Mystery backfire/popping through carb

[tgreese]

The spring should be installed with pre-load, so the accelerator pump rod moves along with the throttle rod rotation. When the accelerator pump rod reaches the end of its travel, the spring winds up further and allows the throttle rod to continue to rotate to WOT.

This old carburetor body shows it pretty well.

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[will e]

You mention compression being 110 to 120 on all cyls. That means you don't have a valve stuck open.

I would double check the firing order.

Also make sure the wires are separated well It shouldn't be jumping spark, especially at idle, but that back fire is frequent and rhythmic.

You might also try removing plug one wire at a time and see if the problem goes away. Try each wire separately.

[jakerc02]

I think we’re gonna try to get a new spring/throttle shaft with it on there.


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[jakerc02]

Is it okay to rev it a little with a cylinder disconnected at a time? Because that’s when the problem is happening.


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[will e]

Yes, it won't damage anything. I drove around for a couple of weeks with a plug wire disconnected once.

[tgreese]

I think we’re gonna try to get a new spring/throttle shaft with it on there.

Looks to me that it's either broken or put together wrong. Look at my example closely, I think you can't get it apart without destroying the linkage.

If you have no accelerator pump action until half throttle, I'd be surprised the engine did not simply stall when you open the throttle. When you open the throttle, it takes a second or two for the vacuum to catch up with the new throttle position. Your carb is a mechanical device that uses the vacuum as both a signal and a source of power to draw the fuel into the air stream and atomize it. No vacuum means no fuel flow, and the mixture goes way lean. Not sure that this is enough to cause backfire - maybe - but certainly the motor should fall flat on its face when you open the throttle. Lacking the accelerator pump, you can still open the throttle very slowly and the engine should catch up.

[will e]

Listening to the video it sounds to be like it has successive small backfires, is this an accurate description?

[jakerc02]

Yes, it's lots of smaller pops and some occasional louder ones mixed ones. It's not big flame spitting backfires.

[will e]

Okay, that probably means a cylinder is firing while the intake valve is still open. This is not a accelerator pump issue or a carb issue. You may have those too but if you get consistent backfire it's either a mis-timed ignition event or a stuck valve. Your compression test probably confirmed it is not a stuck valve. Try the test I suggested where you remove each spark plug wire and see if it goes away. If it does you know which cylinder is causing the backfire.

[jakerc02]

So we did the test and each time it didn’t change, so it’s not an individual cylinder. We have a new throttle shaft/accelerator pump spring coming.


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[jakerc02]

The accelerator pump is fixed. The spring wasn't set right and so it works like it should now. There doesn't seem to be any more lag or hesitation under throttle now, but we'll see when we drive it tonight. The popping is still there tho. Was thinking and could someone have put in the wrong size jets. Thinking this could cause our lean condition once they kick in and then the backfire through intake happens.

[will e]

Lean jets won't normally cause popping when you are not moving.

You mentioned vacuum earlier. What is your vacuum gauge doing during the popping?

[jakerc02]

The manifold drops to around 7-13 and ported goes to about 10-15


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[jakerc02]

So here’s an update. Still haven’t been able to figure this out. Did realize that the vacuum advance canister is bad though. When vacuum advance is connected, the timing mark jumps all over the place sometimes more than 20 degrees. Without the V-advance connected it’s fine. Problem is that I can find the right vacuum advance canister anywhere, but I’m still looking. I don’t think this is related to the popping since the popping still happens when the vacuum advance isn’t connected. Here’s some ideas to what the problem could be. Slack/jumped timing chain, bad valve guides, buildup on back of valves, something with can/lifters/rockers, maybe even bad altitude compensation unit on the back of the carb could be causing a lean mixture? Any advice on these possible causes or others would be great thanks.


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[tgreese]

Buy a reman, rob the canister, turn in the reman distributor with the broken can. You can probably keep the pickup coil as a spare, depending on how picky the store is about your core. They are not typically picky, in my experience. The reman plants are prying the canisters apart and replacing the diaphragms to rebuild these distributors, also in my experience. Pretty sure MSD offers a VR distributor for these engines, which would be another option. Also, though I'm not a fan, there are lots of aftermarket GM HEI clones available for the AMC V8s.

[jakerc02]

Thanks, ordered a reman today. Thinking of pulling the valve covers to see if anything bad is happening there with the lifters and such. Also while the covers are off I could repaint them.


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[jakerc02]

Is it possible that crossfiring between plug wires could be causing this? The wires are only a few months old and I don't see any evidence of it, but just trying to think out of the box.