Ok so i have an 89 camaro that i just did an engine swap on. And i got it running good in park but when i drive it, it runs ok until i hit about 3000 rpms. Then it starts backfiring out of the carb, sometimes with flames. Also it wont let me go over half throttle or it stumbles and backfires. Sometimes its only 1 backfire and sometimes its several very quick ones.

Some background info.. It was a 305 tbi and the new engine is a 350 board .40, flat top pistons, vortec heads, edelbrock rpm intake, summit brand hei distributor, and a brand new holley 670 street avenger. I started it up for the first time and did the cam break-in and set the timing at 10* advanced. I have the fuel pressure set at 6 psi. The floats are set correctly. I have #68 jets in the primarys, and #70 for the secondarys. Shooter size .35. Power valve is 3.5. I checked the number 1 spark plug and it was very black. So i figure it must be running very rich. But i also thought that when a carb backfires that usually means its running lean...?

So i thought maybe one of the exhaust lobes on my cam got eaten up. And i took the intake off and looked at each of the lifters. All of them looked excellent. So i think my cam is ok.

Now i guess i dont know what is wrong. I havent put it back together yet but im guessing its something with the carb or timing. Anyone have any ideas?

 

Timing sounds too retarted. Advance it some more. Are you 100% positive 10 degrees timing on the indicators is actually 10 inside the engine?

Had a similar problem with my 350 swap. My 350 had the timing indicator at the 1'oclock position and the old 305 had it at 12'oclock. I overlooked this and as a result timing was retarded and I got back fires.

Also, the firing order could be off 180 degrees.

Pull dist cap off. manually rotate engine so the rotor points at whatever terminal you have #1 plug wire connected to. Pull #1 plug and check to make sure the piston is almost at the top of its compression stroke. Both valves for #1 should be closed. From this point rotate the crank 90 degrees counter clockwise and the #1 exhaust valve should start to open. Then you can be sure that #1 was on TDC on its compression stroke while the dist was sending spark to it.

Also double check that the spark plug wires got to the correct pistons.

 

You know thats a really good point, because i noticed when i was timing it that it actually idles a lot smoother at like 16* which to me would seem like too much. I didnt hear any detonation but its hard to tell because its just open headers right now. I know its not 180* out or it wouldnt even run

 

I agree - bump up the timing. Mine is set at about 14*BTDC - on my car, I can't go above that due to detonation or I'd go higher (I need heavier weights/lighter springs in the distributor).

I'd also lower the fuel pressure a tad - 6psi is too much - you need closer to 5psi. Edelbrocks like no more than about 5psi, most Holley's like slightly less.

 

According to holley's tech videos, 6psi is ideal. Anything over 7 is too much

 

Might be different on different models then - last time I looked at a Holley instruction it stated slightly lower psi, but I don't remember which model I was looking at.

 

Holley says 5-7 psi fuel inlet pressure ( http://www.holley.com/data/Products/...0219-3rev1.pdf , Page 7). But, as long as you don't have too little or too much pressure, having it in between doesn't affect the way the carb functions - i.e., 5 psi doesn't make it run leaner than 7 psi.

Are you setting the timing with the vacuum advance disconnected? If you have vacuum on while setting it at 10 degrees, it will be very retarded.

You can't have the distributor in 180 degrees off and get the engine to 3000 RPMs. If it's 180 degrees off, it won't start.

Put the stock jets, PV, and squirters back in. Probably isn't your backfiring problem, but you need to get everything baselined before you start messing with those changes. However, a 3.5 PV is probably going the wrong way (what cam are you running?).

 

The vacuum advance was connected now that i think of it. Ill re-time it this weekend without it. Thanks five7
The cam is a comp 270h magnum. The vacuum gauge read about 13 when it was at idle
I have a car show to be at exactly 1 week from now, so im really hoping i can get her running good this weekend

 

If you had it connected to a timed vacuum port, it may not have been advancing, but it's best to disconnect it rather than chance it.

If it was connected to manifold or "full" vacuum, then for sure your timing was set too retarded. It should make a big difference when properly set if that was the case.

Things such as idle speed and idle mixture should be adjusted after the timing and with the vacuum advance connected.

 

I hate it when a thread goes dead after the OP figures out the problem, so it turned out to be just the timing. Waaay to retarded. I had it set with the timing light at 12* advanced and i tried more and less timing, but then i noticed it liked to idle better with a LOT of timing. The timing mark on the balancer was on the other side of the block with the timing light on it, at like where 11:00 would be. So i figure the outer ring on my balancer must have slipped or something. Its running good now, pulls hard, lots of fun