1967 Backfiring Through Carb

[Lee B.]

I have a 1967 327/300 Convertible. I restored it several years ago, but it has always been plagued with problems. The engine was rebuilt and ran fine for a year or two. About a year ago, it started backfiring through the carb. It starts up and idles perfectly. When I start driving it, if I ease into the throttle, it is fine (not great) but if I get into the throttle at all, it stumbles and backfires through the carb and sometimes in the exhaust. The engine has about 3000 miles on it. Everything is original. I just changed the distributor cap, coil, plugs, wires, condenser, points, and rotor. The initial timing is set at 6 degrees, the dwell is set at 30 degrees, and the plugs were gapped at .045. I am at a loss and tired of pouring money into this problem. Just looking for any suggestions.

[Jim D.]

Re: 1967 Backfiring Through Carb

From what you described, it sounds like a bad accelerator pump and you're mistaking a "lean pop" for a back fire. Remove the air cleaner and check the performance of the accelerator pump. Good luck.

[Duke W.]

Re: 1967 Backfiring Through Carb

First you said the engine was "rebuilt". Then you said "everything is original". Please explain along with "always plagued with problems". That could indicate errors during the rebuild. You need to provide more details.

Assuming a manual transmission the engine should idle butter smooth, 500 @ 18" Hg in neutral. What's your engine's idle behavior?

Improper distributor assembly and/or installation can cause backfiring through the carb. If properly assembled and installed the plane of the cap window should be approximately normal to engine centerline and the #1 plug wire should be immediately to the right (pass. side) of the cap window. A photo of your installation would help.

Plug gap should be .035". If it really is .045" that could be a problem. The single point ignition doesn't have enough energy to support that wide a gap.

What is the manufacturer and part number of the installed spark plugs?

Duke

[James W.]

Re: 1967 Backfiring Through Carb

A friend of mine was having the same issue with his '65 327/365. It ended up being bad spark plugs and wires.


James

[Lee B.]

Re: 1967 Backfiring Through Carb

Yes manual transmission and yes it idles really smooth. I installed AC Delco Gold R45S plugs. The problems I had were the following:
The guy who helped with the restoration did not tighten the A-Arm bushings, and the bushing failed immediately.
Pirkle died while he had my electrical parts. I finally got them back but the wires in the solenoid were crossed so I was starting the car on reduced voltage and running on 12 volts. Caused backfiring, coils to burn up, etc... It took a long time and a lot of parts to figure that one out.
And yes the window is facing almost normal tot the engine.

[Lawrence S.]

[Stephen L.]

Re: 1967 Backfiring Through Carb

I had a similiar problem with my 427/390. I had installed a set of "correct" AC plugs (new NOS) for judging. After experiencing the problem, I installed a set of Champion plugs. This fixed the problem. Both sets were gapped at 0.035"

[Timothy B.]

Re: 1967 Backfiring Through Carb

In addition to all the above, it sounds like the engine is going lean when you open the throttle blades, that will cause a backfire through the carburetor. If the accelerator pump is adjusted properly and working I would look for a blockage or dirt inside the carburetor.

How far out are the side screws adjusted and can you kill the engine turning then in.

[Lee B.]

Re: 1967 Backfiring Through Carb

I was wrong. All the plugs were gapped at .035. The were black and dry. I took it out again and no change. The carb has a good accel shot on both sides.

[Leif A.]

Re: 1967 Backfiring Through Carb

First you said the engine was "rebuilt". Then you said "everything is original". Please explain along with "always plagued with problems". That could indicate errors during the rebuild. You need to provide more details.

Assuming a manual transmission the engine should idle butter smooth, 500 @ 18" Hg in neutral. What's your engine's idle behavior?

Improper distributor assembly and/or installation can cause backfiring through the carb. If properly assembled and installed the plane of the cap window should be approximately normal to engine centerline and the #1 plug wire should be immediately to the right (pass. side) of the cap window. A photo of your installation would help.

Plug gap should be .035". If it really is .045" that could be a problem. The single point ignition doesn't have enough energy to support that wide a gap.

What is the manufacturer and part number of the installed spark plugs?

Duke

Lee,
It is difficult to accurately diagnose such an issue over the "web". However, it does sound like a vacuum issue. Do you own a vacuum gauge you could plumb inline between the vacuum cannister and carburetor? Accurate vacuum readings can tell a lot about an engines' condition. Do you own a MityVac you could use to check to see if your vacuum cannister is functioning? Which vacuum cannister is currently installed on your engine?

[Lee B.]

Re: 1967 Backfiring Through Carb

Duke I looked at the distributor and I was off on plug 1. Thank you for that. I rotated all the plugs around into the correct order and reset timing. It definitely runs better but it is still stumbling at WOT and will still backfire from time to time.

[Duke W.]

