C2 Back Fire and Bolt Torque questions...

[Steve Wallach]

I have a 67 427/390 (L36) that I have done quite a bit of restoration work to that I'm trying to dial in now that a lot of it is done. I am having some back fire issues now that I have gotten everything back together. A friend of mine that is relatively familiar with old motors and carberators suggested I re-torque the Intake Manifold which he feels may address the back fire issue. I removed the Intake Manifold, had it cleaned and repainted. I replaced the gaskets, restored the Distributor, had the Carb "show rebuilt", and did several other things at the same time. the car seems to run strong but it is back firing and the vaccuum seems low (11 inches of mercury). And when "cold", it does not want to Idle, but will once warmed up. (still back fires though)

We have checked and rechecked the timing, I think we are ok there.

What is the torque setting of the intake manifold bolts (I'll try re-torquing those)

Any other suggestions?

Thanks for any suggestions!

Steve

[John H.]

Re: C2 Back Fire and Bolt Torque questions...

Steve -

Intake manifold bolt torque is 30 ft-lbs. Is it backfiring (through the carburetor) or after-firing (through the exhaust pipes)?

[Steve Wallach]

Re: C2 Back Fire and Bolt Torque questions...

Hi John,

Through the Exhaust Pipes at about 3,000 RPM and above. The Vaccuum also seems low at about 11 inches of Mercury...

The car was running EXTREMELY Rich and pouring fuel into the oil when I bought it. Everything was carboned up pretty heavily. The Intake Manifold and Carb. have been completely cleaned and gone through as has the Distributor. It has Side Pipes on it and the back fires are Loud Pops in the exhaust pipes.

Thanks Again for any help on this.

Steve

[Duke W.]

Re: C2 Back Fire and Bolt Torque questions...

The low idle vacuum could indicate either a non-OE cam with much more overlap than OE or a leaky valve.

Unfortunately, the combination of idle low vacuum and exhaust afterfiring indicates a leaky valve as a prime suspect.

You need to conduct a compression and/or leakdown test, preferably both.

Basic ignition and fuel system functionality checks should be done first because ignition misfire or something like a clogged fuel filter or a faulty fuel pump can cause backfiring through the carb or afterfiring in the exhaust.

Compression/leak down testing should be done ANY TIME there is a possibility that symptoms are caused by some internal engine fault. It's the equivalent of your doctor checking your blood pressure and heart rate.

Duke

[Terry R.]

Re: C2 Back Fire and Bolt Torque questions...

Perhaps a little Marvel Mystery Oil poured in the carb to loosen some sticky valves?