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I have a backfire issue. When I accelerate at full throttle the backfire starts about 3500 rpm. If I let off on the throttle, the backfire goes away and the car continues to accelerate. If I accelerate at part throttle the backfire doesn't start until higher RPM. 1/2 throttle and the backfire doesn't start until about 5200 RPM.

I hooked up a vacuum gauge today to see what it said. I have a very steady 18" at idle. If I slowly increase the throttle the vacuum increase with increased RPM. If I snap the throttle, it goes to near zero and then up to 21 or so and then back to 18.

Backfire occurs with initial timing set at 4 deg BTDC or at 12 deg BTDC.

Let's just start there and see where it takes us.

Murph

What kind of ignition are you running?

MSD pro-billet Dist., 6AL box, Blaster coil and MSD wires.

Murph

Murph can you borrow another carb and see if that's the problem?

Not specifically Don. I'm sure if I asked around I could find one to borrow. I'm with you on a fuel issue. I'm going to try to clean the carb this weekend and maybe a fuel pump first. Original fuel pump from 1968 is still in there. Carb was new when I installed it 500 miles ago.

The carb does have some signs of issues but I'm not sure what it means. When I try to adjust mixture I get some strange happenings. On the passenger side mixture screw it performs as expected. If I turn it out, idle raises until it just stays flat/smooth. If I close it down too far the idle goes down and starts getting rough. Just as I would expect it to happen. The drivers side screw is a different story. If I turn it in, it starts to stumple and run rough, if I turn it out, it also begins to stumble and run rough. There is a very small window where the idle is smooth. Either direction on that screw the idle speed goes down and gets rough.

Murph

When I built my engine I had a similar problem with the mixture screws. It turned out to be a poor seal between the carb & intake. I replaced the gasket & carb studs it went away. It seems that re-using a carb gasket was false economy.

OK. Here's today's results.

Checked plug wires for proper firing order. OK.
Put different rev limiter in the MSD Box. No change.
Checked fuel pressure. 5.5 psi at idle and up to at least 1500 rpm. Ran out of fuel in the carb bowl at this point, end of test.
Pulled the carb off, took it apart, cleaned it out, back togther. No change. (Evaluated fuel flow to carb after forgetting to tighten fuel lines at carb and starting engine)

So here's the answer to through the exhaust or through the carb. I don't know. I can't tell and don't have enough experience with backfires I guess. It is not coming from the tailpipe area. Defenitley under the seats or in the engine compartment. I would best describe it as popping. It is rapid. Sorry I don't have any better detail on this one.

Tomorrow I was planning on trying compression testing.

When I pulled the plugs the other day they all looked the same except number 3 and 5. Both had oil fouling. Not real bad but it was there. I guess I could post pic's of the plugs if anyone thought that would help.

Thanks for the help so far.

Murph

Have you filled up with gas lately?

2 or 3 times.

I had this same problem at one time. It would idle pefectly and slowly accelerate but open it up and it would spit and raise all kind of hell. I ended up finding I had about a half tank of WATER.