The Stovebolt Site Forums The Shop Area Engine Shop backfire thru carb at downshift

Good to be back. Haven't visited this board in a couple years.

I got the 57 running again. I was in the middle of a minor overhaul and then got married and and had to remodle our 120 year old house. The truck got hte short end of the stick.

it drives great now except it gives a bunch of quick back fires when I down shift and sometimes when I let my foot of the pedal. Timing maybe?
Its been timed w/ the light, but the mark jumps around a bit, New points, vac advance, fresh rebuild on orig. carb, cap, rotor, plugs plus other new things not rlated to problem.

It drives great besides that, just trying to work out the little things as they pop up.

Any ideas on that backfire.

Thanks

glad to hear from you,, could be in the time or may even been a cracked dist cap...........you gotta watch when the little things pop up, sometimes they become big things good luck,,,,Fred

acme,

Are the backfires through the carburetor, or out the tailpipe. With the timing light on the engine, the mark should only react to a change in RPM. You sound knowledgeable, so you know your mark is being moved, first by vacuum, then by mechanical.

I think I would take a look at the throttle plate in the carburetor and make sure it closes properly. If the timing was swinging way advanced with the burst of RPM's on a downshift, it could backfire through the carburetor.

The problem happens under high vacuum, closed throttle and no load!

You didn't mention if your rig has solid lifters.


Stuart

yes...check your timing. Most likely to be the problem.

An exhaust leak is the most common cause of exhaust popping on decelleration. No way could the timing be far enough off to fire while the exhaust valve was open and still run.

If your timing is jumping around you may have worn dist. bushings or loose breaker plate that holds the points. Lean idle mixture will also cause backfire on deceleration.

Wow! Thanks for the responses.

I'll check those problems.

Atomarc- Solid lifters.

Baldy-new manifold gasket b/c of a previous exhaust leak. There are two holes in the pipe where the heat riser used to be. Would that be enough?

Wrenchbender- The dist does move a little when it shouldn't I'll re-check the mixture/ timing w/ vac guage maybe.

I would take a serious look at the entire ignition system, including the spark plug wires.

Older resistive wires (carbon core) can develop air gaps. The air gaps act as a capacitor and require a miniscule (but finite) amount of time to charge, thus delaying the spark through a defective wire. This can cause an explosion with the intake valve open on the next sequence, thus popping through the carb.

Jon.

Jon,

If the intake valve is only open on the intake stroke, and the plug were to fire later (delayed), wouldn't that mean that the intake valve would have been closed for a longer period of time. Advanced timing means "advanced before TDC"..if too advanced, you fire when the intake is still open. Later, or more delayed firing would be classified as less advanced, or almost ATDC.

Does this theory make sense, or did I misunderstand the post.

Stuart

Stuart - both. The delay causes the firing to occur on the intake stroke of the NEXT cycle, not the current cycle: long (if milliseconds can be long) after the cylinder should have fired on the current cycle.

Often, backfires from this issue are quite dramatic.

Jon.

I retimed it with the light and the vac guage. The bb jumps on both sides of the needle unless i hold the distributor. Vac reading of 21. No backfire on the first couple downshifts after the adjustment. Then it started again. I made the carb mixture a little richer (1/2 turn ) no change. Maybe i need a new dist.? Thinking about HEI.

acme,

The vacuum advance unit should have zero play (truck not running) and if you take the cap off and twist the rotor, it should move with moderate effort, and spring positively back to a rest position.

The timing mark should only move in the advance direction in reference to the pointer, but should not go more retarded than its at rest position.

Sort of hard to understand I know..using a clock as a reference, at idle, the mark is as 12 o'clock. As you increase RPM, it will move 1 or 2 or 3 o'clock. As RPM decreases it will move back to 12 o'clock. BUT it should never move to 11 o'clock or 10 o'clock.

Find the problem first..make it run with the ignition it has, THEN experiment with the HEI or Pertronix or whatever. Keep at it.

