Need help with mystery engine miss

[fifteenduth]

Mysterious engine miss shows up after running very well for all summer. So this is the data I have;

- 74 Westy, mostly stock
- stock Solex dual carbs, stock muffler
- ACN SVDA dizzy, with Computronix ignition
- 60,000 miles on original engine

On a trip with my son for a few days a couple weeks ago, we noticed a rough idle after filling up at a remote gas station with premium (91 octane) plus I add Lucas Octane Booster to each tank. Wasn't sure if I got bad gas maybe so topped up tank on the way home.
Limped it home with a couple stops to check plug wires, possible vacuum leak...
Got home and found the vacuum booster check valve connection with the line had come off so reconnected and thought I had found the issue. It was a bit better but still a miss. We had driven some rough roads so thought I would check timing. Before that I thought I would see if there was a specific cylinder that was the culprit and found that when I pulled #3 wire there was no change in engine idle. Hmm... I thought. Maybe bad plug or bad plug wire? Pulled all plugs and no issues but replaced with new. Checked wires, all good, replaced anyways since I had a spare set. Replaced dizzy cap with good clean spare I had and cleaned rotor. No improvement. So I'm sitting behind the engine (back is back!) as it is idling and I'm watching sparks jumping from the two coil terminals to the tower on the coil where the main wire is connected. Hmmm....thinking bad coil. So change coil from Bosch Blue that I've been running since 2009 back to original coil. No improvement.
- when I pull the #3 plug wire, I get a good spark and the engine idle does not change but when I put it back, there's a popping in the exhaust, like a vacuum leak would produce, but I can't find a vacuum leak.
Hmmm... What next? Checked timing....right on, all good. Inspected guts in dizzy and no indication of problems. Check fuel cut off solenoid. No problem.
Hmm.... so now I'm thinking of getting out the incense sticks to look deeper for a vacuum leak and also checking my #3 valve clearances, which were fine last tune up.

Can anyone think of something I may have missed here?
She's been running great since I got her back in 2009 and I have had a couple issues related to vacuum leaks which I have been able to isolate and solve but this is mystifying.

Thanks in advance for any help.
Ronnie C
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[skills@eurocarsplus]

sounds like a possible plugged jet by your description.
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[fifteenduth]

So if it's a plugged jet why would the issue only be #3 and not #4 cylinder too? This is the struggle I'm having with fuel delivery since both cylinders are fuelled by the same carb.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[skills@eurocarsplus]

a quick and dirty test is to start the bus, CAREFULLY fog/dribble some carb clean into the carb. if it smooths out, I would suspect the carb or a vac leak
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[SGKent]

[Starbucket]

Check the valve clearance, you probably got a chunk of carbon stuck on a valve seat or you burned a valve. Do a compression test.

[Wildthings]

[fifteenduth]

Thanks for the replies.

I forgot to say that I did a compression test on #3 and it was fine (~110 PSI).

SGKent - what I saw was arcing between the negative and positive posts of the coil and the centre post of the coil. Assumed the centre post of my coil was cracked but can't see any sign of crack with naked eye. Also when this happened I realized the #3 plug wire was not connected to the spark plug. Not sure if that means anything with respect to what I was seeing. Hope that clarifies my statement.

Bosch wires, and cap were the replacement. Using NGK BP5E plugs
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Tcash]

Ignition High Tension Leaks

[busdaddy]

[SGKent]

you have some exceptionally high voltage there. I have never seen a coil arc from the center to the negative. My first thought would be failing insulation on the plug wires at that point. I'd go for a new set of wires, regap the plugs and replace the rotor and cap with genuine Bosch ones. If you have points and a condenser replace both.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[fifteenduth]

Thanks again for more replies.

Busdaddy - the drive home from when I first noticed the rough idle, while stopped in construction zone, was about 400 kms or 250 miles. Compression test was done semi-warm I'd say, if I remember correctly. And I agree that valve adjustment should be next step. May be able to get to it today.

SG - I've never seen that arcing either. I was kinda shocked!!! Pun intended!! Gotta keep a smile through all this, eh? I did change out the Bosch Blue coil to the original Bosch coil and I changed and gapped new plugs, new Bosch wires, cleaned rotor but didn't replace. The arcing was not present after changing coil. I'm running Computronix ignition which has been running great since installed a few years ago. Not sure if something can go wrong with components of that unit but I may check with John at ACN about that since I got it from him. I thought about switching back to points and condenser as a test since I carry a set with me always anyways.

Again the puzzle for me is why it's isolated to only one cylinder, which is making me think it's a valve problem at that cylinder. I thought if it was a stuck valve I'd have harder starts and that's not the case, nor have I noticed a significant power loss.

Thanks folks.
Ronnie C
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[fifteenduth]

Also I did replace the cap with my original one, well the last one I had put on before switching to the SVDA dizzy a few years ago. Obviously cleaned the contacts prior to installing.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Tcash]

[fifteenduth]

Didn't do the test, since I replaced coil, but will include on my list of tasks as I continue troubleshooting. I'll reinstall the coil in question and run the test.

Thanks,
Ronnie C
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[fifteenduth]

So I checked #3 valve clearance and all ok. I'm down to my last troubleshoot idea of going back to points, assuming my Computronix has an issue. I would welcome any other ideas.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Wildthings]

Buying the premium gas in a remote location may have caused your problems. The stuff may have been sitting in the tank with water on the bottom for months. You may have ended up with a tank of watery ethanol and not gas. If you haven't done so yet try some FI cleaner in the fuel and see if that will clear things up. I have found with old alcohol laced fuel that I sometimes have to remove the jets and use a toothpick to break the crud caused by the gasohol loose. I have had to do this with both main and idle jets.

[Starbucket]

Try sticking the fuel pump pick up hose in a can of fresh gas and if it starts to run well, drain and refill your tank.

[fifteenduth]

Thanks. The fuel question has been in my mind the whole time, but again, why isn't #4 affected when it is fed fuel by the left carb just like #3? I have added some Sea Foam magic potion but have not driven far since so maybe I will go and fill up today with fresh fuel and drive a bit to see if it will clear up.

If I go back to points, or try my Pertronix that is still in the box from several years ago, and it doesn't fix it I will have tried everything I can think of and then the sad face will show up.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[SGKent]

tonight go out to the bus at dark and start it. There is a good chance that in the dark you will see sparking and halos on the wires. be careful of the fan belt in the dark.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin