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Post by RichH on Sept 5, 2014 15:22:06 GMT -5
Dan, what PSI do you use for your leak down test ? Way back in he day when I was tracking stuff on my race motor( car) I used 95psi TDC, mid bore, and bottom bore. Would just like to compare apples to apples if and when the topic comes up.
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Post by Simon on Sept 5, 2014 16:34:59 GMT -5
And how do you hold the crank and stop it spinning for the mid bore measurement ?
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Post by RichH on Sept 5, 2014 22:41:38 GMT -5
In a car is was not much of problem because the other cylinders were always fighting the rotation. With a single cylinder I would probably use a strap wrench on the flywheel. Or an old clutch hub with the key and some vice grips
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Post by norcalacr on Sept 8, 2014 8:48:45 GMT -5
I've had success with a box end wrench on the clutch bolt.
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Post by Dan Pell on Sept 8, 2014 14:51:56 GMT -5
Sorry guys, away from the screen for the weekend. Set the pressure regulator at 40-60 pounds, that's plenty of air to establish where leakage might be. First, loosen the rocker arms (remove from push rods) so the easy-spin doesn't influence the exhaust valve. As for holding the piston, a lesser issue with the rockers off. Do not need to hold the crank, unless you want to see if there's a measurable amount of loss past the rings as the piston travels through it's stroke.
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Post by RichH on Sept 8, 2014 16:14:41 GMT -5
Thank You Dan.
A couple more question I should have thought of;
Engine, warm or cold?
At what percentage leakage do you consider it a problem?
I am now at seven races on this motor and do not want to be caught flatfooted ( forgive the pun )
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Post by Dan Pell on Sept 8, 2014 23:57:06 GMT -5
Warm or cold, doesn't matter since valves & rings won't know the difference. You might see a slight improvement with a warm motor, but what we're trying to decipher, are the parts & pieces functioning at acceptable levels. What can help, is a small squirt can of oil, and shoot it in through the sparkplug hole. Do this AFTER the first leak-down test, it will help to better explain where you're losing air. The oil will help to seal the rings, and more accurately demonstrate valve leakage.
As for "an acceptable level" of leak-down, that's tougher. some say less than 10%. I'd first conclude...is it rings or valves. If rings are the reason for the loss, and you're seeing 4%, some would say that percentage is unacceptable, others, perfectly fine. After the initial testing, add oil, then test again. The oil does not influence valve seal (2-3 tablespoons can't reach the valves) so you get a true measurement with the oil as it relates to valve seal.
If it's valves, i want zero because it's so easy to remedy. Reality is, you might lose a little bit past the valves, and with a small hose from either the carb or exhaust pipe, to your ear, it's easy to hear air which one it is. Obviously, if one valve is leaking, you'll want to reseal them both.
Hope this helps!
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Post by RichH on Sept 15, 2014 22:51:50 GMT -5
Whoa!!!! Just did a leak down test tonight. 10+% at 50 psi Only a drop of maybe .5% with oil on the rings. Can feel and hear the air thru both valves on the head.
Guess I am changing the head before this weekend
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Post by Dan Pell on Sept 16, 2014 19:16:50 GMT -5
Hey Rich, I think we predicted that well before we did the valve job on that spare head??
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