Compression test
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Cruiser
Joined: Jun 2018
Posts: 119
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From: Monroe
Compression test
Howdy! So Ive never done a compression test ever till today. I was reading past threads a bit to know why Harley Davidson says 90lbs is sufficient but alot of guys think 150 or more is what it should read. Anyway i did mine and the front read 103 and the rear 120. I thought that was a bit off so I read the threads to add a bit of oil in the off cylinder and it actually read 110 after.
Any ideas on why these are so different between the cylinders and is it that big of deal?
Recap:
1997 FLHR road king carbed with andrews cam, rejet, v and h pipes
55,555 miles
Any ideas on why these are so different between the cylinders and is it that big of deal?
Recap:
1997 FLHR road king carbed with andrews cam, rejet, v and h pipes
55,555 miles
Road Warrior
Joined: Dec 2012
Posts: 1,686
Likes: 242
From: Glen Mills, PA
Hey. My .02 as I recently went through this on a bike that sat for almost 10 years. The oil will seal the rings and check the compression as if the rings were sealed. A large change in your results with oil indicates worn rings and a loss of compression for that reason from what I have read.
I think up to 10% variation is normal or acceptable. If you google search this you will educate yourself on compression tests, leak down tests and how to diagnose bad rings or bad cave seating.
Good luck and there will be smarter folks that chime in. The manual also walks you through this.
I think up to 10% variation is normal or acceptable. If you google search this you will educate yourself on compression tests, leak down tests and how to diagnose bad rings or bad cave seating.
Good luck and there will be smarter folks that chime in. The manual also walks you through this.
Banned
Joined: May 2018
Posts: 4,576
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From: Viet Vet, crossroads
1, -Fully Charged battery
Ignition disabled (wires grounded)
Throttle wide open .
.The max difference would be 15 % difference between the two.
On a V8 that might be OK
On a 2 cylinder engine, I would not be happy with that
the closer the better of course.
SO -your readings are just barely in the allowable tolerances
Ignition disabled (wires grounded)
Throttle wide open .
.The max difference would be 15 % difference between the two.
On a V8 that might be OK
On a 2 cylinder engine, I would not be happy with that
the closer the better of course.
SO -your readings are just barely in the allowable tolerances
Here are some numbers I came up with a while back. Short version is (assuming the stock Keihin CV carb), much depends on the state of the throttle and the slide. https://www.hdforums.com/forum/evo-c...n-testing.html
I agree with the conclusion that your difference per cylinder is on the edge of being a point of concern.
I agree with the conclusion that your difference per cylinder is on the edge of being a point of concern.
Dirt don't hurt
Joined: Jan 2011
Posts: 21,059
Likes: 4,534
From: Haslet Texas
Howdy! So Ive never done a compression test ever till today. I was reading past threads a bit to know why Harley Davidson says 90lbs is sufficient but alot of guys think 150 or more is what it should read. Anyway i did mine and the front read 103 and the rear 120. I thought that was a bit off so I read the threads to add a bit of oil in the off cylinder and it actually read 110 after.
Any ideas on why these are so different between the cylinders and is it that big of deal?
Recap:
1997 FLHR road king carbed with andrews cam, rejet, v and h pipes
55,555 miles
Any ideas on why these are so different between the cylinders and is it that big of deal?
Recap:
1997 FLHR road king carbed with andrews cam, rejet, v and h pipes
55,555 miles
Doing that lets air in to be compressed.
The numbers you posted look like your throttle was closed when you did the test.
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Cruiser
Joined: Jun 2018
Posts: 119
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From: Monroe
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Grand HDF Member
Joined: Apr 2007
Posts: 3,706
Likes: 3,017
From: Brighton, MI
Remove air cleaner to get access to carb. Use something to prop open the slide but make sure it i not something that can get sucked into the engine.
Twist the throttle wide open and lock it in the full throttle position with the thumb screw on the throttle (poor mans cruise control).
Then do the compression check based on instructions provided by the previous posts. Don't forget to ground the plug wires.
Just an FYI, my evo had 37,000 miles on it and comp check was 140 each cylinder prior to tear down. The only reason I tore it down was because of base gasket leak. Decided to do a 10 over bore while it was apart. YD
Twist the throttle wide open and lock it in the full throttle position with the thumb screw on the throttle (poor mans cruise control).
Then do the compression check based on instructions provided by the previous posts. Don't forget to ground the plug wires.
Just an FYI, my evo had 37,000 miles on it and comp check was 140 each cylinder prior to tear down. The only reason I tore it down was because of base gasket leak. Decided to do a 10 over bore while it was apart. YD