Need help with mystery engine miss

[fifteenduth]

Thanks SG, I'll try that. With brand new Bosch wires I'll be surprised but you never know.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Tcash]

[fifteenduth]

Shoot no! Thanks for the reminder. I'll do that today for sure.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[fifteenduth]

Ok so the high tension leaks show no leaks and moving plugs and wires result in no change. So my spider sense is telling me that maybe I have missed a small vacuum leak after all, because the way it's missing feels very much like a time a couple years ago where I found a small vacuum leak in the left carb where one of the "dummy port plugs" (I don't know what the actual term is for this plug that is in place where there is an unused port, near the idle air screw at the front of the top carb body). I will now take a closer look at the vacuum line from the front engine compartment up to the brake booster including a closer inspection of the vacuum line check valve as it may have been bouncing around while it was disconnected during my recent trip. A friend also suggested maybe the intake manifold gasket has sprung a leak but my smoke test hasn't shown any vacuum leaks in the engine compartment.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[fifteenduth]

Wanted to add that when I pull #3 plug wire while engine running, I have good spark at connector and engine idle speed doesn't change but the popping in exhaust disappears and then when I reconnect the plug wire connector, the popping returns and the idle speed seems to be unchanged. Any other three plug wires I pull reduces engine idle lots, to almost stall engine.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Wildthings]

[fifteenduth]

Compression test on #3 was 115. Last time I checked compression on all cylinders which was a couple years ago, #3 was 120, with others as follows: #1-110; 2-125; 4-120
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[fifteenduth]

Pulled my vacuum check valve. Blow test shows it works fine. Will reinstall with new rubber inserts at both connections just to be sure, in case there was a tiny leak.

I've been reading a lot of comments about poor quality Pertronix stuff but not much about the Computronix though I haven't specifically searched by product. Does anyone have any experience with components of this system wearing out or failing? Like maybe the magnetic disk?
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Tcash]

Hook your timing light to #3 and see if it flashes the same as 1-2-4. Or does it flash intermittently.

May be time to hook up a vacuum gauge.
Vacuum gauge

Good luck
Tcash

[fifteenduth]

Good idea. I'll try it once I get the check valve back in.

I'll go borrow a vacuum gauge from my brother-in-law.

Thanks Cash!
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Wildthings]

I am going to guess that you have a valve hanging open because it is sticking in the guide. At some times it works fine while at other times it doesn't shut fully. Maybe add some Seafoam, MMO, 2-cycle oil, or even synthetic motor oil to your fuel tank and see what effect that has. Today's crappy fuel can gum up the valve stems and cause this, happens frequently to vehicles that don't get a lot of use and if the valve gets slapped by a piston things can get expensive.

Do check your valve lash. AMC heads can also cause these kinds of symptoms as their junky parts fail.

[fifteenduth]

Thanks Wildthing, I added some SeaFoam already but could add more, as this morning I went and topped up my fuel tank again to dilute potential bad gas from recent trip. Heads are original with just over 60,000 miles so I know it's time to keep a closer ear on potential wear noises.

It has been running so well up until my last long trip that I don't know what may have happened all of a sudden.

I just finished cleaning up my brake vacuum check valve and when I find my new rubber inserts for the connections I'll reinstall and carry on with troubleshooting
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[fifteenduth]

Forgot to mention last post Wildthings that I checked the valve clearance a couple days ago on #3 and both intake and exhaust were .006" so ok in my books.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Wildthings]

It would be possible that the magnets for the Compufire are fouled with ferrous debris and the debris is causing it to fire at the wrong time. Never seen this happen on a gas engine, but have seen similar problems develop on other equipment.

Maybe a vacuum leak effecting only #3, check that the manifold to head bolts are tight and that the bolts between the carb and manifold are tight. Spray some starter fluid around the joints and see if anything changes.

You could be looking at the well known problem of the throttle shaft bushings getting loose.

[Tcash]

You talked about putting the points back in, did you try this?

[fifteenduth]

Will try vacuum gauge first then decide about points
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Amskeptic]

Hot off the 2017 Itinerant Air-Cooled presses!

NGK plugs have been causing all kinds of grief. Please install Bosch WR8CC plugs and get back to us.

Inquire with tommu (IAC forum) and Ich Bin Kurt.
If the stupid little misses appear to go away when you lift the center coil wire slightly out of the cap at idle (don't overdo it or you will get shocked or damage the electronic ignition systems), then change out the NGKs.
Both customers noted a cessation of misfire symptoms with the Bosch in place of the NGKs.
ColinInWyomingChasingEclipse
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www.itinerant-air-cooled.com

[fifteenduth]

Been running NGK plugs for at least 5 years and no problems. Changed out to new ones and didn't have any impact on issue.
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'74 Westy - "Josée"
2014 Beetle GSR - "Ozzy"
2006 Jetta TDI - "Jerry"

[Starbucket]

Jack up the left side, remove valve cover, hook up timing light to #3, lay down some cardboard and newspaper, put on an old shirt and safety glasses, start the motor and watch the #3 valves as you slowly increase rpms and watch for a sticking valve.

[SGKent]