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jwilsman 
Samba Member
Joined: December 19, 2009
Posts: 10
Location: St Louis, MO
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 Posted: Tue Jul 15, 2025 7:59 am Post subject: Very rough idle and backfire |
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Hi folks, I'm needing some suggestions. I have a 67 bug, original as I was told last fall when I bought it. 1500cc, 30Pic-1 which has been running like a charm. Well last weekend I was going to a friends house and she started to buck and backfire and loss of power. She would idle fine and start right back up. I haven't changed anything since her last tune-up, valve adjustment(.006), points and condenser , plugs, wires, dist cap. It had a Empi carb on it so I put on the rebuilt 30pic-1 instead and after all that she ran great. This was done last fall and I've been driving her off and on and not let sit for long periods of time.
Any ideas I'm missing here?
I appreciate any input.
-Jeff
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1970 Kamann Ghia AutoStick
1970 Kamann Ghia 4spd
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VW_Jimbo
Samba Member
Joined: May 22, 2016
Posts: 11127
Location: Huntington Beach, CA
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| Posted: Tue Jul 15, 2025 8:39 am Post subject: Re: Very rough idle and backfire |
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So with the newer carb it is running good, or…?
Valves at .006”, cold? Did you double check them?
Points at .016?
Plugs at .028”?
Throttle plate and throat clean?
Timing, idle mixture and speed set to?
Fuel filter is old? New?
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Jeff Geisen
Samba Chaplain
Joined: December 21, 2004
Posts: 1930
Location: N.W. Georgia
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| Posted: Tue Jul 15, 2025 8:41 am Post subject: Re: Very rough idle and backfire |
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I’m guessing ignition issue, check points for dwell, condenser can cause this type of problem, coil failing or bad connection at coil spades. Vacuum leak?
New parts often aren’t good, sometimes the original part is better than the new replacement.
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Cusser
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Joined: October 02, 2006
Posts: 33049
Location: Hot Arizona
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| Posted: Tue Jul 15, 2025 8:46 am Post subject: Re: Very rough idle and backfire |
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| Jeff Geisen wrote: |
| I’m guessing ignition issue, check points for dwell, condenser can cause this type of problem, coil failing or bad connection at coil spades. Vacuum leak? |
Did you add a dab of grease to the distributor cam where the points' rubbing block would rub? I'd check points gap and add tad of grease there, as first guess.
Second guess might be bad condenser, as I've read about new ones not being best quality. I use a German one that has been on my German 009 since I bought it in 1976 !
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Bobs67vwagen
Samba Member
Joined: March 27, 2005
Posts: 277
Location: Eastern north carolina
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| Posted: Tue Jul 15, 2025 10:29 am Post subject: Re: Very rough idle and backfire |
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| X2 on starting with the ignition especially with new parts in there. There have been lots of problems with poor quality points and condensers. If you still have the old ignition parts try replacing 1 item at a time to isolate the problem. Good luck-Bob |
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aquifer
Samba Member
Joined: August 30, 2021
Posts: 328
Location: Nebraska
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| Posted: Tue Jul 15, 2025 10:43 am Post subject: Re: Very rough idle and backfire |
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Maybe the distributor clamp wasn't tight and it finally decided to rotate on you a little bit, throwing off the timing?
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KTPhil
Samba Member
Joined: April 06, 2006
Posts: 35858
Location: Conejo Valley, CA
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| Posted: Tue Jul 15, 2025 10:52 am Post subject: Re: Very rough idle and backfire |
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Start with a tune-up, including valve adjustment. This is cheap or free, and eliminates potential causes, and may lead you to find the culprit.
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zerotofifty
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Joined: December 27, 2003
Posts: 3748
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| Posted: Tue Jul 15, 2025 4:36 pm Post subject: Re: Very rough idle and backfire |
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Check your point gap first, maybe it drifted. You have a vacuum advance, make sure it operates, by pulling the hose off the carb and sucking on it, see that the plate in the distributor moves with the vacuum. Next id ohm out the spark wires, and then inspect the four plugs, they should all look the same, if not, you either have a bad ignition wire on the odd plug, bad cap or plug itself, or you got an intake leak But intake leaks usually manifest poor running at idle and not at high speed. Then check the valve adjust.
Finally do a carb tune.
