Compression

[Bikeboy]

Hi guys, well I did a compression check today. Piston #1 had 123psi, #2- 127, #3- 129, #4- 128. My question is, what should it be. And if these numbers are good then why want she go over about 100 mph WOT. This can't be right. I would expect a bike of this size to go 125-135 mph. Do you think the head might need to be rebuilt. Engine seems to run fine other than a little top end noise, (chatter).

[steelcat]

from my Yamaha book:
Standard 142 psi
Minimum 128 psi
Maxium 156 psi

[Redbaron]

Engine Chatter?

Valve job maybe, check before rebuild. people are notorious for neglecting their valves.

[TopCatGr58]

Hey there Bikeboy,

What RPM are you able to reach? If you're reaching redline in 5th but only going 100mph, then you've got some clutch slippage going on there!! If it just won't reach redline in 4th or 5th, then you've got some tuning issues there, perhaps too lean?

Your comps are very close to each other, that's good. Yes, stock at sea level is listed at 142. Interesting maximum....I've got a big bore, and had 175-185 range across mine!! I'm sure you did it with the engine warmed up, and throttle plate at FULL OPEN, and plugs out of the other cylinders!?

Are your exhaust pipes clear, not clogged? Good luck!
T.C.

[ratbyk]

Compression

Bikeboy, make sure that you warm engine up first. Remove all plugs. Hold throttle wide open so the cylinders have something to compress. This is called a dry test. Next squirt a small amount of oil in each cylinder. Turn the engine over a few times to distribute the oil around all rings/pistons/cylinders and repeat the test. If there is a significant increase in compression then your problems are in the rings.
If you are still getting low readings try another compression gauge. These gauges are notorious for giving false readings.
Readings on all cylinders being close together is a good sign.
Before opening up the engine I would suggest a leak down test to try and pin point where the compression is leaking to.
One tooth out on a camshaft can also give low readings on all cylinders.
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"

[crawlspace2112]

also consider checking valve lash you could have valves staying partially open or open too late causing a lower compression.

[nashville_bill]

While your compression is at the lower limit, your bike should still have plenty of power. Check the spark plugs for color and tune to correct lean or rich condition. Check the air filter, exhaust for obstructions. Either of these will cause a drop in power. Redline in third gear should be around 85 MPH. Will your bike get to redline with power in the lower gears or does it just run out of power in the higher gears?

[Bikeboy]

Thanks guys

Hey Topcat, Ratbyk, you might have something there. I did not hold throttle wide open when I did the test, but the engine was warmed up . Did something right. All of you fellows have some good ideas. I'am going to try this again, the right way. And also try it with an oz. or so of oil in each cylinder to see if I get a different reading. Plugs look fine, I think, kind of grey on the tips, but black down around the inside, white insulator in the center is light grey brown. Plugs are NGK BP-6ES, and have about 300 miles on them. Does this sound about right.

[ratbyk]

Compression

From what you describe your plug appearance is, I truly think that you are in good shape. Retest with that throttle open and I can see you get readings around 150 PSI.
Good Luck!
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"

[eltoroloco]

compression question

my bike tested at 140 all around +or- 4psi. the question is, are the psi supposed to hold for a certian amount of time. My manual says nothing about the duration or how many psi lost over a certian amond of time is acceptable. anyone have some insight on that one.

[Ken Talbot]

That's what a leak-down test works on. You supply a metered amount of air into the cylindres one at a time and see how well it holds. You cannot replicate this test with just a simple compression gauge.

[ratbyk]

Compression Question

Never thought or had that question ever asked before. I am quite sure the peak compression pressure would leak down quite quickly. But in real life it only has to hold this pressure for milliseconds. It then has to contain combustion pressure that is likely several hundreds pounds higher again only for milliseconds before it starts all over again.
There is a test known as a leak down test that should be performed if you have low compression. This will allow you to isolate were the leak is so that an intelligent decision can be made on what path to follow doing repairs.
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"