Re: 1967 Backfiring Through Carb

You may have an incorrectly assembled or installed distributor. If the wires were improperly indexed that is an indication of other possible issues and that the last guy who messed with the distributor was clueless! Do you have a 1967 CSM and COM? You NEED these documents.

The following thread should also help.

https://www.forums.ncrs.org/showthre...tion-corrected

Pay particular attention to where I indexed the crank to remove the distributor and how I "static timed" it at installation so it would start right up with the initial timing in the ballpark if not right on (once I manually closed the inoperative choke). Also pay attention to proper installation of the distributor gear, specifically indexing of the "dimple" on the side of the gear.

Improper assembly and/or installation of the distributor is a common problem and next year will mark the 60th year that I have helped guys out getting their distributor assembly and installation squared away after I screwed up mine in 1965 because the Chevrolet documentation says NOTHING about proper indexing of the distributor drive gear. After the third time I removed the distributor I noticed the dimple. It was a Eureka moment!

As I previously suggested post a photo of the distributor with the rear half of the engine in the view. The orientation of the cap window is a major indicator of proper assembly/installation.

Duke

[Lee B.]

Re: 1967 Backfiring Through Carb

Thank you Duke. I will take a Pic when I get home and I will look through the information you linked to make sure it is right. Will let you know the results.

[Mark E.]

Re: 1967 Backfiring Through Carb

Installing the distributor gear 180 degrees off or indexing the plug wires one or two terminals clockwise or counter clockwise on the cap will NOT cause any problems, including backfiring. These changes only affect the orientation of the distributor housing and cables in relation to the engine compartment. Orientation is important and specified in the shop manual because if altered, you may not be able to rotate the housing to properly set timing because the VAC hits the bulkhead or something else. But a different orientation in itself will not cause backfiring or other drivability issues.

When you re-indexed the cables, the engine ran better probably because you changed static timing, not because the cables are in different holes.

Echoing Leif and Timothy, it's almost impossible to diagnose this on-line but check the usual suspects: vacuum leaks, bad or wrong VAC, static timing set improperly, binding or stuck centrifugal advance mechanism, wrong distributor advance curve, VAC improperly connected, cracked or damaged cap/rotor, poor or loose primary-ignition connections (including wire below breaker plate), lean condition. It may even be a bad tank of fuel (don't laugh, this caused a bucking, poor drivability issue one time that took me forever to diagnose. The fuel was not old; it was bad right from the pump).

This may not be an option for you if you don't have these parts, but a quick way to narrow this down is to install a spare carburetor, then a spare distributor as tests.

[Duke W.]

Re: 1967 Backfiring Through Carb

Installing the distributor gear 180 degrees off or indexing the plug wires one or two terminals clockwise or counter clockwise on the cap will NOT cause any problems, including backfiring. These changes only affect the orientation of the distributor housing and cables in relation to the engine compartment. Orientation is important and specified in the shop manual because if altered, you may not be able to rotate the housing to properly set timing because the VAC hits the bulkhead or something else. But a different orientation in itself will not cause backfiring or other drivability issues.

You apparently didn't read the thread I referenced in post #12. The dimple was correctly oriented but the wires were improperly indexed being one terminal CCW from correct, and I immediately spotted this. Once corrected along with proper distributor installation, which oriented the plane of the cap window to be near normal to engine centerline at the correct initial timing, the low speed "trailer hitching", which made the car virutually undriveable was eliminated.

I learned 60 years ago that of the 208 combinations of dimple orientation, wire indexing, and final distributor orientation there is only ONE that allows the correct initial timing to be achieved on a C2 Corvette either small or big block, especially SHP versions. This is the way it was done by Chevrolet during the assembly process, and this is the way I've always done it once I learned 60 years ago.

I've lost track of how many improper distributor assembly and/or installation errors I've helped guys correct or done myself, and I don't ever recall the issues being anything other than improper dimple orientation, wire indexing or final distributor orientation. The way GM did it is how it must be done in the field if you don't want to fall into a black hole. How many have you done?

Decades ago I learned an aphorism from guys with more experience than me that went something like: "90 percent of fuel injection problems are the ignition system", and I eventually learned that one can replace "fuel injection" with "carburetor".

One of Keith Duckworth's favorite aphorisms was: "It is better to be uniformed than ill-informed."

So Lee, I highly recommend that you thoroughly investigate the entire ignition system and correct any problems before you cast an accusing eye on the carburetor other than going through the idle speed, mixture procedure and checking for leaks, external or internal such as fuel dripping into the venturis after engine shutdown from a leaky needle and seat or leaky float. In my experience the mixture screws should be out about one turn for the 300 HP engine and 1.5 turns for L-78 at or near sea level.

A thorough ignition system check includes the secondary components, cap, rotor, plug wires - no more than about 5K ohms per foot for the OE type RFI suppression wires.

Duke