compression test

geezerbiker · Full Member Joined: Sep 15, 2007 Posts: 49

Finally got around to purchasing a compression tester. I got to the point in my project where I didn't want to go any further until I could determine if it would be necessary to pull the engine from the frame and tear it down. The Hayne's manual says the bike should be at normal operating temperature to test compression. Of course, that isn't possible when the bike is in the state that mine is now. The best I can do is crank it over with the kickstarter and check the numbers. As it's winter here in the northeast USA and temps rarely climb out of the 20's, I figured I'd warm it up as much as I could. I cranked up the shop heater full for about 8 hours so the bike got thoroughly warmed through. Then I took the readings. 135 right cylinder, 115 left. Not good. I think the right is okay, but there shouldn't be that much difference between the two. Seems unlikely the rings would be worn a lot more in one cylinder than the other, so I'm suspecting valve problems. Looks like I'm diving into a total rebuild.

jayel · Posted: January 28, 2008, 11:13 pm Post subject: Re: compression test

hi geezer I'd try to get it to run first, it may knock some of the carbon/rust off the valves and be fine, anytime these bikes set for a period of time it more than likely one valve is held open, the valve face and seat are susceptible to rust forming, should be less than 10 lbs. difference, to check rings add a small amount of oil to the cylinder and re-test, but try this first to check valves and rings set cylinder to TDC with valves closed (compression stroke) use a rubber tipped air nozzle to blow a small amount of air (blow gently but steadly) into sparkplug hole if air escapes thru crankcase=bad rings, if out the carb=bad intake valve, out the exhaust pipe=bad exhaust valve ----- kind of a leak down test some air will leak but if it blows straight thru you've found the problem

geezerbiker · Full Member Joined: Sep 15, 2007 Posts: 49

Great ideas. I will try them. At this point, I have the bike stripped down as much as I can without tearing into the engine. If I start putting it back together, only to find later that the engine needs a rebuild, then I'll be taking it all back down again. Of course we try to avoid redundancy, but I suppose sometimes it can't be helped. I will follow your advice and try to fire it up and see how it does. Thanks.

geezerbiker · Full Member Joined: Sep 15, 2007 Posts: 49

Another test: this 150psi on the right and 60psi on the left! I shoot some oil into the left cylinder and try it again. The engine is much harder to kick over. In fact, I have to throw my entire body weight onto it to get it to turn over. But then it does, and the pressure on the left shoots right up to 150psi. I figure adding oil would bring the pressure up a bit if the rings were bad, but not that much. I'm thinking there was probably a valve seized in the open position, which is now functioning normally. I can't think of anything else that would account for it. It was really kicking over too easily before. Now it seems about like what a 650 twin should be like to kick.

jayel · Posted: January 30, 2008, 12:31 am Post subject: Re: compression test

you are checking it with throttle open and slides blocked up? one cylinder at a time sparkplug out of other

nj1639 · Posted: January 30, 2008, 8:15 am Post subject: Re: compression test

geezerbiker wrote:

Another test: this 150psi on the right and 60psi on the left! I shoot some oil into the left cylinder and try it again. The engine is much harder to kick over. In fact, I have to throw my entire body weight onto it to get it to turn over. But then it does, and the pressure on the left shoots right up to 150psi. I figure adding oil would bring the pressure up a bit if the rings were bad, but not that much. I'm thinking there was probably a valve seized in the open position, which is now functioning normally. I can't think of anything else that would account for it. It was really kicking over too easily before. Now it seems about like what a 650 twin should be like to kick.

I did that test, having problems with the right cylinder. Got the same results as you're getting. My problem was rings/scored cylinder. If a valve was open you'd get no change in the psi. Bad rings or a score? The oil will fill the gap and give you a higher reading.

geezerbiker · Full Member Joined: Sep 15, 2007 Posts: 49

jayel wrote:

you are checking it with throttle open and slides blocked up? one cylinder at a time sparkplug out of other

Right now, I basically have an engine on a frame; no throttle, no carbs, no exhaust system. I'm testing it with the gauge in one cylinder, the other open.

geezerbiker · Full Member Joined: Sep 15, 2007 Posts: 49

I did that test, having problems with the right cylinder. Got the same results as you're getting. My problem was rings/scored cylinder. If a valve was open you'd get no change in the psi. Bad rings or a score? The oil will fill the gap and give you a higher reading.

