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one cylinder not pulling its weight? (video)
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airkooledchris
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PostPosted: Thu Jul 16, 2009 2:53 pm    Post subject: one cylinder not pulling its weight? (video) Reply with quote

short history. a recent compression test showed Cyl 1&2 were very low in compression, while 3&4 seemed to be almost brand new....

what im a little fuzzy on, and perhaps I need to just ignore this whole fact, but why is it that if Cyl #1 and #2 have the lowest compression and #3 and 4 the highest, why does pulling the spark from 1 or 2 nearly kill the engine and when you pull it from #3 the motor can barely tell the difference?
I took a little video of it just now:



Link



you can see it's getting spark, it just seems like the engine isn't seeing much benifit from the #3 side even being there....
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1975 Kombi
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PostPosted: Thu Jul 16, 2009 4:12 pm    Post subject: Reply with quote

What were the numbers and were the valves adjusted before the test.
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airkooledchris
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PostPosted: Thu Jul 16, 2009 4:15 pm    Post subject: Reply with quote

the first time we checked compression, after a valve adjustment, was:

#1 - 66
#2 - 85
#3 - 110
#4 - 120

with a bit of oil we got

#1 - 90
#2 - 110
#3 - 150
#4 - 160

after we ran it to burn up the oil inside

#1 - 90
#2 - 85
#3 - 90
#4 - 120
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1975 Kombi
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PostPosted: Thu Jul 16, 2009 4:21 pm    Post subject: Reply with quote

There is a procedure somewhere. I think on Ratwell's site. The engine warm the throttle body wide open, I do normally five or six rotations with all the plugs out, watch the pumps on the gauge to see if it jumps well to the max or if it slowly builds, battery full, Just to make sure you have the most accurate test.
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1975 Kombi
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PostPosted: Thu Jul 16, 2009 4:24 pm    Post subject: Reply with quote

I say 3 is not firing. Pull the plug and check spark grounded on the case.
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PostPosted: Thu Jul 16, 2009 4:27 pm    Post subject: Reply with quote

I wonder if there is a short along the plug wire somewhere, and it's not reaching the plug.

[/quote]
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airkooledchris
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PostPosted: Thu Jul 16, 2009 4:32 pm    Post subject: Reply with quote

I had Colin the aircooled itinerant help me with the compression test, using his gauge so I know those numbers are good.

I actually swapped out that plug wire with a used bosch and had the same result. if you look really close in the video, you can see that when I hold the wire close to the dist cap there are sparks, and when you see them you an hear a very faint change in the sound of the motor, but almost none.

are you saying to pull the plug itself and start it and run it without any plug in the #3 side, then hold the end of the plug near the case to verify there is spark there as well?
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1975 Kombi
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PostPosted: Thu Jul 16, 2009 4:37 pm    Post subject: Reply with quote

Well just have someone turn it over and you watch the plug for spark. The plug could be bad. And later run it in the dark and look closely for spark jumping somewhere in the system.
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airkooledchris
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PostPosted: Thu Jul 16, 2009 4:42 pm    Post subject: Reply with quote

airkooledchris wrote:
are you saying to pull the plug itself and start it and run it without any plug in the #3 side, then hold the end of the plug near the case to verify there is spark there as well?


1975 Kombi wrote:
Well just have someone turn it over and you watch the plug for spark. The plug could be bad.


aka, yes - ? or did I not describe what your referring to correctly?
it's not a procedure I want to F up by following a vague guideline.

but for a cyl with good compression to behave like this isn't normal in any circumstances, and this is something I need to continue to track?

as long as I know im not chasing ghosts and this is something that may be seen in normal operating conditions, ill go hunting.
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1975 Kombi
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PostPosted: Thu Jul 16, 2009 4:46 pm    Post subject: Reply with quote

I am not sure but a spark from the cap to the wire may not translate into a spark at the gap. If it was firing there would be a difference in the motor.
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Wildthings
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PostPosted: Thu Jul 16, 2009 6:06 pm    Post subject: Reply with quote

If you have a shorted plug you will see a spark at the cap, but get no power from that cylinder. You could also have a problem with the injector for that cylinder, or an air leak that is affecting #3 worse than the others.
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1975 Kombi
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PostPosted: Thu Jul 16, 2009 6:32 pm    Post subject: Reply with quote

The lack of #3 probably balances out the opposite side cylinders.
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airkooledchris
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PostPosted: Thu Jul 16, 2009 7:19 pm    Post subject: Reply with quote

Wildthings wrote:
If you have a shorted plug you will see a spark at the cap, but get no power from that cylinder. You could also have a problem with the injector for that cylinder, or an air leak that is affecting #3 worse than the others.


ill try what 75kombi mentioned and verify the plug itself sparks as it should, or even try swapping it out with new.
I think im ok with vacuum leaks, I just did the intake runner gasket's on both sides.
i also have one brand new injector I could swap out on the #3...

thanks again for the tips. at the very least, I know it's not something to be ignored.... (for now!)
Wink
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PostPosted: Thu Jul 16, 2009 7:48 pm    Post subject: Reply with quote

I would check the rubber boots on the plenum to see if #3 has a leak. If it doesn't then I would have the injectors cleaned.
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