Tracking down a backfire problem - SOLVED - Thank you all!

[61SNRF]

Unless you're going downhill in neutral, there is usually engine braking when you decelerate.
So, what do you mean, it backfires when you downshift?
Maybe backfire is the wrong word. This word is usually used to describe a problem with the spark timing as in when the fuel air charge is ignited with the intake valve still open. This causes a "backfire" through the intake and if the air cleaner if off you will see a flame shoot out the carburetor.
What I thought you were referring to is when the mixture is overly rich, and you're coasting in gear, there is a strong vacuum that will pull extra fuel into the intake via the idle circuit and into the muffler filling it with a fuel air mixture. If there is an exhaust leak, then that introduces extra oxygen which can spontaneously ignite the mixture inside the muffler causing a popping or backfire noise through the exhaust. This will cause flames to shoot out the muffler.
You might have someone follow you or stand beside the road as you drive by and "engine brake" to listen where the noise emanates from.
When you're sure of the source, then maybe we can help you make a better diagnosis.

[Culito]

I get quite a pit of popping/backfiring on decell when driving my bug with the mechanical - only distributor. Not so much with the SVDA. I figured it was not so much a "problem" as just a characteristic of the mechanical only distributor, since it doesn't retard timing at all during coasting.
My mixture seems fine by the plugs but I may have a few minor exhaust leaks.
It doesn't bother me much, and I figure it probably keeps other drivers awake.
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Copyright CJ Industries, Inc.
'64 standard w/2.0L type 4
'62 bug

[79SuperVert]

[61SNRF]

Culito makes a good point. I have heard popping from the exhaust on deceleration from more than few Beetles and Buses with upright engines.
I would say more typical (or louder) from cars with aftermarket QP exhaust though. I would still say lean it out a little though. My Bentley book doesn't have much driveability troubleshooting, so I can't back that up right now.
Like he said too, if it doesn't bother you too much it's probably not much of a problem.
Do you know Jeffery H.? If so tell him Bruce F. said Hello if you see him . We traded '67 Beetles way back in '82-'83. He got my rusty 1200A and I got his wrecked Cal-Look!

[Harris]

usrname Wrote:

[Goge]

My bug lets out a little squeaky fart noise on deceleration, I wouldn't call it a backfire as it's not really much of a 'bang' sound. It only does it when the revs get down to a pretty low number, probably just a bit above an idle speed. Quite repeatable too. Sometimes, if I take it out of gear before it gets to that point, it does it when the revs drop down to idle.

I hate it. Because other than that, the thing runs near perfect.

This doesn't describe your situation 79SuperVert , does it?
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'64 Beetle
'73 Westfalia
Too many kids
Not enough hair on my head
Not enough beer in my fridge

[Harris]

[Goge]

[79SuperVert]

[79SuperVert]

This Saturday I'll take the Beetle to a park with a hilly 2 mile circuit and try changing the idle air mixture per the Bentley.

But before I take it over there, I'm going to try what I should have done last weekend: blocking the exhaust pipes with the engine running and listening for exhaust leaks. That should tell me pretty quickly if there are leaks.
_________________
Central Jersey VW Society

Wanted: Art Collins VW (Savannah, Georgia) items - license plate surrounds and other items. Also ivory "AM", "FM" and "SW" buttons for a US Blaupunkt Frankfurt.

[79SuperVert]

I tried blocking the exhaust - no effect at all. I crawled underneath with the engine running and found that both connections between the muffler and the front exhaust pipes ("J-pipes") were leaking a lot. I could feel the exhaust blowing past my fingers as I held them near the connection. I had tightened the connections but never felt them while the engine was running.

I replaced the connections, which also involved removing and replacing the rear exhaust outlet gaskets and the heat riser gaskets as well since the muffler had to be removed. But no matter how much I tried, I could not get the muffler-to-exhaust-pipe connections to stop leaking. I used new clamps, new asbestos gaskets (thanks, Lou), new bolts and nuts, and I tightened the clamps to the point where the two pieces of each clamp were touching each other. They still leaked; not a lot, but each of them leaked just a bit.

