Leak Down Test

[jumpjg]

I pulled the engine out of my Goat yesterday to take care of some issues - throttle linkage, belt tension, clean-up, ect. I was curious about the condition of the valves/rings because it doesn't seem to have a whole lot of compression. I have a leak-down tester so I thought I'd give it a try.

Fot those not familiar, a leak-down tester uses compressed air pumped into the combustion chamber to determine how well valves/rings are sealing. The engine is put at TDC on compression stroke, and pressurized air is introduced throught the spark plug hole. The tester has 2 pressure gages with an orifice between them and a pressure regulator. The first gage reads the air pressure supplied & the second reads the pressure in the cylinder. Generally 10-15% leakdown is considered ok, but any more and you need to look into the problem. The beauty with this type of test is that it allows you to locate where the leakage is by hearing the hiss at the intake or exhaust port (valves not sealing) or through the oil fill port (rings leaking).

My engine leak-down test showed 90/100 or 90%, which seems ok. The valves are tight and the leakage was past the rings.

Any feed back appreciated - thanks!

Joe in St Louis

 

[joshhoward]

Hey Jump


If you really like the engine, and dont want to have to pull it again at the end of the yaer to mess with it. Just rebuild it. Finish hone the block, New rings, lapp the valves. And your good to go! Got any pics of it?



Josh

 

[Yellowhand]

Excellent tip, Joe...thanks for the info on a really useful, inexpensive tool we can all benefit from.

How's the Goat lookin'?...going to start a photo album?

 

[jumpjg]

Yellowhand - I believe Harbor Freight now sells a leak-down tester kit for around $35 - $40. Here is a link to a do-it-yourself build article...

http://www.xs11.com/tips/misc/misc3.shtml



The Ol' Goat is looking pretty good, and I'm having some fun working on it. Spent some time yesterday working on the front & rear brakes, and I think they will probably work now. I had to elongate the engine mounting holes in the frame so I could put a little more slack in the drive belt - it's much better now & probably close to being right.

I don't have a digital camera, but I'll try to get my neighbor's boy to take some pics this week.

Thanks,

Joe in St Louis

Excellent tip, Joe...thanks for the info on a really useful, inexpensive tool we can all benefit from.

How's the Goat lookin'?...going to start a photo album?

 

[RuppParts]

All leak down tests need to be run with engine warm. Cylinder walls and pistons and rings need to be fully expanded to get best accuracy. it is not uncommon to see 10% leakage on a cold engine and 5% when warm due to expansion. If I have 3% that is too much in my hot rod engines. When dealing with single digit HP ratings it doesn't take much leakage to quickly hamper engine performance. 10% is no issue to worry about for the leisure bikes

 

[minidragbike]

If you tested the motor and it does good it's most likely something to do with the cylinder. At TDC the the cylinder can be sealed very well. I have seen this with a lot of briggs motors. Some come factory with a slight taper in the bore. Or you can even have a groove somewhere in the bore causing it to leak. A leak test mostly helps valves, intake, and head gaskets.

Stock motors don't fire at TDC they fire BTDC and usually around .250"+ in the hole depending on the motor.

 

[RuppParts]

mindrag......lay off the beer before replying!! LOL

You obviously got your decimal point wrong for timing....a 1/4"btdc??

a leak down test wont help anything...all it will do is tell you leakage.

You kinda missed the whole discussion....LOL...what you drinking Budweiser? and how many you had? LOL

 

[minidragbike]

Ruppparts what are you talking about? I only said 1/4" btdc to put the thought into a person head that doesn't know about firing points.

I maybe wasn't into to much detail. But other then that, that's all.

He said it don't seem to have that much compression. Normally when a motor fires it's not at TDC. It's anywhere from .100" -.250" in the hole depending on the stock ignition timing, and motor used.

Most motors leak before the motor comes up to TDC. It leaks on the way down, and usually really bad on the way up.(Which is probably what he's feeling)

Example: On the new race motor I'm building I have a custom flywheel, and custom stroker setup. So I have to do in the hole timing. The have to measure the timing in the hole to get the correct ignition timing. I will have mine set at .215" in the hole to start. And I might go as high as .300" when I'm done.

Most of the motors I checked don't leak to bad at TDC. But it's a good way to check the valves, intake, and head gasket. Most motors leak at the firing point, or lower into the bore.

 

[jumpjg]

Good discussion here-thanks for the input. Minidrag, your point about a scratch in the cylinder or taper certainly would support what I'm seeing with the weak compression. I might have to pull the head to take a look.

I do have quite a lot of experience with ignition timing, setting up degree wheels using a piston stop, and degree-ing in cams. But none of this experience is with lawn mower type engines. Certainly all this can be accomplished on them, but the data is not all that easy to come by. Shoot, I can't even find out exactly what motor I have because the model/serial #s are only stamped onto the shroud, and my shroud has been replaced with one from a vertical crankshaft model. That's the reason I joined this group - to get info. I've learned a whole lot so far & hope to learn more. Any recommendations for resources (books, web sites, etc) that would help me get up to speed are appreciated. Seem as if most of your knowledge comes from experience, which is the best teacher.

Thanks for your patience,

Joe in St Louis