Backfire through carburator

[sh011956@aol.com]

Initial dist setup at 14 degrees. I retarded the timing far enough to kill the engine. No change. Reset to 14 degrees and advanced the timing until the engine was idling at 3500. No Change with the spitting.

Surprisingly, the new sparkplugs are now black with soot.

As long as it is idling, it sounds good. Give it gas and it spits out the carbs.

Carbs have #52 jets. Does it need larger jets?

Accelerator pumps are new and blue. They work correctly.

I have disassembled the carbs and cleaned them in my ultrasonic cleaner. No change.

Vacuum advance works as it should.

I may have to put the Monza engine back in it.

[erco]

Surprisingly, the new sparkplugs are now black with soot.

Classic sign of running fat (rich).

Carbs have #52 jets. Does it need larger jets?

Likely you need smaller.

[Dennis66]

If you can increase the speed from idle to 3,500 RPM by advancing the timing (if I read this right) there is another problem here.
New questions:
Have carbs been properly set up? static settings for closed throttles + feeler gauge, + turns in (I don't remember exactly off the top of my head) and the mixture seated then backed out (what?) 1-1/2 turns
Do you have a vacuum gauge?
What carbs are you running?

[Wagon Master]

At what timing setting did the engine develop spark knock?

[SkidRo]

I have been following this thread and something finally clicked in my head. I had the same exact issue with my 110 Rampside. I beat my head against the wall for months trying to figure it out. In no particular order, I drained my gas tank, changed the plugs, set the timing (hundreds of times), set the points, etc. In the end I had a bad (stuck) vacuum advance unit. I switched it with a 140 to test and the issue went away immediately. I bought the correct vacuum advance afterwards once my theory was proved.

[sh011956@aol.com]

I have had 2 different vacuum advance units in it. Both of them were tested with a vacuum pump. No change.

I cant advance the timing enough to make it knock.

I have changed the carburetors with 2 other sets of known good carbs. No change.

3 different distributors, no change.

I have been trying to figure this out for many weeks. I'm done with it. Its going to a friend's garage and he will check it out.

Sounding like the cam-crank gears aren't set correctly.

Thanks for everybody's input. When it gets solved, I will update the post.

[Frank DuVal]

I cant advance the timing enough to make it knock.

We do mean ping while driving it, not revving the engine in the garage. If you can't make it ping while accelerating in second or third from a low speed, then something basic is wrong. One can advance a distributor on a Corvair to make even the low HP 80 ping!

For years, and on many different distributor (no ecm control) type vehicles, I would advance the distributor to get ping then back off, like Wagon mentioned. Who needed a timing light? I just left distributors loose enough to do this while on a drive, never tightened one, never had one slip.

We can assume a harmonic balancer, since it is a 140, unless someone put on a solid pulley at some time before you. Do the alignment marks on the balancer still line up? These are lines on both sides of the rubber section. If they have slipped, then new balancer time. Before it chews through the oil filter!

Still line up? Then maybe cam to crank timing is way off. Corvair has the unusual crankshaft driven distributor, so crank to firing timing can be fine but the cam can be advanced or late.

[RexJohnson]

I have seen a broken valve spring cause popping thru the carbs the same as a bad cam lobe. Might be time to pull the valve covers off and have a look inside.

[erco]

Sounding like the cam-crank gears aren't set correctly.

You could check this by pulling the pushrod tubes on cylinder #1. Press your fingers on both lifters while someone rocks the engine back and forth at TDC on the intake stroke. With split overlap, you should feel both lifters barely open at 0 degrees TDC. One tooth off would be noticeable.

[RexJohnson]

You can check the cam timing by removing the oil pan. With the balancer timing marks on zero the timing mark of the cam will be up or down. With it down it should be on the block parting line. If the crank gear slipped it would not be lined up using the balancer to compare it to.

[66vairguy]

You can check the cam timing by removing the oil pan. With the balancer timing marks on zero the timing mark of the cam will be up or down. With it down it should be on the block parting line. If the crank gear slipped it would not be lined up using the balancer to compare it to.

Agreed - EXCEPT, over the years I've seen the "o" timing mark under the tooth, under the adjacent valley, and in between. Apparently stamping the timing mark was not always exact. I now mark a line that crosses the CENTER of the key way slot and cam center. I use the line as a reference to the block parting line. Just me. To confirm I always degree the cam, but that is a lot easier when you are "building" up an engine.

[aircooledairhead]

You mentioned doing a running valve adjustment. You might try redoing this especially if the backfire started after you did the adjustment.
It was over 50 years ago, but I recall this happening to me on a turbo Corsa. Redoing the valve adjustment solved the problem.

[aircooledairhead]

Oops! I mixed up 2 events. In both cases re-doing the valve adjustment solved the problem. The popping through the carb was on a 140 Corsa. Valve adjustment fixed it.

The turbo car was just running roughly and the valve adjustment took care of that. Unfortunately when I tightened down one valve too far, raw gas must have gotten into the muffler and it exploded, fortunately outwards away from the center of the car.

After that, I would remove the muffler, attach flex pipe so it was a good distance from the car then do the valve adjustment. That also helped me hear whether the rockers were tapping or not as I tightened them down.

[66vairguy]

Oops! I mixed up 2 events. In both cases re-doing the valve adjustment solved the problem. The popping through the carb was on a 140 Corsa. Valve adjustment fixed it.

The turbo car was just running roughly and the valve adjustment took care of that. Unfortunately when I tightened down one valve too far, raw gas must have gotten into the muffler and it exploded, fortunately outwards away from the center of the car.

After that, I would remove the muffler, attach flex pipe so it was a good distance from the car then do the valve adjustment. That also helped me hear whether the rockers were tapping or not as I tightened them down.

Interesting -- the only way the muffler should get raw gas is if the exhaust valve is held OPEN. Normally you back off the rocker nut SLOWLY until you get clatter then SLOWLY turn down the nut (slow to allow the lifter to bleed down - if engine idle gets rough then STOP turning the rocker nut until lifter bleeds down and idle becomes smooth). Once clatter stop then SLOWLY turn another 1/2, some like 3/4 turn with NEW cam and lifters to allow for break-in wear. Just 1/4 turn after clatter was considered fine at one time, but SOME lifters do not seem to pump adequate oil up the push rod at 1/4 turn. I've seen this "valve train oiling problem" and going to 1/2 turn always solved it.

[sh011956@aol.com]

Problem solved, not by me but by a couple of club members.

It was the distributor. I put 3 different distributors in it with no change. The guys put in another 330 dist (#4) and the backfiring went away.

Then they put the points and point plate from #4 dist into #3 housing and the backfire came back. Moved the points and point plate back to #4 and the backfire went away.

We think it has something to do with the mechanical advance. I know that original distributor had modified weights and springs but don't know about the others.

I want to thank Kevin and Bill for finally solving this problem!

[Wagon Master]

Glad you found it.