Backfiring out one side.. Need some help..

commercialcobb · #1 (**permalink**) 08-19-2015, 01:03 PM

So, I've got a 429 with MSD 6al set-up. We just put on a Holley double pumper 750 carb to stop the awful QuadraJet carb issues. It has literally doubled in horsepower and feel from what I can tell, however we are getting backfiring. AND it ONLY backfires out the passenger side pipe..

The backfires only happen at WOT high RMPs (its constant small backfiring) and also backfires when I let the RPMs drop down to idle and get back on it then. (It will big backfire there).

Is there a way to diagnose the problem?
We have done the following trouble shooting so far:
1. Timing is set at 16deg advance at idle
2. The Holley 750 is tuned in so its not running lean
3. We re-adjusted the valves to check for sticking open. (all close up tight)
4. The engine still has its break-in oil so plenty of zinc for the Flat-tapping
5. I pulled the rev limiter from the MSD 6al and nothing changed
6. MSD distributor points look good inside.

We are trying to avoid taking down the intake and everything to check for damage.

CHANMADD · #2 (**permalink**) 08-19-2015, 01:23 PM

Leaky header, vacuum leak somewhere ....most likely header

commercialcobb · #3 (**permalink**) 08-19-2015, 02:16 PM

Compression test on the passenger side revealed a low cylinder..
#1 140 psi
#2 140 psi
#3 130 psi
#4 140 psi

Detroit Bill · #4 (**permalink**) 08-19-2015, 02:41 PM

That's not too low. Is it a dual plane manifold that does not have a cut in between the sides? If so may be a vacc leak. Is it popping through the carb?

commercialcobb · #5 (**permalink**) 08-19-2015, 03:09 PM

Quote:

Originally Posted by CHANMADD

Leaky header, vacuum leak somewhere ....most likely header

A header can create a backfire??

commercialcobb · #6 (**permalink**) 08-19-2015, 03:32 PM

Quote:

Originally Posted by Detroit Bill

That's not too low. Is it a dual plane manifold that does not have a cut in between the sides? If so may be a vacc leak. Is it popping through the carb?

Not popping through the carb only out the passenger side pipe..

Tommy · #7 (**permalink**) 08-19-2015, 03:59 PM

Popping through one exhaust, particularly when backing off the throttle is often a sign of an exhaust leak in the header/coupling/side pipe. The leak lets outside air get into the hot pipe with the unburned fuel and it burns in the pipe (pops).

commercialcobb · #8 (**permalink**) 08-19-2015, 04:36 PM

Oh, it's not backfiring when letting off the throttle.. it's only at WOT and hammering it at idle..

It really appears to be 1 cylinder because it does do it all the time.

commercialcobb · #9 (**permalink**) 08-19-2015, 07:15 PM

sorry, I meant it does NOT backfire all the time, and when it does, its under load or WOT

Slo-poke · #10 (**permalink**) 08-19-2015, 08:38 PM

Double check Dizzy cap . Strange things can lead to crossfiring inside the cap. Look for tracks from one cyl to another and the coil to a cyl.

You can also short one plug at a time to perhaps isolate it to a particular cyl. ,when you find which one , then the fun starts. Look at all the wires in the dark, see if a wire is sparking over to another. The wrong wires such as solid core with a msd sys can lead to a lot of grief.

Others who have more experiance with Msd systems please chime in !!

Good luck
now back to my ghost water leak
Gary. aka Slo-poke.
Classic 429 enhanced with lots of expensive parts !

Gaz64 · #11 (**permalink**) 08-20-2015, 03:28 AM

I have used MSD and had similar issues.

Just check the MSD website forum.

ICE ignition will fix your issue.

Have bought ICE twice for 2 cars and never had an issue.

BAsque1 · #12 (**permalink**) 08-20-2015, 05:20 AM

Header gasket

commercialcobb · #13 (**permalink**) 08-20-2015, 10:45 AM

I still don't understand how a header gasket leak could cause a backfire out the tip of the tail pipe while accelerating?

commercialcobb · #14 (**permalink**) 08-20-2015, 11:10 AM

I will try to re-wire everything today and run it again

SFSDFGT · #15 (**permalink**) 08-20-2015, 11:39 AM

Quote:

Originally Posted by commercialcobb

I still don't understand how a header gasket leak could cause a backfire out the tip of the tail pipe while accelerating?

As I understand it, if there is a leaky header gasket, exhaust pulsations can draw some atmospheric air/oxygen INTO the exhaust system through the gasket, allowing any unburned fuel in the exhaust to ignite (which otherwise wouldn't occur without enough oxygen), hence the "popping" in the exhaust. Somebody, correct me if I am wrong.

twobjshelbys · #16 (**permalink**) 08-20-2015, 01:03 PM

Have you pulled the plugs on the side that is backfiring? I would guess this is an ignition problem. Distributor or plug wire cable or plug itself.

Slo-poke · #17 (**permalink**) 12-03-2015, 07:02 PM

Have you ever found an answer to the backfiring ?

Ray1960 · #18 (**permalink**) 12-03-2015, 07:39 PM

Hi, did the car have the back fire problem before you changed the carb?

66gtk · #19 (**permalink**) 12-04-2015, 07:33 AM

I had a similar problem, and it was the header gasket. Cheap fix, and now the problem is gone. A better solution is to not even use a header gasket, but make sure your mating surfaces are machined to match up nicely with proper sealant as the gaskets generally fail quite easily.

kevins2 · #20 (**permalink**) 12-04-2015, 08:42 AM

I had the same exact symptoms with my side oiler w/MSD. After a few days the coil went completely. A new coil solved the problem. I still can't explain why the backfire was always on one side. My best guess is that one of the contacts in the distributor cap was the weakest, or something similar (plug wire, etc.). Regardless, a new coil completely eliminated the backfiring.