying pooling in the inlet and detonating when she's firing. Valves all ok - not sticking or a burnt valve?

[101RRS]

Thanks for the confirmation - engine is a new build so valves etc are all OK.

I need to retard the ignition which is a bit hard without a distributor and timing marks.

Cheers

Garry

[PhilipA]

Have you got the firing order correct. wrong firing order will do that.

Regards Philip A

[101RRS]

Thanks Philip - I will double check - right now.

[bee utey]

Ignition advance/retard is less critical than correct mixtures. With waste spark style coil packs what happens is that a cylinder at firing point misfires for whatever reason, then the plug fires the unburnt mixture one revolution later when both the inlet and exhaust valves are slightly open. When you have port injection any backfire is usually limited to a small pocket of fuel/air mix and merely makes a small "pop".

About the only thing that will help you is to keep the plug gaps down to around 0.6mm and hope it gets enough fuel to fire straight off.

Reminds me of a bloke who bought a damaged V6 Mercedes to play with, some bloke had used aerostart on it and blew up the plastic inlet manifold.