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Old 10-11-2009, 09:20 PM   #1
Prostreet71
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Could low compression be tight valves

Compression readings are 175 - 165 - 110 - 50. Could the valve clearance being too tight cause the low reading? Would you gamble and pay to have the valve clearance checked to see if that fixes it? Don't know if the valves have ever been checked. Bike has about 30,000 miles on it. Seems to run good just has a low compression reading. Let me know what you think. Thanks

Last edited by Prostreet71; 10-11-2009 at 09:53 PM.
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Old 10-11-2009, 09:49 PM   #2
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Re: Could low compression be tight valves

It can be ,but not likely unless someone adjusted them wrong !

Last edited by luker669; 10-12-2009 at 12:13 AM.
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Old 10-11-2009, 11:10 PM   #3
utp216
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Re: Could low compression be tight valves

I would think the valves being loose not tight causing it. Like one or more of the valves is not seating all of the way.

I would pull the valve cover and the plugs and the signal generator cover and check the valve clearances. It's not hard to do. Do you have a PDF of the shop manual that you can print the valve adjustment parts out from?
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Old 10-11-2009, 11:42 PM   #4
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Re: Could low compression be tight valves

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Originally Posted by utp216 View Post
I would think the valves being loose not tight causing it. Like one or more of the valves is not seating all of the way.
I used to think that too, until it was pointed out that I had it backwards.

This would be tight valves, not loose. "Tight" refers to the clearances between the cam lobe and the bucket. If they're too small then the valve can be held off of its seat when it should be closed. This can result in loss of compression and, at worst, burned valves because they cannot dissipate accumulated heat back to the cylinder head.
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Old 10-12-2009, 12:47 AM   #5
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Re: Could low compression be tight valves

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Originally Posted by d207gp View Post
I used to think that too, until it was pointed out that I had it backwards.

This would be tight valves, not loose. "Tight" refers to the clearances between the cam lobe and the bucket. If they're too small then the valve can be held off of its seat when it should be closed. This can result in loss of compression and, at worst, burned valves because they cannot dissipate accumulated heat back to the cylinder head.
I gotcha. I'm thinking backwards then myself. Whatever the correct terminology I think at least one of the valves is not closing all the way and in doing so not allowing the cylinder to build the proper compression.
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Old 10-13-2009, 01:15 PM   #6
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Re: Could low compression be tight valves

If the valve is sticking open, there should be no compression. It'll pump back thru the head into the intake system. Went thru this w/ my HD and adjustable pushrods. More likely the compression is bleeding off due to worn rings and sleeves. 30k is a decent amount of mileage on a beast like ours, especially for higher RPM motors. I'm guessing he's not the 1st owner since it's a '93 and there's no telling what kind of abuse the bike suffered from previous owners. My '98 1100 only had 22k miles on it when I lost compression in a cylinder. When I pulled the motor down the cylinders were wallowed out so badly the rings lost all seat. Moral is the bike had been run veeeeeeeeeeeery hard by someone before me. Luckily for me, the guy that rebuilt my top end (Dan at Cafe Racer in KCKS) checked cams for wear and replaced all the valves for good measure. Now she's nice and tight... kinda reminds me of a girl I dated once back in college. She was tight.
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Old 10-13-2009, 03:37 PM   #7
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Re: Could low compression be tight valves

You should check the valve clearances. No clearance can mean that valve does not seat causing loss of compression on cranking. High clearance can mean bent valve with same effect. Burnt valves, same thing. It could also be a blown head gasket between 3/4, as both have low cranking pressures . . . look for gases coming out of the coolant overflow bottle while running, or in extreme cases there would be a lot of steam coming from the exhaust with that distinct coolant smell. Find someone with a cylinder leakage tester and they will be able to find the problem quickly.
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Old 10-14-2009, 01:12 AM   #8
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Re: Could low compression be tight valves

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Originally Posted by wombat58 View Post
You should check the valve clearances. No clearance can mean that valve does not seat causing loss of compression on cranking. High clearance can mean bent valve with same effect. Burnt valves, same thing. It could also be a blown head gasket between 3/4, as both have low cranking pressures.
Very likely. But normally you'll see an equilization of the compression pressure between the 2 cylinders if the head gasket blew out between them. More likely as you said 1st that a valve is hanging up or open or that the motor is just plain worn out and needs a top end rebuild. Either way, motor needs to come down for inspection.
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