new engine, backfiring

[minkis18]

Well I just helped my dad put a new goodwrench 350 engine in his 86 sierra 3/4 ton, and now we can't get it to stop backfiring. Also, bare with me here, I am definitely not a professional mechanic

to start, the engine is brand new not a rebuild. We reused everything else though including the intake manifold, carb and distributor. We also had a problem that the bracket that they supplied us for timing does not match! The piece they gave us sits off to the right side, but TDC seems to be nearly vertical on the damper. Our reason for replacing the engine was because it had about 210k miles on it, burnt oil and had low compression.

So like I said, the engine backfires, and thru the carb. It only backfires on acceleration. We have tried tweaking the timing all over the place, everywhere from TDC to +8 and I can not really say which is best. It was working fine at +4, but as I came home from work it was backfiring endlessly after stops, even at low throttle. If the timing is any more advanced, it backfires more frequently, while if it's retarded it idles rough and responds slowly.

The sparkplugs are of course new, however the cables are not, but they worked fine 5 days ago on my old engine. The same goes with the distributor and really everything else. We checked all the vacuum hoses and they are connected correctly and I made sure that all spark plugs were in the correct order. one thing worth mentioning is since the timing mark on the damper and the bracket supplied with the engine don't match up, we made a new mark based on TDC so it could be possible that it is off a bit, but timing it based on the idle performance and backfiring alone says that it is correct.



so, what can/should I do? is it possible that it the vaccum advance is advancing it too far, or is it another issue? any help would be appreciated, this is my first engine swap.

[maxwedge]

Welcome to AF. This is an out of time issue either plug firing order or engine timing, another possibility is an valve not opening such as would happen with a bad cam lobe, if you exhaust the engine timing/ign timing possibilities pull the valve covers and make sure all the valves are actuating. Get the engine on tdc #1, by pulling the plug and getting the piston to the top, use a screw driver thru the plug hole and carefully monitor when it is up at the highest on the compression stroke, then check the rotor position on the cap and the tdc mark on the damper and indicator.

[fuzzypuppy]

This tread is a bit old but I was wondering if you have found your problem?
I had a similar problem with backfiring through the intake only after warm and under load, turned out to be my plug wires, though they were fairly new and tested good on a meter, on a hunch after testing everyother possible cause I changed the wires.

[minkis18]

yes we found the problem...it was actually a ground wire that wasn't connected to the back of the engine!

the timing is still a bit off on the thing, but it runs quite well and has no problem reaching highway speeds.