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bhartwell59 Samba Member
Joined: December 28, 2014 Posts: 791 Location: dallas, tx
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Posted: Mon Oct 07, 2019 12:28 pm Post subject: Exhaust backfire when starting... sometimes |
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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? _________________ '74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k |
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jwold Samba Member
Joined: September 30, 2008 Posts: 2088 Location: Portland, OR
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Posted: Mon Oct 07, 2019 1:16 pm Post subject: Re: Exhaust backfire when starting... sometimes |
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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. |
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bhartwell59 Samba Member
Joined: December 28, 2014 Posts: 791 Location: dallas, tx
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Posted: Mon Oct 07, 2019 2:10 pm Post subject: Re: Exhaust backfire when starting... sometimes |
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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)? _________________ '74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k |
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SchnuppiePup Samba Member
Joined: November 07, 2014 Posts: 67 Location: San Jose, California
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Posted: Sat Oct 12, 2019 11:53 pm Post subject: Re: Exhaust backfire when starting... sometimes |
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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". _________________ Better a life-long learner than a life-long know-it-all.
1964 Ghia coupe
1968 Honda CT 90
1980 Honda CT 110 |
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bhartwell59 Samba Member
Joined: December 28, 2014 Posts: 791 Location: dallas, tx
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Posted: Sun Oct 13, 2019 5:54 am Post subject: Re: Exhaust backfire when starting... sometimes |
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Good stuff, Sir! A nice list to start the elimination process from.
New boots=new intake manifold boots _________________ '74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k |
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djkeev Samba Moderator
Joined: September 30, 2007 Posts: 32625 Location: Reading Pennsylvania
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bhartwell59 Samba Member
Joined: December 28, 2014 Posts: 791 Location: dallas, tx
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Posted: Fri Oct 18, 2019 2:03 pm Post subject: Re: Exhaust backfire when starting... sometimes |
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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! _________________ '74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k |
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djkeev Samba Moderator
Joined: September 30, 2007 Posts: 32625 Location: Reading Pennsylvania
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bhartwell59 Samba Member
Joined: December 28, 2014 Posts: 791 Location: dallas, tx
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Posted: Mon Dec 09, 2019 7:15 am Post subject: Re: Exhaust backfire when starting... sometimes |
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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. _________________ '74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k |
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myghiarocks Samba Member
Joined: June 02, 2009 Posts: 143 Location: DFW area Texas
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Posted: Mon Dec 09, 2019 10:51 am Post subject: Re: Exhaust backfire when starting... sometimes |
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Did you have it done locally? Who did it?
Thanks |
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bhartwell59 Samba Member
Joined: December 28, 2014 Posts: 791 Location: dallas, tx
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Posted: Mon Dec 09, 2019 11:28 am Post subject: Re: Exhaust backfire when starting... sometimes |
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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 _________________ '74 Zambezi green Ghia vert, autostick, 101k
'87 Jeep Wrangler, 4.2, 330k |
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myghiarocks Samba Member
Joined: June 02, 2009 Posts: 143 Location: DFW area Texas
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Posted: Mon Dec 09, 2019 1:02 pm Post subject: Re: Exhaust backfire when starting... sometimes |
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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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