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  View original topic: Exhaust backfire when starting... sometimes
bhartwell59 Mon Oct 07, 2019 12:28 pm

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?

jwold Mon Oct 07, 2019 1:16 pm

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 Mon Oct 07, 2019 2:10 pm

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)?

SchnuppiePup Sat Oct 12, 2019 11:53 pm

Sounds like youíre 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).

Iím a big fan of the compression check. Itís 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 plugís 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".

bhartwell59 Sun Oct 13, 2019 5:54 am

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

New boots=new intake manifold boots

djkeev Sun Oct 13, 2019 6:37 am

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

bhartwell59 Fri Oct 18, 2019 2:03 pm

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!

djkeev Fri Oct 18, 2019 2:44 pm

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

bhartwell59 Mon Dec 09, 2019 7:15 am

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.

myghiarocks Mon Dec 09, 2019 10:51 am

Did you have it done locally? Who did it?

Thanks

bhartwell59 Mon Dec 09, 2019 11:28 am

myghiarocks wrote: Did you have it done locally? Who did it?

Thanks

Yep, Doug Crotts over at Qualitat in Grand Prairie: http://www.qualitatvw.com/

Tell him Billy sent you

myghiarocks Mon Dec 09, 2019 1:02 pm

bhartwell59 wrote: myghiarocks wrote: Did you have it done locally? Who did it?

Thanks

Yep, Doug Crotts over at Qualitat in Grand Prairie: http://www.qualitatvw.com/

Tell him Billy sent you

Thanks.



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