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Discussion Starter · #1 ·
I've got a1981 cb900c and have no compression in one cylinder except when the engine is running. Then the cylinder has some compression in it. I'm assuming this means that the head gasket is bad. I've ordered the parts and tools already so if anyone doubts this diagnosis please let me know so I can cancel the order. I don't have a lot of extra money but I need the bike running since I have no other transportation. The exhaust valves have no clearance either so I'm going to have to adjust that. Waiting on tool so I can figure out what size shims to order. I'm thinking this isn't the problem with the compression.
 

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Discussion Starter · #4 ·
I'm only taking the head off to replace the gasket. Does this seem like the issue if I'm getting compression out of the cylinder only when the engine is running. This doesn't seem like it could happen with leaky valves.
 

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If the engine starts when you're doing a compression check, you're doing it wrong.
You should be checking to see how much pressure you can pump up by just bumping the starter a few times.
 

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Discussion Starter · #6 ·
You misunderstand. There is no compression when I do the check. But when the engine is running and the spark plug is not in there is a lot of air coming out of the cylinder. I'm including this as a symptom to help diagnose the problem. I'm assuming the pressure is coming from one of the other cylinders due to a bad head gasket.
 

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Zip
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Even if there is low compression in one cylinder you will still get puffs of air coming out of the spark plug hole. The cylinder needs almost zero compression to make that puff of air. Imagine a 1/16" hole in one of the valves. That's not enough of a leak to prevent that puff of air coming out the spark plug hole. But it's way more than enough to ruin compression in the cylinder. Yoh may well have a burned valve.
 

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Discussion Starter · #8 ·
There is more than a puff of air when it's running. There is a slight puff air when it's not running but turning over. I guess either way I've got to take the head off which means taking the engine out of the bike. Either way it's a big job and I'll have no transportation while I'm doing it. And I live alone in the middle of nowhere. I was just hoping it would be something simpler.
 

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Zip
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Yeah, if it's failing the compression test you have no choice but to take the head off. And thanks to Honda's lovely design that means removing that one part of the frame and pulling the engine out.

I supported the engine on a floor jack and had my brother move the bike away from it.
 

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Discussion Starter · #10 ·
It's outside and I'm on my own and I don't have a floor jack. Concrete blocks maybe? Or 2x4s?I'm going to watch some YouTube videos and read the manual instructions. It's a little overwhelming; but, I can't afford to replace it or pay someone else to fix it. I just can't understand why there is pressure in the cylinder when it's running but not when it's just turning over.
 

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Zip
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The cylinder needs to be absolutely air tight for the engine to run right, but it can have significant leaks and still blow air out of an open spark plug hole. Prove this to yourself with an empty plastic milk jug. Take off the cap and poke a small hole in the bottom of the jug. Then point the top of the jug at your face and squeeze it hard and quick. You'll get a blast of air even though the jug won't hold water.
 

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Discussion Starter · #12 ·
I may have to just keep riding it like it is. 20 mpg sucks. And I don't know how long it will keep going. I'm not sure I can actually do a full tear down with limited tools and no help.
 

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Might be worth giving your engine a good dose of decarbon treatment like Kreen or Seafoam. This won't repair a warped valve or fix a leaking head gasket, but it might soften carbon that is built-up on the seat of the exhaust valve and may well loosen rings that might be cemented into their slots by carbon buildup.

Another trick that can help with valve sealing on those machines that have adjustable valves is to temporarily add extra tappet clearance. This makes sure the valves are seating firmly and puts a little extra vibration into the valve train to pulverize carbon buildup.

Since you are talking about riding the machine on 3/4 cylinders, you might want to block off gas flow to the carb for the affected cylinder. This will prevent that gas from being wasted and ending up in your exhaust system, where it can cause overheating and backfiring.

What you described sounds like a valve issue to me, but could be cylinder wear, cracked or frozen rings, or even worse. Have you followed the procedure for a pressure test, including putting oil in the cylinder after getting low readings on the initial test, or was your testing more along the lines of "stick a finger over the spark plug hole"?
 

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Discussion Starter · #14 ·
I've run seafoam through it to no effect. I will adjust the valves. I have to wait for the valve shim tool and then I'll have to order shims when I figure out what size I need. And yes, I did try putting oil in the cylinder and the compression didn't change.

Any idea why the cylinder has more compression when the engine is running vs. just turning over. Is it just an rpm effect or is pressure coming from one of the other cylinders through a hole in the head gasket? And if this is the case, shouldn't I have two cylinders with low compression? This is really confusing me. But maybe it doesn't matter.

I'm not sure how to shut off gas to one carb.
 

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Any idea why the cylinder has more compression when the engine is running vs. just turning over.
I might be wrong but it seems to me, if the engine is running it means the crank is spinning much faster than you can spin it which results in what "seems like" more compression. No?
 

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I've run seafoam through it to no effect. I will adjust the valves. I have to wait for the valve shim tool and then I'll have to order shims when I figure out what size I need. And yes, I did try putting oil in the cylinder and the compression didn't change.

Any idea why the cylinder has more compression when the engine is running vs. just turning over. Is it just an rpm effect or is pressure coming from one of the other cylinders through a hole in the head gasket? And if this is the case, shouldn't I have two cylinders with low compression? This is really confusing me. But maybe it doesn't matter.

I'm not sure how to shut off gas to one carb.
johnnyvee asked how you are testing compression. Have you tested the compression with a gauge while holding the throttle in the Wide Full Open position? If so what numbers are you getting on the other cylinders and is the gauge actually reading zero on the bad cylinder?

" The exhaust valves have no clearance either so I'm going to have to adjust that "
That ^ could burn valves or maybe, if you're lucky, only cause a loss of compression pressure.
I'd adjust valve lash first, then see what I've got...

" Any idea why the cylinder has more compression when the engine is running vs. just turning over "

Yes, with the other three cylinders running that piston is moving up and down so fast that no amount of valve leaks, or whatever leaks, could "bleed off" very much of the strong stream of air (wind) that will shoot out the open spark plug hole.

S F
 

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Discussion Starter · #18 ·
Thanks. That helps. I will try to adjust the valves first and pray I don't have to tear it all down.

The compression tester doesn't seem to be working right but I am getting significant pressure{about 100) on the other cylinders and zero on the one cylinder. And the bike runs fairly well once it warms up but terrible gas mileage.
 
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