Stuart

Thanks for the reply Ato. I understand the vac advance. The dist. just has play in it. Running or not I can turn it 1/8 inch or so in either direction, like its just worn. I can't understand why it would be fine the first couple of downshifts after timing and then backfire again. Seems like a timing issue huh?

I could reinstall the manifold gaskest again, maybe its a small exhaust leak.

I keeping at it.

acme,

Maybe I still don't understand. With the truck sitting in the driveway...you grab the distributor and twist it with light moderate force..it should not move at all. The vacuum advance unit has a spring in it that is suppose to hold it very firmly in a base position. If you can freely turn it as you indicate, either you are eating too many Cheerios, or the vacuum advance unit went south. This is the CPU on the truck, it must be accurate and tight!

Everything on the exhaust side is "after the fact"..I wouldn't waste a lot of energy in this area.

Stuart

acme,

I don't know how intense you want to get on this issue, but to 'prove' the vacuum advance unit, you could disconnect the vacuum line and plug it at the carburetor. Use some good old duct tape or a rubber band and restrain the distributor housing from movement. This would keep the distributor in the static timed position and bypass the advance unit.

Take the rig out on the road and see what happens. You will notice some reduced low speed response because the vacuum unit is disabled.

Wouldn't it be great if the backfiring quit!

Stuart

you dont want to much back-fire around here,, you might get returned fire Fred

The vac advance works. It's new and I've watched it turn when I pull on ythe throttle at the carb. I'm just saying that the dist has a little play. When I grab the dist it will turn either way about 1/8-1/4 in or so with no effort and without activating the vac advance. This if its running or not. It's like the dist is loose in its clamp. But the screw and the nut can'y be tightened any more.
Something I noticed today is that sometimes it would backfire and sometimes not at downshift and/or decel.

Atomarc thanks for stickin w/ me on this.

A tight distributor makes a huge difference.
I had also a lose dizzy for years with the timing marks not steady under a timing light.
The vacuum advance started to act too soon and too fast right after coming off idle and I never could time the engine right across the rpm range.

Last year the timing started to retard or advance by itself at times.
I replaced the old lose fitting dizzy with a remanufactured one which was very tight and much harder to get inside the block.
Day and night - now the engine sounds better, the timing marks are steady in the light and I could finally get the timing perfectly lean across the rpm range.
The vacuum advance now only kicks in after 1500-2000 rpm and much more slower.

I heard there are different distributors for the early and later blocks, because of the difference in camshaft metals over the years.
Typically parts suppliers don't know and would sell one to go for any year.

Bottom line, if your distributor is too lose, the timing marks move around and the vacuum advance is too snappy and moves too quickly, replace it.

acme,

I'm like a bulldog..I'm not going to let go! If you draw a cartoon of the 4 strokes of the engine, it becomes obvious that in order to pop back through the carburetor, the intake valve must be open. On this stroke, ignition would not be an issue.

Did the truck not do this before, and now it does it..was something changed or replaced. You did a whole bunch of work to the engine. Were you trying to solve this problem.

I am assuming you have an air cleaner on the truck..is the backfiring through the carburetor severe, as in loud and flaming! Is it an actual explosion, or just a puff of moving air.

The slop in the distributor just isn't enough to cause what you describe. Have you ever thought about dumping some top end cleaner like Berryman Chemtool into the carb to douche the carbon!

As I usually state when I am confused, something is really rotten in Denmark on this one!

This backfiring scenario would require some fuel, and an ignition source...timed when the intake valve was open. Should be easy to figure out HUH! :confused:

Stuart

Think it's fixed!! I put two pieces of cork in the holes where the heat riser used to be just to see what it what do and drove it @20 minutes and no backfire. Then the corks burnt out and it started again. must have been the exhaust leak (Baldy). Now I'll rig something to patch it for good. Thanks for all the help.

acme,

Congratulations..looks like the old carburetor was an innocent bystander. Ain't that a corker!


Stuart