I loosened the clamps and rotated them partially and re-tightened them, which changed the location of the leaks but didn't eliminate them, and although they are now leaking much less than before, they have not become completely airtight. So I have to assume there is a problem with either the muffler pipe ends (it's an original VW muffler) or with the exhaust pipe ends.

I took the Beetle out for a drive and played a lot with the idle air mixture, noting the number of turns and the direction (in or out) in a notebook so as not to lose track. I found that changing the idle air mixture via the volume control screw didn't really make that much of a difference. I think the only time it will make a difference is when the connections are all fully airtight already.

In the end my idle air mixture is now 1/4 turn leaner than before because I kept making the mixture richer and richer and the engine was starting to run crappy. I re-adjusted the mixture per Bentley (screw it in until the engine rpm's slow down, screw it out until the engine runs well and then 1/4 turn more out).

So the backfire problem has gotten better, as long as I don't push the engine to high rpm's downhill for long periods of time. But since I can't get rid of the exhaust leak completely, I guess I will have to live with some degree of backfires, at least until I change either the muffler or the front exhaust pipes.

A side benefit of replacing the connections was that the "fweem" has returned. It had disappeared as the muffler - J pipe connection leak had gotten worse.
_________________
Central Jersey VW Society

Wanted: Art Collins VW (Savannah, Georgia) items - license plate surrounds and other items. Also ivory "AM", "FM" and "SW" buttons for a US Blaupunkt Frankfurt.

[61SNRF]

At this point you're probably tired of crawling around under it, but there is a wonder product that might cure 99% of your remaining exhaust leaks. Next time your at the auto parts store get some Permatex Copper Hi-Temp silicone made for sealing exhaust leaks. It will fill gaps up to 1/8" and is heat resistant. I can testify it has held up under badly warped exhaust manifolds on a big block Ford V/8 and many other applications.
I might suggest you loosen up and remove the clamps, slide the gasket rings back, clean the area and pipe as best as practical, then apply a good 3/16" bead of silicone around the union between the muffler and J-pipe forcing some into the voids and onto the rusty J-pipe, then slide the gasket back. Wipe up the excess and pack it in like caulking for a neat look. Reinstall the clamps and let it cure overnight.
I'll eat my hat if it doesn't work

[zozo]

I was having the same problem on my bus, and found that I needed to torque my heads to spec. The tops on both sides were barely finger tight. Once torqued to spec, the popping/backfiring on deceleration, etc stopped completely.



NOTE: This is what my problem was and what worked for me. I'm not a mechanic and can't defend the why's or why nots. If you disagree that this was my problem, or that was the fix, that's fine. You may be right. As I said, I had the same problem, or similar problem, and this is what worked for me.

[79SuperVert]

[79SuperVert]

[79SuperVert]

Did the copper Permatex thing this afternoon. It will be curing all week. Let's see what happens next Saturday.

By the way, 61SNRF, could you post a picture of the hat you offered to eat in case this doesn't work?
_________________
Central Jersey VW Society

Wanted: Art Collins VW (Savannah, Georgia) items - license plate surrounds and other items. Also ivory "AM", "FM" and "SW" buttons for a US Blaupunkt Frankfurt.

[61SNRF]

Opps, I forgot and ate it Sunday night!
Come on, it'll work, have some faith in silicone.
I'll be sitting on the edge of my seat waiting to hear the results!

[tb03830]

[79SuperVert]

[79SuperVert]

Fast forward to late summer 2011: the backfire returned. This time I removed the muffler, cleaned all the Permatex off thoroughly with solvent, rounded the end openings of the J-pipes as best I could, then coated the ends of the pipes with a thin smear of Permatex copper, and slid the gaskets on over the Permatex. This time I used the woven metal ones, not the asbestos ones, and coated them too with Permatex prior to putting the muffler back on.

Then I reassembled everything using all new gaskets and hardware, let it cure a week, and drove it. The backfire is 99% gone (sadly, very little fweem, though), but on steep downhills and on hard downshifts it returns. When the backfire gets really bad again, I will put on NOS heater boxes and a NOS muffler that I have in my stash of spare parts and see if that cures it finally.
_________________
Central Jersey VW Society

Wanted: Art Collins VW (Savannah, Georgia) items - license plate surrounds and other items. Also ivory "AM", "FM" and "SW" buttons for a US Blaupunkt Frankfurt.