1971Ultra

Here's one for the Forum.

Just installed the ZZ4 Fastburn. I have about 300 miles on the motor.

The car runs great at 55mph/2500rpm in 4th gear. As soon as I take it to about 60/65mph and just about 3000 rpm's, the engine backfires bigtime, plus a few other noises that I can't convey (really scary). Once I get past the 3000 rpm barrier it seems to be OK.

Furthermore, I noticed that it does the same thing in 1st, 2nd and 3rd gear at right around 3000 rpm.

Too rich? Too lean? Electrical? Any ideas?

Kipp


[Modified by 1971Ultra, 1:59 AM 2/17/2003]


[Modified by 1971Ultra, 2:00 AM 2/17/2003]


[Modified by 1971Ultra, 2:00 AM 2/17/2003]

The Money Pit

If I were to guess I'd start by checking the distributer and spark plug wires.Crossfire maybe,or a dead spot in the ignition module.

Smokehouse69

I think it is too lean, it could be from fuel starvation. Perhaps your pump is failing or the one of your fuel filters is partly plugged. I guess it could be a timing issue of some kind.

LAvetteman

I am having almost the exact same problem. I hope we both find our problem, because if you are like me, it will drive you crazy. Mine for the most part is a pop/miss, but last week it did backfire in the exhaust which is really odd. I have changed everything except the plug wires. I am thinking it is not fuel related but who knows. Keep us posted.

BTW the wires I have now are 1 yr old TAYLOR wires, from what I have seen so far I will never buy Taylor wires again.

Alwyn678

:withstupid:

1971Ultra

By checking the distributer, what should I be looking for? Points? Timing? :confused:

Do you think maybe the ignition box (MSD 6A) may be bad?

The sparkplugs wires are brand new (MSD)

Thanks

Denmech

Backfire out the intake or exhaust?


[Modified by Denmech, 6:27 AM 2/17/2003]

1971Ultra

There is much popping coming from under the hood. And to be honest it sounds like backfire is coming from both locations. I am going to have to pay a little more attention.

Turbo-Jet

Afterfire (thru exhaust) under acceleration is usually caused by lean a/f ratio, or timing, assuming valvetrain mechanics are functioning normally. Afterfire on deceleration is usually caused by rich idle or part throttle a/f ratio, our leaking fuel nozzles due to high vacuum signal, or too high of a float height.

Backfire (thru intake) is almost always a result of timing errors, either over advanced or inconsistency in spark voltage or distribution.

Have you pulled your ALL the plugs? How do they look?

I’m no expert, but it sounds as if your MSD brain is whacked.
:skep:

lou

Is this a new or old distributor?

1971Ultra

Not sure on the age of the distributer. The MSD came with the car when I bought it and the previous owner had not record of its purchase. Same goes for the MSD6A ignition box.

If I replace the box is there any preference for the 6A or 6AL. As far as I know the only difference (other than price) is that 6AL has rev limiting capabilities.

As for the the plugs, they are also brand new. I will check them all again.

And as always, the Forum is full individuals willing to share information, experiences and ideas. THANKS

LAvetteman

I sent my box to MSD and they gave it the ALL OK. But, who to say thats not where my problem is.......

Bearcat

Sounds to me an awful lot like busted valve springs. Pull the valve covers for a quick look-see.

turtlevette

This has happened to me. I went through 2 sets of pistons before i figured it out. The carb is jetted too lean. Its lean backfire thru the carb. Are you using your stock Q-Jet? If so, it would need bigger jets to accomodate the much higher breathing capacity of your new hi-po engine. You should take care of this ASAP before you burn a piston.

jim

Denmech

:iagree: I would check this first! If dist is old I would check for play in the shaft.

1971Ultra

WOW - the last thing I want, is to go through psitons on a brand new motor.

The carb is also brand new - Speed Demon 750. I will address a potential lean issue and go from there.

turtlevette

You wouldn't think that a high performance carb would run lean unless it was purposely jetted that way on your old engine. I would also check your fuel pressure as a previous poster suggested.

How does the engine pull under heavy acceleration?

jim

KNEVER

I would try another carb just to see if the problems go away. I had a new speed demon doing the same thing. check out lars's post about BG tuning.

LAvetteman

My vette started doing it with my old holley (750) and continues to do it with my new speed demon(850). Makes you want to rule out a carb problem.

Fevre

:yesnod:

When I changed cams and tried jetting with a O2 sensor I was reading it wrong and kept jetting it up thinking it was running lean. :bb Car backfired when dec and shoot flames. :eek: Once I figured out i was an idiot and started reading the O2 sensor right, I leaned out all four jets and have not had a problem with backfire since.