back firing 340 hp

[Bruce W.]

Hey guys,
have a strange problem. when driving under hard acceleration i will get a back fire at about 5,400 rpm. Acts like valve float but it is a 340 hp with solid lifters. Has new plugs,valves have been adjusted, timing is 8 degrees BFTDC. has electronic distributor. other than that it runs perfect. Wondering if it could the gas. I am running 93 octane pump gas. appreciate any help
Bruce

[Frank D.]

Re: back firing 340 hp

If its backfiring out the rear (after fire) and not the carb then raw gas is igniting in the hot tail pipe - often, its from an exhaust manifold leak so check those bolts and also a lean mixture condition causes it...

[Bruce W.]

Re: back firing 340 hp

Frank, thanks for your response. Back fire is at carburetor, not out the exhaust. Any other thoughts?
Bruce

[Frank D.]

Re: back firing 340 hp

Frank, thanks for your response. Back fire is at carburetor, not out the exhaust. Any other thoughts?
Bruce

Some - make sure your distributor hold down bolt is tight, make sure that during the plug change no wires got swapped (if the right wires are crossed the car may seem fine at low RPMs), make sure the wire at the distributor breaker plate is solid and well attached. I'm assuming its a C2 and if so check the bulkhead connectors....some friends have had issues similar to yours when the infamous "red wire" problem showed up.

Gas would be a long shot unless it is contaminated (e.g. water)...

I'm not sure what "electronic distributor" you're running but I wouldn't totally discount a problem there...

[Terry D.]

Re: back firing 340 hp

How many miles on the engine? Have you done a compression test lately? What distributor? When does total advance occur? Does distributor have vacuum advance, if so is it working properly? As you see we need a little more info

[Bruce W.]

Re: back firing 340 hp

Terry, engine was rebuilt about 10,000 miles ago. it has 50,000 miles on it now. was bored to 60 over. has delco 1110985 distributor. no vacuum advance. no compression test. Car runs very strong. Don't know when total advance occurs. timing is set at about 8 degrees BTDC. It seem to be pretty much wound out about 5,500 rpm that when the pop occurs?
thanks Bruce

[Gene M.]

Re: back firing 340 hp

Dual point distributor?

Based on description you give at 5500, sounds like ignition timing, possibly points need reset.

How is the advance curve working and what does the points dwell show?

What have you done to correct it?

[Bruce W.]

Re: back firing 340 hp

Gene, does not have points was converted to electronic. what timing would you recommend? not familiar with advance curve? How do I check it?
Thanks Bruce

[Terry D.]

Re: back firing 340 hp

I'm confused, you say it was rebuilt 10,000 miles ago but has 50,000 miles on it?? you should be pulling more than 5500rpm from a good 340hp engine. Why no vacuum advance? do you know what weights were used in distributor? You need to find out total advance and at what rpm. You can either do some research on how to do this ( many threads on this forum or google ) or get someone to help you. How about the engine rebuilder? Did he set the distributor up without vacuum advance? Did you do a compression test yet?

[Duke W.]

Re: back firing 340 hp

... could be distributor issue. Check dwell at idle then free rev it to about 6000. How much does it drop off.

Also, remove the fuel filter, empty it and blow through. There should be very little resistance.

Duke

[Gene M.]

Re: back firing 340 hp

Gene, does not have points was converted to electronic. what timing would you recommend? not familiar with advance curve? How do I check it?
Thanks Bruce

[Bruce W.]

Re: back firing 340 hp

Terry, sorry, i meant it had 40,000 miles when the engine was rebuilt which was 10,000 miles ago. now has 50,000 miles. This engine doe not have a vacuum advance on the distributor. Had it judged several times was never gigged for that. Is it suppose to have vacuum advance? it is a 62 340hp engine
Bruce

[Richard G.]

Re: back firing 340 hp

The lack of vacuum advance, in the 1962 motor, is one of the few differences between the 1962 and 1963 340HP motors.
If there are other differences it would be interesting to know what they are.
Rick

[Bruce W.]

Re: back firing 340 hp

Gene, i was wondering if it could be a weak coil? How do I check it?this coil it is 25 years old?
Bruce

[Gene M.]

Re: back firing 340 hp

Gene, i was wondering if it could be a weak coil? How do I check it?this coil it is 25 years old?
Bruce

Anything is possible but not likely since 5400 RPM is achieved before it happens. Electronic ignition has a predetermined dwell. If not intended for higher RPM use it is more possibly the culprit. Are you using an HEI type that is known for early versions to be unacceptable as RPM’s exceed 5000.

What ignition module do you have? Full voltage or ballast resistor? Carbon impregnated or solid plug wires? Old longer or current short replacement rotor? More info would help myself and the guys help you fix the problem.