Compression test question


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phoenixtubs
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Motorcycle: 1985 Interstate
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1978 GL 1000

Compression test question

Post by phoenixtubs »



Hi, I just bought a 78 GL 1000 and I'm in the process of getting her road ready.
Short history here: viewtopic.php?f=3&t=36612

So far I've changed the timing belts and adjusted the valves. I want to do a compression test but I have the radiator off the bike. Will it hurt anything if I do a compression test without coolant running through the system?




Thanks!


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kepfordj
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Re: Compression test question

Post by kepfordj »

Nope it will not hurt a thing.
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Maz
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Re: Compression test question

Post by Maz »

phoenixtubs wrote:Hi, I just bought a 78 GL 1000 and I'm in the process of getting her road ready.
Short history here: viewtopic.php?f=3&t=36612

So far I've changed the timing belts and adjusted the valves. I want to do a compression test but I have the radiator off the bike. Will it hurt anything if I do a compression test without coolant running through the system?

GL 1000 new timing belts.jpg
GL 1000.jpg

Thanks!
It won't hurt anything. You're only spinning the motor on the starter, with the throttle wide open, briefly for each cylinder.
Maz
Ironically, Common Sense is the LEAST common of all senses!
phoenixtubs
Posts: 48
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Motorcycle: 1985 Interstate
1987 VT700C Shadow
1978 GL 1000

Re: Compression test question

Post by phoenixtubs »

Thanks for answering. Not sure if this belongs in a new thread, but I went ahead and did the compression test, and cylinders 2, 3 and 4 read 120, and cylinder 1 read 65. I put a little oil in 1 and it came up to 90. The bike hasn't been running for over a year, maybe two, so my thought is to go ahead and get it running and try to ride it for a bit and see where I'm at after that. Any advice as to how I should proceed would be welcome.

Thanks!
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SnoBrdr
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Re: Compression test question

Post by SnoBrdr »

phoenixtubs wrote:Thanks for answering. Not sure if this belongs in a new thread, but I went ahead and did the compression test, and cylinders 2, 3 and 4 read 120, and cylinder 1 read 65. I put a little oil in 1 and it came up to 90. The bike hasn't been running for over a year, maybe two, so my thought is to go ahead and get it running and try to ride it for a bit and see where I'm at after that. Any advice as to how I should proceed would be welcome.

Thanks!
Ring could still be stuck.

120 is low.

http://www.randakksblog.com/engine-compression-testing/
phoenixtubs
Posts: 48
Joined: Mon Jun 01, 2015 2:56 pm
Location: Chicago, IL
Motorcycle: 1985 Interstate
1987 VT700C Shadow
1978 GL 1000

Re: Compression test question

Post by phoenixtubs »

Thanks, I have seen that and I agree 120 is low. I'm going to follow Randakk's advice on starting after a long layup here:

http://www.randakksblog.com/starting-a- ... ng-lay-up/

Hopefully I can get it running and free up the rings a bit.
barrettmusicman1
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Re: Compression test question

Post by barrettmusicman1 »

Where do you get a pressure gauge?
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SnoBrdr
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Re: Compression test question

Post by SnoBrdr »

barrettmusicman1 wrote:Where do you get a pressure gauge?
If you don't want to buy one and why should you, most auto parts stores lend tools.
phoenixtubs
Posts: 48
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1978 GL 1000

Re: Compression test question

Post by phoenixtubs »

I did buy one, Harbor Freight.
sfruechte
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Re: Compression test question

Post by sfruechte »

A tight valve that doesn't close can lower your compression.
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badandy
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Re: Compression test question

Post by badandy »

Compression numbers should be between 140-180 psi with no more than a 15% difference across the cylinders. A number close to 100 psi and you definitely have a problem in that cylinder. Running it may bring up the compression, but your reading of under 100 on the single cylinder even when wet is indicative of a major problem.
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WingAdmin
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Re: Compression test question

Post by WingAdmin »

sfruechte wrote:A tight valve that doesn't close can lower your compression.
This is when you do an oil test.

If you have low compression in a cylinder, take a tablespoon of oil and pour it into the cylinder through the plug hole.

If the compression goes up (because the oil has helped seal the piston rings) then your issue is worn piston rings or cylinder wall. If the compression doesn't change, then the issue is a valve.


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