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Valves?

N

NightRider

Well-known member
Hey guys, looking for a little insight on a problem I've got with an 0540 300 A/V. A leak down test showed low compression on 3 cylinders. The engine was rebuilt about 5 yrs. ago and has 110 hrs. since. As of right now engine idles cold at 60-65 psi and hot at 35psi. It is a new engine to me and I have 20 hrs. run time on it and am not blowing any smoke and burning app. 1 qt. of oil per tank (30 gallons). Does it sound like valve issues? Engine runs and sounds good to my non-mechanical ear, but does feel down on power (readings on low cylinders was 30-40). Any insight and information is greatly appreciated.


Just went out and fired it up. Definately valve leaking on port side. (Exhaust sucking back in.)
 
Of course it is always better have the max compression wise, for performance reasons but if it is running well gets you where you want and doesn't leave you stranded run it till you have a real problem, unless you like to race or have money to burn.
 
If your compression is below 60 over 80, you have some big problems that should be addressed. The problem is not going to get better with time. 30 pounds of compression isn't enough. Could you hear the excess air coming out the intake or the exhaust? Warm the engine and check it again. You usually get higher readings when the engine is warm.

You may have a valve stuck open. You could try to stake it to beat the carbon off of it. I'd pull a cylinder with that low of a reading.
 
If you can hear it blowing by, its just a matter of time and it will let you down. Could be something as simple as adjustment, broken spring or such.Deal with it now or you will most likely deal with something worse pretty soon.
 
Are you talking about a cyl leakage test or a compression test. A cyl leakage test will show you weather it is a valve or rings that are the problem. If it is a compression test you dont know which it is but should do a leakage test to determine weather it is a valve or rings. A valve problem is not that hard to fix if found early.
Tim
 
What ever the problem is, it will not heal. Fix a small problem now,
( and spend a little money) or fix a big problem later, and spend
a bunch.
 
General rule of thumb it takes 90 pounds of cylinder pressure to ignite gasoline. Are you talking leak down % or cylinder pressure. Anything below 90 pounds is pretty much a dead hole. You have some major issues do a leak down that will tell you if it’s valves or pistons or rings. 30 to 40 pounds she is hurt bad! If it's 30 to 40% you still have a hurt motor but it will run. I try to keep leak down less then 8% on a car motor and most big bor A/C motors below 20% leak down.
 
I have seen motors run fine but fail a leak-down, Don't get me wrong there is something there that needs to be fixed, When the motor is running it's turning way faster than when just spinning off the starter, When it's running there might be enough pressure to better seat the valve or valve seat. Whatever is wrong with the motor isn't gonna fix itself.
 
Thanks for the responses, this is a great forum! A little more history on the engine may help. When I bought the engine it had good cyl. compression. After pulling it off another boat and mounting it on mine, we fired it up to find that it had a valve stuck. I took the boat to an A/C mech. who did a leak down test. The readings were 0. 30, 40, 45, 58, and 62. The cyl. that registered the 0 was pulled and a valve job done. The valves in that cylinder were ground excessively and it had really fine cracks around the edge of it whitch caused it to stick. I'm thinking that may be the case with the other cylinders. The A/C mechanic suggested I run the engine for 50 Hrs. and monitor oil consumption and oil pressure, due to the fact that it only had 90 hrs. on it in the last 5 yrs. since last top end job. He said there is a possibility that the rings may never had seated properly do to the low hrs. over an extended time period. But now that I have a valve sticking again I think its time to find a good A/B mechanic before I swallow a valve or do more damage. Again, thanks for all the great responses.

P.S. Hey Thunder, you are building an engine for my little bro, (R.S. from Vero). He's chomping at the bit, so you can see why it's imperative I get this thing right, can't have the kid brother smoking me! :twisted: :twisted:
 
Thanks for the responses, this is a great forum! A little more history on the engine may help. When I bought the engine it had good cyl. compression. After pulling it off another boat and mounting it on mine, we fired it up to find that it had a valve stuck. I took the boat to an A/C mech. who did a leak down test. The readings were 0. 30, 40, 45, 58, and 62. The cyl. that registered the 0 was pulled and a valve job done. The valves in that cylinder were ground excessively and it had really fine cracks around the edge of it whitch caused it to stick. I'm thinking that may be the case with the other cylinders. The A/C mechanic suggested I run the engine for 50 Hrs. and monitor oil consumption and oil pressure, due to the fact that it only had 90 hrs. on it in the last 5 yrs. since last top end job. He said there is a possibility that the rings may never had seated properly do to the low hrs. over an extended time period. But now that I have a valve sticking again I think its time to find a good A/B mechanic before I swallow a valve or do more damage. Again, thanks for all the great responses.

P.S. Hey Thunder, you are building an engine for my little bro, (R.S. from Vero). He's chomping at the bit, so you can see why it's imperative I get this thing right, can't have the kid brother smoking me! :twisted: :twisted:
 
I'm not sure what those numbers represent or how the test was conducted. I measure all the cylinders over 80 psi. I had one cylinder that had a valve stuck wide open and I still got almost 20 psi on the test. The only time I saw zero was on one with a hole in the piston.

On my aircraft I'm good to go as long as I get 60 over 80 on the test. Personally if I see anything less than 70 over 80 I replace the cylinder. Even on the airboat I wouldn't go less than 60. Those numbers you gave show that all of the cylinders are bad. In fact they are so low I can't believe the engine runs. Since it runs, my guess is the test wasn't done correctly. Have an aircraft person give you a hand and try it again.
 
My suggestion is while you have ALL of the cylinders off, do a detailed inspection of the cam.

jim
 
a compression test is when you get a certain psi while spinning the motor over. a leakdown test is down by pressurizing the cylinder and measuring the percentage of leakdown over a period of time. they are not the same. The compression test will tell you how compression is there, the leakdown test will tell you where the loss in compression is going via the rings, valves, or head gasket.
 
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