Outer Limits 4x4 Board

Driving home yesterday my patrol (308, LPG) with trailer and car on the back, up hills as soon as you get under load it starts back firing.

Then I back off and its ok but car loses speed of course so on a couple of hills i was back to 1st and 2nd low ratio to get up the hill without putting so much load as to have it back fire.

Anyone got any ideas. Im thinking it could be too lean under load?

Check your leads , if your not using spiral woung steel ones , otherwise check for cracking of the porcelain on the plugs.

John.

im getting this in my 253 dunny wagon , the only things i can think of is, its the lifters (cause mine have seen better days) or the dizzy (ive got a electrick dizzy). have you got pionts or a electronic dizzy?

Its points dizzy and they are normal looking old leads :p

I have recently put in new plugs which are one heat range up (i think) on standard for gas use. The car had stock plugs in it before this..

Ill double check the ignition system and see what the plugs are looking like and see how we go.

through exhaust or through carby?

Through carby

I have had that occur 3 times during the life of my 1989 Maverick and each time I tossed a new coil in it fixed it straight away.

its either firing too soon or the valves are opening too soon. check both, backfires through the carby are really not good for the system.
ive destroyed 2 carbys and 2 convertors via backfires through the intake. although, they were big enough to be heard a km away and blow off the intercooler piping at 3 places

it sounds like you might need a new dissy and coil and leads i have all this for sale but just not coil if you need or want it

The dissy is electronic bosh hei and brand new eagle high perfomance 9mm leads the dissy if re graft by hurcules race engines for good power and efficnecy..........


Just pm me if you are inerested in them..........

too lean a mixture will make it back fire through a carbie

surely it would have to be very lean to detonate early enough for the intake valves to still be open. especially with LPG and its higher octane ?

Faulty(high resistance) leads and/or overly lean mixtures cause backfires in the intake due to crossfiring in the distributor. A compressed mixture is difficult to ignite, an uncompressed on easy, this is why a spark plug will easily spark a 1/2 inch gap in free air but often struggle at .20 thou in the compression state. Lean mixtures have less carbon, as LPG has less carbon than Petrol, carbon is the semi conductor that the spark travels through as it ionizes the plug gap, this is also why LPG is cleaner and has lower carbon emissions but is harder to ignite. The spark will jump in the distributor to a non-compressed cylinder if the inlet valve is open the air/fuel mix in the intake manifold ignites, hence the big bang and destruction of parts.
Occasionally a non-opening exhaust valve due to a very worn camshaft can cause an intake backfire but that happens consistantly at 1/2 engine speed as the combustion cant exhaust through the exhaust valve and comes out the intake.
Waste spark coils have a similar issue and can ignite on the off cycle cylinder, ie 153624 no 1 and 6 are both ignited at the same tim if no.1 is on compression then no.6 is on overlap and is prone to backfire.

Joel