A higher reading yes, but that much higher, from 60 to 150?
I'm wondering is a valve was staying open before, but now it's not. That's why the sudden big jump in psi. Maybe adding oil to the cylinder somehow got the sticky valve to free up. Now the two cylinders have almost equal compression. It's just a little hard to believe that adding the oil could do that.

nj1639 · Posted: January 30, 2008, 8:55 pm Post subject: Re: compression test

geezerbiker wrote:

I did that test, having problems with the right cylinder. Got the same results as you're getting. My problem was rings/scored cylinder. If a valve was open you'd get no change in the psi. Bad rings or a score? The oil will fill the gap and give you a higher reading.

A higher reading yes, but that much higher, from 60 to 150?
I'm wondering is a valve was staying open before, but now it's not. That's why the sudden big jump in psi. Maybe adding oil to the cylinder somehow got the sticky valve to free up. Now the two cylinders have almost equal compression. It's just a little hard to believe that adding the oil could do that.

Yep, that much difference.Mine was around 30 psi before the tablespoon of oil and around 140 psi after. Let it sit a while and try it again. If you still have pretty equal psi, then it may have been a valve. Otherwise.....

geezerbiker · Full Member Joined: Sep 15, 2007 Posts: 49

Yep, that much difference.Mine was around 30 psi before the tablespoon of oil and around 140 psi after. Let it sit a while and try it again. If you still have pretty equal psi, then it may have been a valve. Otherwise

Yes, I thought that if it is just the oil doing that, it's only a very temporary fix. After it sits for a while, I'll give it a good crank again, and see what happens.

oldskoolcool · Full Member Joined: Sep 08, 2007 Posts: 372

leakdown tester is a much more accurate way. Thought i had bad rings, but it was a stuck valve

geezerbiker · Full Member Joined: Sep 15, 2007 Posts: 49

I guess I've got my answer. This morning I checked it again; still 150psi. Then I fed a spoonful of straight gas into the cylinder, figuring that would wash away the oil I had added earlier. It did. Pressure dropped right back down to 60psi. Looks like I'm in for a total rebuild, or at least the top end.

grizld1 · Posted: January 31, 2008, 5:46 pm Post subject: Re: compression test

Or maybe just rings. Take the pistons and cylinders to a machine shop and have 'em miked out (usual charge is between cheap and free). If you're lucky, deglazing and new rings may get it done.

geezerbiker · Full Member Joined: Sep 15, 2007 Posts: 49

grizld1 wrote:

Or maybe just rings. Take the pistons and cylinders to a machine shop and have 'em miked out (usual charge is between cheap and free). If you're lucky, deglazing and new rings may get it done.

I guess that would be best case scenario at this point. I've been thinking of purchasing some micrometers anyway. Since the manual lists all of the tolerances, maybe I'll check it out myself, see what it needs, and take it from there.

grizld1 · Posted: February 1, 2008, 3:57 pm Post subject: Re: compression test

Good plan. It's like I keep telling my wife--buying tools isn't an expense, it's an investment--it pays! (No, Hon, I can't show a return on that, but without it I'd have had to spend a LOT MORE!)

geezerbiker · Full Member Joined: Sep 15, 2007 Posts: 49

grizld1 wrote:

Good plan. It's like I keep telling my wife--buying tools isn't an expense, it's an investment--it pays! (No, Hon, I can't show a return on that, but without it I'd have had to spend a LOT MORE!)

I hear that! I actually was able to show a profit, when I sold the KZ440 I rebuilt and used the profit to buy this computer, which we badly needed. That satisfied her for a while, but now? There's always a list of tools (she calls them toys) that I absolutely must have for the next project!

pumps · Posted: February 1, 2008, 10:31 pm Post subject: Re: compression test

I tell mine at least it's better than being in the bars. If I buy a tool I can make money with it.

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