Intolerant1

In anticipation of new heads, cam and exhaust upgrades this winter, I went ahead and did a compression test last night. here are the numbers:

#1 - 140
#2 - 140
#3 - 140
#4 - 135
#5 - 135
#6 - 130
#7 - 140
#8 - 145

Manual states cranking compression should be 125. Im thinking 10 years of carbon buildup is raising number.

Not bad for a 10 year old motor but here is the part I dont understand. The engine was tested cold with the air cleaner off and throttle wide open. I cranked the engine until the needle stopped moving, a few compression strokes. After checking the gauge on each cylinder, I would go back and crank it one more time to make sure I had it right. On cylinders 1 and 8, the gauge would jump to 160. This only happened on those two cylinders and only if I waited at least 15 seconds before cranking it again. I do have some oil on the spark plug threads. Im hoping this is just valve seals which Im sure are shot on this old of a motor. Could it be that on those two cylinders that oil is seeping in during the 15 second pause and making the compression higher when I crank it again?

Ill do it again on a warm motor and do a leakdown as well when I get the chance. Any thoughts on this appreciated.

stevejohns

Those look very similar to my 10 year old 502. I cant explain the jumps in #s tho. sj

Iggy

From what I read the cranking compression on a pump gas motor should be around 160-200 psi. Probably talking about a freshly rebuilt or new assembly.
For those that don't have access to a leak-down tester there's an old trick to see if the rings or valves are causing a loss of compression. First pull all the plugs, take a reading on each cylinder then squirt a small amount of oil in each cylinder, crank it over a bit to distribute the oil and recheck the compression. If you get no change in the readings, bad valves. Just the opposite, if the readings spike up then the rings are worn.

Cord

I had simular compression numbers on a engine that had 60% leakage on one cylinder. If you really want to know the health of the engine, do a leak down test.

patrick22309

Here is a link to make your own tester.
http://www.650motorcycles.com/LeakDownTester.html

Intolerant1

Thanx for the link. I have a leakdown tester. Been drag racing bikes for many years. I need to run some definitive tests before going ahead with cam and heads. With any luck, there will not be a need to pull the motor.

Still hoping somebody has a thought on the 140 - 160 jump.

blackhawk

When you do a leakdown test, how long do you need to wait to see how much pressure you will lose?

Intolerant1

Leakdown will show you immediatly. You control the pressure going in with a regulator shown on one dial. The amout the cylinder holds is shown on another dial.

stevejohns

160-200 on a stk 502... I dont think so.

Intolerant1

I think that would put your compression numbers around 12:1.