Backfire through carburator

[sh011956@aol.com]

I have been working on Corvairs and old cars since 1973, and I can usually find and solve issues within a reasonable time, however...

I recently installed a 140 engine in my 66 Monza convertible, and it backfires through the carbs on acceleration, especially when cold.

I've always been told that backfire through carbs was caused by either timing or a lean condition. I have adjusted the timing multiple times, installed 3 different distributors (2 140's and a 110) and changed to pertronix ignition. New plugs, new wires, new cap and rotor. Harmonic balancer has not slipped.

I have been through the carbs multiple times and have installed 2 other sets of known good running carbs. I am running a mechanical fuel pump with an electric pump in line also. No change.

This is supposed to be a good engine from an "expert". Runs strong at intervals but runs poorly consistently.

What am I missing?

[Dennis66]

Can you elaborate on "this is SUPPOSED to be a good engine"? My initial thoughts would also lean towards timing or carb issues. Beyond this, is it one side only that backfires? Both? Swaps sides? If it is one side, time to focus on that side. Pull rocker cover and make sure all valves are opening correctly and the same. I had a straight six that started backfiring through the carb and that turned out to be a worn cam lobe. (one possibility). Cam to crank timing is another. Just my thoughts.

[sh011956@aol.com]

The engine came in a car that was purchased by a friend and is supposed to be "fresh'". It looks like it has been recently worked on, that is everything is extremely clean and painted with no leaking seals.

I failed to mention that I adjusted the valves with engine running.

The backfire is not limited to one side. Both sides will pop.

Still baffled.

[sethracer]

Have you run a compression test?

[Frank DuVal]

In hard cases I put blanks on the secondary ports and run just two carburetors while troubleshooting. Eliminates the possible lean from open secondary carburetor throttle plates. :tu

I know, need fuel plumbing from a two carburetor engine. And some scrap metal to make the plates.

I agree, a compression test should be next.

[sh011956@aol.com]

Compression check results
2---155
4---160
6---150
1---145
3---155
5---145

Found vacuum leaks on the secondaries. Next step is to block them off (again) and check the results.

[sh011956@aol.com]

I blocked off the secondary carbs using aluminum plates under the base. Reset timing.

Still pops through both carbs on acceleration. If you are extremely easy on the throttle, it will not spit.

Still baffled in Alabama.

[Wittsend]

I don't see a great variation in compression that would indicate an intake valve hanging open. Thus valve wise the only possibility I see is that at idle the oil pressure is low and a leaking lifter (adjusted to compensate for the leak) might pump up at higher RPM/oil pressure??? High enough to not fully close the valve?

I don't want to send you on a wild chase but perhaps removing one plug at a time would indicate which cylinder is causing the backfire. The other thought is that try running the timing as low as you can get it (with the engine still running) and see if that helps. It might be that this supposed "fresh" engine has the cam timing off a tooth?

[Wagon Master]

Where do you have your initial ignition timing set at with vacuum advance disconnected and plugged? What is your ignition timing at full advance at say 3,000 rpm withe vacuum advance connected?
Do you ever experience pre ignition aka spark knock?

[Dennis66]

Given the "new information", a few more things to consider: Do you have a vacuum gauge? What's vacuum at idle? What about a UniSyn gauge? The sudden acceleration backfire makes me lean towards a lean mixture (no pun intended). I originally skipped this dur to multiple carbs being tested, but what main jets are you running? Have you checked accelerator pump streams?
Beyond this, again skipped because of multiple distributors being used, have vacuum advance units been verified to be working correctly (and correct units), what about mechanical advance units?
What carbs? Some have the enrichment valve that can stick causing a ... lean mixture. Just more suggestions Dennis

[aj_ed]

Hey sh011956@aol.com, If you get this figgured out, I am in the same position, and have done and gotten the same results.
Still popping thru both sides, good compression, tried blocking off 2 carbs, new vacuum advance, no leaks, adv 16Deg (140)
gets better when warmed up. Let us know.

PS mechanic and machinist since 1970, and have built several Corvair engines (stock, and race)

Ed

[sh011956@aol.com]

We are still looking and haven't found the problem. Maybe the cam-crank gears 1 tooth off? If you find something let us know.

I have a couple more things to try.

[RexJohnson]

Since a lean mixture seems like the most likely cause, what have you done to confirm or deny this theory?

[Dennis66]

After running, what do the plugs look like? White and blistery? Maybe a picture of a few plugs

[Wagon Master]

What is the initial and total (all in/full advance) ignition timing on this engine?
Let's rule out the easy things first!
P.S. Keep testing while moving the distributor an 1/8" at a time counter clockwise till it exhibits spark knock/pinging. Then reduce, turn clockwise, to eliminate ALL spark knock. Post results.

[sh011956@aol.com]

I will be testing again today. Yesterday I was helping my neighbor vacuum water, due to frozen pipes, out of his house.