Note the carb tune is always done last, it never makes sense to tune the carb if the valves aint right or if the timing, or ignition is at fault. If those other things are wrong, then tuning the carb is an exercise in futility.
Also dont discount a bad coil, the coil does a lot more work at high rpm than low, so issue will manifest themselves more at high rpm in so far as the coil is concerned.
But the easy first thing is check the points, timing and vacuum advance operation, confirm spark wires, cap, rotor and plugs be good then valves, and finally carb adjust.
Oh and one more thing, make sure the fuel pump is putting out correct pressure and flow. if the pump is low on flow, you will notice it more at high speed than low
But backfire in the muffler indicate unburnt fuel in the muffler. That means either over rich running or weak ignition.
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jwilsman
Samba Member
Joined: December 19, 2009
Posts: 10
Location: St Louis, MO
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| Posted: Sun Jul 20, 2025 12:04 pm Post subject: Re: Very rough idle and backfire and stalls |
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Thanks everyone for the tips. I checked my valves for correct adjustment and they all were good. I checked the vacuum advance and I noticed the plate would not return all the way after suction on the diaphragm. I replaced the spring inside the distributor that pulls back the plate and now that's working good. The points are about 100 miles on them so I'm guessing (?) they are still good, I checked the gap and that was good also. I also changed the condenser just for grins. Adjusted the timing and dwell and all that is good now.
She starts right up but idles poor and I mean real poor. I have to give it a little throttle to keep running, the idle smooths out and them drops like its going to stall and it will go up and down like this without any throttle from my help. You think there is an wire shorting out? I'm stumped. How can I test for a bad coil, just replace it with a new one?
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tasb
The Distributor Distributor
Joined: April 27, 2002
Posts: 6735
Location: Pentwater, Michigan
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| Posted: Sun Jul 20, 2025 12:42 pm Post subject: Re: Very rough idle and backfire |
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It is not very likely that changing the spring in your distributor actually resolved the sticking points plate. Replacing the spring may have temporarily loosened the plate. You need to add a couple of drops of oil to the points plate where it meets the distributor shaft to make a permanent fix. Don’t spray a lube into the distributor.
There are several topics on the website on testing ignition coils. Click on the forums tab above and then search. Enter coil testing in the search window. You will need a VOM or multimeter.
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Bobs67vwagen
Samba Member
Joined: March 27, 2005
Posts: 277
Location: Eastern north carolina
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| Posted: Sun Jul 20, 2025 2:23 pm Post subject: Re: Very rough idle and backfire |
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| If you have not done so already check the fuel pump for fuel delivery to the carb.. pop the coil wire off so it cannot start. Take fuel line off carb and place it into a plastic container. Crank the engine over a little and see what you are getting for fuel to the carb. You can rule out fuel starvation this way. Just another thing you can check. |
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jwilsman
Samba Member
Joined: December 19, 2009
Posts: 10
Location: St Louis, MO
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| Posted: Sun Jul 20, 2025 4:43 pm Post subject: Re: Very rough idle and backfire |
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I'll give that fuel pump output a try. Thanks for the input.
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1970 Kamann Ghia AutoStick
1970 Kamann Ghia 4spd
1967 Beetle |
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Jeff Geisen
Samba Chaplain
Joined: December 21, 2004
Posts: 1930
Location: N.W. Georgia
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| Posted: Sun Jul 20, 2025 4:48 pm Post subject: Re: Very rough idle and backfire |
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adjust idle mixture when at operating temp
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ashman40
Samba Member
Joined: February 16, 2007
Posts: 16555
Location: North Florida, USA
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| Posted: Sun Jul 20, 2025 5:47 pm Post subject: Re: Very rough idle and backfire and stalls |
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| jwilsman wrote: |
| I checked my valves for correct adjustment and they all were good. I checked the vacuum advance and I noticed the plate would not return all the way after suction on the diaphragm. I replaced the spring inside the distributor that pulls back the plate and now that's working good. The points are about 100 miles on them so I'm guessing (?) they are still good, I checked the gap and that was good also. I also changed the condenser just for grins. Adjusted the timing and dwell and all that is good now. |
Not to be harsh, but you saying "it is good" does not provide any information.
What was the valve lash/gap value you measured on cold valves?
