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Uhoh...Low Compression

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Uhoh...Low Compression

PostAuthor: SammyB71 » Wed Sep 13, 2017 2:36 pm

Hey all,
Question regarding low compression on my '82 XV750...
Both cylinders showing 122 lbs or so on compression test.
Compression test done on warm engine at WOT.
Put a teaspoon of oil down each jug...no change..122 per cyl.
What's next? Valve clearance check?
Bike runs ok but I do think it prob should have a little more snap than it does...

Thanks
Sam

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Re: Uhoh...Low Compression

PostAuthor: martyjay7 » Wed Sep 13, 2017 7:04 pm

SammyB71 wrote:Hey all,
Question regarding low compression on my '82 XV750...
Both cylinders showing 122 lbs or so on compression test.
Compression test done on warm engine at WOT.
Put a teaspoon of oil down each jug...no change..122 per cyl.
What's next? Valve clearance check?
Bike runs ok but I do think it prob should have a little more snap than it does...

Thanks
Sam


Any idea what that pressure measurement is in PSI?
My understanding was you do a leak down test to see where the air is escaping.
For that you'll need an air compressor and a leak down adapted kit.
--- I just want her to sound nice, ride nice, look nice and do what she's told.
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Re: Uhoh...Low Compression

PostAuthor: SammyB71 » Wed Sep 13, 2017 7:13 pm

Apologies if ambiguous.
By 122 lbs I mean 122 pounds per square inch...psi...
I'll have to check out the leak down test procedure. Wondering if it's valves that need to be lapped, or if it's just normal piston ring wear...
I did do another check today and had 121 psi front and 129 psi on back...I removed the air filter this time as it was fairly dirty...
Bike came to me in pieces and odometer shows 39000 km on it, but there is likely more mileage on it than that...PO had an Acewll speedo on it temporarily.

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Re: Uhoh...Low Compression

PostAuthor: Flyingdog » Thu Sep 14, 2017 1:44 am

Never hurts to check valve clearances at the beging of engine work, since it's cold at this point. Eliminates that factor. Just the fact your psi didn't change with the oil in cylinders usually says rings/seating...ok. Then points to valves. As long as a person does the wet test correctly. A teaspoon of oil lube wouldn't distribute itself over the top of the piston, then down into the ring cavities. You would need to put a couple of ounces in the low reading cylinders, wait a about five minutes, then slowly turn the engine over without the plugs in it. I normally use a socket on the crank nut to turn it so I don't blow oil all over . Catch the excess oil on a shop towel. Once you've turned it over three or four times you'll have the excess oil out of the cylinders and you can do the compression check, then determine if it's ring or valve related. This is just my method.
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Re: Uhoh...Low Compression

PostAuthor: SammyB71 » Thu Sep 14, 2017 2:11 am

Thanks Dog..l will give this a try.
Not that big a deal anyway...riding season is all but over here, and the bike stills runs great.

And...I did get the motor from my donor bike unlocked, so over the winter, I think I will rip that one down and have a spare drop in engine ready to roll by spring...
Will be great practice to then move on to the engine I'm running now...
Thanks.
Sam

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