Exhaust backfire when starting... sometimes

[bhartwell59]

Sometimes I'll start the rig (cold or hot, usually hot) and will get a loud single bang from the exhaust. Sounds like the drivers side.

Auto stick, dual port, 34pict3.

Just redid the DP exhaust gaskets, new boots and cleaned out the intake manifold. Static timed. Valves adjusted 760 miles ago.

Car runs fine otherwise... thoughts?
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'74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k

[jwold]

Is the "freeze plug" on the carb firmly in place? (the thing that looks like a dime) I know those can cause a backfiring condition if not firmly in place, and iirc are there to prevent damaging the carb in a backfire situation.
I'd recon a guess to say your timing is out.
Iirc also, autosticks had a special carb that was designed to work with that system, probably something to do with the vacuum, but beyond my VW understanding.

[bhartwell59]

Off timing make's sense. I'll check that out.

Would a bad condenser cause this (it's just the backfiring does not happen when its running)?
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'74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k

[SchnuppiePup]

Sounds like you’re not getting detonation in every cylinder and that unburned fuel is thereby getting to the exhaust system and detonating there with a bang.

Check the compression in each cylinder. It's done easily using a $30(?) tool (from your FLAPS*) inserted in each spark-plug port. Compare the compression values you get with the specification values listed in your repair manual (from your FLAPS).

Compression that's way too low could be due to a crack in a cylinder head, which could cause an improper fuel mixture that won't detonate in a cylinder but can in the exhaust system (then, Bang).

I’m a big fan of the compression check. It’s so easy and quick and has told me so much about my engines that I consider it an almost fun task.

Plus, while each spark plug is out with the compression check on each cylinder, you can check each plug’s condition. Maybe you just need a new set of spark plugs.

Next: Additional tune-up stuff, like, is the distributor cap clean of arcing deposits, and is the rotor clean? Is a new set of spark-plug wires needed (are they old)? Then: Was/is the valve adjustment 760 miles ago truly correct? Last (last resort): Could the carburetor fuel-mixture adjustment be off? Is the choke functioning correctly?

*FLAPS = friendly local automotive parts store. Also, it's unclear what you mean by "new boots".
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Better a life-long learner than a life-long know-it-all.

1964 Ghia coupe
1968 Honda CT 90
1980 Honda CT 110

[bhartwell59]

Good stuff, Sir! A nice list to start the elimination process from.

New boots=new intake manifold boots
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'74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k

[djkeev]

Check your electric choke, not only for operation but also adjustment.

If the coil spring is lazy (old) the choke plate may not be all the way open on a hot start providing not needed extra fuel, not so much to flood it but more than can be burned on start up.

The extra fuel vapors in the exhaust combust giving you a backfire.

Once the engine is running the intake vacuum holds the choke plate wide open, hence no more extra unburned fuel.

Dave
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Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[bhartwell59]

Checked the compression and have 120 in all cylinders except #4 which is 30. Which would explain why the backfire sounds like it's on the drivers side.

What does this mean? Engine rebuild time?

Except for that one initial startup backfire the car runs (not at it's best, no doubt).

Thanks!
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'74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k

[djkeev]

You probably have a burned exhaust valve.
Combustible fuel vapors in the wrong place at the wrong time.

Pull the engine, pull that head and see what you have.

Dave
_________________
Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[bhartwell59]

An update:
Had the engine rebuilt after getting those crappy compression numbers... and it nows runs like a top. It had 89,500 so it was time.
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'74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k

[myghiarocks]

Did you have it done locally? Who did it?

Thanks

[bhartwell59]

[myghiarocks]