Did you compare the old and new spring in the distributor to see that the old one was slightly longer/stretched? Did you lube the advance plate so the diaphragm and spring have less resistance to work against?
You guess the points are good? Did you look to see if they are pitted? A failed condenser would cause points to go bad quickly. Replacing the bad condenser is good but matching them with now pitted points is not ideal.
The proper point gap on pitted points means it is less likely to be properly adjusted.
You checked the dwell and ignition timing... but didn't say what their measured value?? Many folks will look up the stock ignition timing based on the car's model year and set their engine to that value even when they are no longer running the original distributor. Ignition timing should typically be set based on the model of the distributor installed. You should only use the model year spec if your are running the OE distributor installed when the car left the factory. Can you provide more details pls. Actual values including the model of your distributor.
| jwilsman wrote: |
| She starts right up but idles poor and I mean real poor. I have to give it a little throttle to keep running, the idle smooths out and them drops like its going to stall and it will go up and down like this without any throttle from my help. You think there is an wire shorting out? I'm stumped. How can I test for a bad coil, just replace it with a new one? |
Two things I would suggest you check...
When did you last replace the fuel filter or check the fuel sock at the bottom of the tank? If it was more than 2yrs ago or you cannot recall... replace/check both. Fuel filters are consumables. Replace them regularly even if they "appear" to be clean. Most fuel filters in history have been metal canisters which you could not see thru. You just replace them at the mileage or monthly recommended interval even if they were spotless inside (which you could not tell). It is actually too late if you are waiting for the filter element to "appear" dirty.
Check for a vacuum leak.
Warm up the engine until the choke is OFF. If needed, rotate the fast idle cam to the 1st step to keep the engine idling. Remove the air filter housing and cover half the opening of the carb with your flat hand. Slowly slide your hand to cover more and more of the opening. At some point your will start to choke off the air flow. This is functionally the same as closing off the throttle plate. The engine rpms should go down.
If instead the engine seems to run smoother and rpms increase before they go down... you have a vacuum leak causing the engine to run lean (run rough). This is why the engine idles rough. By cutting off the air flow down the carb you are forcing the mixture to go from lean towards rich (less air). When the mixture reaches closer to ideal the engine runs smoother.
Using a can of spray carb cleaner with the red straw attached to focus the spray... spray around all the places air could leak into the intake air flow. All vacuum hoses and vacuum caps. All gaskets for the carb, intake and head. Spray the opening at the sides of the carb where the throttle shaft enters the carb body. When the throttle shaft bores wear out air leaks in thru the bore openings around the shaft. You need to focus the straw in the gap between the throttle shaft flange and the opening in the carb. If spraying anywhere causes the engine idle to change... you have found a leak!
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zerotofifty
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Joined: December 27, 2003
Posts: 3748
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| Posted: Sun Jul 20, 2025 7:53 pm Post subject: Re: Very rough idle and backfire |
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Under the rotor, there is a felt disc inside the distributor shaft. This felt disk is designed for oil, so place a few drops of motor oil on this felt, the oil will then ooze out of the disc and flow down to lubricate the advance plate. Make this a part of your regular maintenance procedure. This oil point is oft neglected.
Check your spark, get a spare spark plug, and connect it to one of the spark wires, grounding the spark plug threads to the engine case, then fire her up and run on three spark plug and note the spark on the loose plug, you should see a fat hot spark, blue in color. A weak yellow spark is bad. You can also remove that plug and hold the wire near the case, and see a fat blue spark from the wire to the case, it should be able to jump near a half inch gap. Rev the motor and note how it sparks, it should be a fat blue spark.
Also did you ohm out all five spark wires? Ohm out the rotor and the cap?
A high resistance in only of these will cause running problems, which may seem like a bad coil, but it is instead a bad wire, rotor or cap.
As for the points, make sure they aint pitted
Also take a look at all four plug, they should all be the same color, not sooty,
Also run here, rev here then shut down and remove top of carb and assure the fuel level in the bowl is correct height from top of bowl. wrong height is an indication of incorrect float valve adjustment, or incorrect fuel pressure or delivery volume.
You can also measure the accelerator pump volume, make sure that is correct. You need a graduated glass vial for this. ALthough this does not appear to me to be the correct issue, as when this is bad, your acceleration is poor, and she stubbles when accelerator is pumped.
Report back with test results.
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