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Old 04-10-2011, 11:59 AM   #1
The Raven OP
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airhead cyl compression question

This question is in my thread, but figured I would toss it out to all to get some more responses.

My r80 when cold has 80lbs compression in the left cylinder and will not immediately fire. My right cyl when cold is 115lbs

The valves are in tune at last check and the tapping sounds fine when started.

Today I tossed in some marvels mystery oil and let it sit a few hours. Blasted it out, tossed the plug in and she started fine. Well with a bunch of smoke

I let it run a couple minutes to burn the oil out then did another test. Left cylinder went from 80 to 122, right cyl went from 115 to 125.

Thoughts?

I'm guess a stuck and rusty ring from winter condensation?
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Old 04-10-2011, 12:03 PM   #2
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carbs off, or at least sliders held up and open?
if so, and oil brings up the readings that much, RINGS.
but even still it is a little low.
How many miles on the valve job?
What's the leak down test tell you?
It's actually the better, more important of the two tests.
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Old 04-10-2011, 12:13 PM   #3
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Quote:
Originally Posted by wirewrkr View Post
carbs off, or at least sliders held up and open?
if so, and oil brings up the readings that much, RINGS.
but even still it is a little low.
How many miles on the valve job?
What's the leak down test tell you?
It's actually the better, more important of the two tests.
Oil actually brought it to about 165 from 80

After I burnt the oil out and got it to stop smoking it dropped to the 120s and inline with the RHS.

Not sure how to do a leak down test?

10k on the valves
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Old 04-10-2011, 12:39 PM   #4
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OK, did the compression test with the cyl at 90 degrees F, carbs off, both plugs out and grounded.

Results:

L&R 132lbs of compression.

This still does not explain the the low compression while cold. Will be interesting to see the result tomorrow after it's stone cold.
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Old 04-10-2011, 12:49 PM   #5
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My r75/7 after not running for 20+ years before I got it, I tested it cold since its not running yet at there even at 90psi. its on the low side but I hope it increases like your bike did.... I dont feel like another engine project.

Maybe a good highway run will even your compression out.
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Old 04-10-2011, 02:56 PM   #6
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As far as I know...

Compression should be checked when the engine is hot, not "stone cold".
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Old 04-10-2011, 07:32 PM   #7
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The G/S doesn't have a high compression ratio, so 130 doesn't sound bad to me. The more important thing is comparable figures side to side, which you now have.

My guess is there was carbon build up you blew out, that was sticking the rings.
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Old 04-10-2011, 10:51 PM   #8
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Another thing to consider is that when you introduce a bunch of extra oil to the cylinders, it will take a good brisk ride to burn/blow it out of the ring grooves. I have heard up to 100 miles. I think that is a little excessive, but even if it doesn't take near that long, a good hard ride is needed.
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Old 04-10-2011, 11:56 PM   #9
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Quote:
Originally Posted by The Raven View Post
...both plugs out and grounded.
If you don't mind, next time you're doing this, could you please take a picture of how you ground the plugs? This is something that I haven't got my head around yet. many thanks
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Old 04-11-2011, 04:19 AM   #10
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Quote:
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If you don't mind, next time you're doing this, could you please take a picture of how you ground the plugs? This is something that I haven't got my head around yet. many thanks
No need for a photo.....take plugs out, insert into wires, then rest plug on the head, ground accomplished.
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Old 04-11-2011, 08:49 AM   #11
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self healing?

My Norton came to me after sitting for 10 years. I did an initial compression test and had 80# in each. Fired it and rode it a bit and have had 128# in each ever since. Your rings just need to get friendly with the barrels again.
Leakdown is the best diagnostic test.
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Old 04-11-2011, 09:19 AM   #12
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Quote:
Originally Posted by mykill View Post
My Norton came to me after sitting for 10 years. I did an initial compression test and had 80# in each. Fired it and rode it a bit and have had 128# in each ever since. Your rings just need to get friendly with the barrels again.
Leakdown is the best diagnostic test.
I'll second that! I would not obsess over it... though I know how we feel after a winter's wait....
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Old 04-11-2011, 11:34 AM   #13
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Quote:
Originally Posted by jimbee View Post
If you don't mind, next time you're doing this, could you please take a picture of how you ground the plugs? This is something that I haven't got my head around yet. many thanks
just pull the hot wire from the coil, no need to worry about loosing the ground
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Old 04-12-2011, 10:03 AM   #14
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Quote:
Originally Posted by One Less Harley View Post
just pull the hot wire from the coil, no need to worry about loosing the ground
Hey neat idea.

The reason I was asking because I've always been curious about snowbums "other" method for carb sychronization, but after reviewing his tome, I see now that it does not involve removing the plugs.
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Old 04-12-2011, 11:05 AM   #15
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Of course you know I'm not referring to the high tension lead from coil to spark plug, but the actual power leads to the coil!! Also for compression test not synch.
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