318 backfiring

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rmoore0088

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I just swapped my slant six for a 318. It runs great at idle or at low rpms. You can rev it in neutral and it sounds fine, but when it is in gear, it backfires real bad right before it shifts. After it shifts it doesn't backfire again until you run the rpms up again. It's a 650 holley dpeardbore and I timed it with a light at about 5 degrees advanced. It starts with ease and gives me no other problems. I'm about to my wits end here. Please help.
 
check the accelerator pump adjustment on the carb....if the throttle linkage moves..it should be squirting gas...any delay is causing a lean condition ...backfire
 
When you say right before it shifts are you talking about full throttle @ high rpms or just normal cruising?
 
Well i have to be away for a bit so i'll throw out a few thought. I'm assuming it's happening under heavy throttle and it's backfiring out the exhaust. 1st thought, it could be going dead lean because of a fuel system that's not keeping up. 2nd thought, not sure if it's a fresh build or a engine with some miles on it but the valve springs may be on there way out. Also the possibility of too much total advance although that's not likely running only 5* initial. That engine should be plenty happy with initial in the 12-14 BTDC range. I'll be back later if there's anything i can add.
 
Okay, I have a couple tricks that may help. First, timing should be set with vacuum to distributor disconnected and plugged. Set to 8 degrees BTDC then reconnect vacuum to the distributor. Switch the other end of vacuum hose to ported vacuum, above throttle plates, instead of the mainfold vacuum plugged in base plate. There are some timing lights that are adjustable to check if the advance that the weights and the vacuum create. Mine is a snap-on brand, By 2000 rpm you should have full advance around 30 plus degrees. Ten to twelve vacuum, twenty plus mechanical. You can change out the springs or weights for the mechanical while the vacuum advance is not adjustable. It either works or it don't. Obviously, replace if it don't. Any pinging at W.O.T. means too much advance. Good luck, hope it helps.
 
I about a cheep set of ign wires. Every 6 mounts i would get a bad wire that would cause a back fire at high rpms.

Get my meter out and would find one with vary high resistance. replace it and back fire gone.

Unfortunately i lost a eng before i figure this one out!

get a volt ohm meter and compare the resistance of all your ign wires. as well as the coil wire.

I when the weak valve spring way and installed shims under the spring to fix problem. Didn't get the valve keeper in properly and dropped a intake valve thru piston and cracked cyl wall.

a cheep meter is like $10 at the parts store.
 
I think The Wall is gonna get this one. I have a very badly worn exhaust lobe on my camshaft. I ordered a new cam .454/.454 lift, all new lifters and double roller timing chain to get her all going again. Thanks for the help everyone!!!
 
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