I've got a stock crate 1999 350 with a 4brrl Carter AFB Comp. Series carb...I just put a new distributer in and tried to dial in the timing by ear as best as I could. I also put some new bosch two prong plugs in, but I told autozone that it was a 1999 350 and they plugged in a 99 model camaro (the difference in plugs wouldn't be the problem would it?). They said they didn't need to be gapped.

I've also installed a set of headers...and the header flange doesn't look like it's mating with the surface of the heads quite right but I can't see/hear an exhaust leak. With just headers running down the road at full throttle the engines rpm's run higher than normal and it sounds like it starts misfiring...then when you let off the gas it backfires through the pipes EXTREMELY loudly. It's had a backfiring problem that seems to coexist with exhaust leaks. The cast manifold developed a leak it you could hear the backfire increase as the leak got bigger.

What could be the possible cause? Since it's a stock 350 is it having problems because it doesn't have any backpressure from the exhaust? It idles great, but when you floor it...it sounds like it's about to explode. Then you let off on the gas and it backfires.

Could it be in the ignition? Could it be in the carb? Could it be a vaccume leak? Do I need to put the pipes on it and shut-up?

Thanks
Brian

 

Backfiring in the exhaust is usually too much fuel, or retarded timing. Other possibilities are valve that isnt closing, cylinder not firing (dumping fuel into exhaust). The leak makes backfiring worse because you are adding fresh oxygen into the exhaust. Another thing to keep in mind is that headers transfer a LOT more noise through them than manifolds. As for the plugs I wouldnt bother with those fancy plugs. Id get some good Bosch or NGK plugs and gap them to .045 (for an HEI). Pull out some sparkplugs and let us know what they look like (describe the colour and if they are wet or not).
CoryM

 

A factory 99 350 would be an LS1, so you probably have the wrong plugs in there. Which crate engine is it? If it's a Targetmaster first gen replacement (55-86 or so), use A/C R44TS.

You should invest in a good timing light, or borrow one until you can. Your timing is probably way off.

Fix those two obvious problems first, then see where you are at.

If your headers weren't bolting all the way up to the head, you would get an exaust leak for sure, but since your headers are open, you might not hear it. If an engine is right on the edge of being lean with a full exaust, going to open headers would lean it out enough to cause problems and require a jet change to compensate.

But like I said, fix the two obvious problems first before you start messing with the carb.

 

I checked the timing...it's dead on the money 8 btdc...it seems like it runs better when it's more advanced though. I checked all the plug wires with the timing light and they're all firing. It seems to idle fine but when you hit the gas and hold it...it sounds like it is stumbling and misfiring. Also when it's idling I can hear a faint ticking sound that sounds like it's coming from the 5 and 7 cylinder rockers or the exhaust opening on the drivers side. It's hard to pin point it because of the noise.

Are spark plugs that much different for an HEI distributer vs. a 1999 distributer on a fuel injected engine? I pulled the plugs and the majority of them are a dingy dark black color and smell like gasoline. I hesitate to take the plugs off and return them, but if the plugs are wrong it's their own fault right! So if it's misfiring at throttle, but not when it's at idle that's got to be the plugs though...everything else on the ignition is functional.

But since it runs better advanced doesn't that suggest a dead cylinder...and the engine is compensating for the lost power by running faster?

I just thought of something...if the plugs are gapped for a 1999 350 with efi...isn't that the wrong gap? Those plugs are obviously gapped closer than .45...they're right up on the electrodes. I'd say you couldn't get a piece of dental floss in there. What if I regapped the plugs to .45?

 

Well, as you may have guessed, the wet/black plugs indicate unburnt fuel. Yes, re gap the plugs to .045 first. If your gap is too small it wont ignite enough fuel. Far as I know the LT1 engines use a .035 gap. Fix this before going any further. After that if you still have black/wet plugs you are likely too rich. Im sure there are people more knowledgable with the carters than me so I will let them help you there (I like my simple holleys ) Keep us posted on your progress.
CoryM

 

The 99 LS1 doesn't even have a distributor. The difference is in the reach of the plug (how much thread length) and the tip extension. I don't know shinola from LS1 plugs, but if they have the wrong reach, your plugs might be recessed into the cylinder head. If the plug isn't all the way to the combustion chamber, it's going to have a tough time getting the flame front going. Different plugs have different length center tips. The center tip length makes a huge difference in the plug's characteristics. Especially how "hot" or "cold" the plug is.

As far as the timing, 8 is ok, but is it really 8? I've seen engines that seemed to want a lot of advance, and it turned out there was a mismatch beteen the balancer and the timing tab. So it threw the indicated timing off. But since you bought a crate engine, I presume the balancer and tab came with it. Other possibility: are you disconnecting the distributor advance when you set the base timing? Have to ask. I've seen guys set timing with advance hooked up and instead of having 8 degrees, they actually have zero or are firing atdc.

Other non timing causes of backfiring: vacuum leak, lifters adjusted too tight not alowing valves to close.