View Full Version : how to do a leakdown?

26th May 2006, 18:31
I have a compression tester that is just a pressure guage, a hose, a spark plug adapter, a quick connect and a little schrader (tire) valve in the line...

could i run a leak down test, or home brew equivalent by removing the valve and just manually cranking it to TDC and seeing if it bleeds off?

if not, what does a pro-leakdown tester do differently? and what do they cost?

Moved On / My Own Choice
26th May 2006, 19:00
A leak down tester is a set of gauges used along with an air compressor.

You warm up the bike to normal operating temperature then you shut her down (just like a compression test). You set the cylinder to be tested at TDC so the valves are closed, then you thread the adaptor into the spark plug hole.

THEN you pressurize the cylinder with compressed air and watch the gauges while listening.

IF the pressure is bleeding off The gauges will read amout of leak down.

And you then feel/listen for where the air is leaking.

Into the engine - rings
Into the carb - intake valve
Into the pipes - exhaust valve

don't know how to jury rig this.

Testers aren't usually cheap enough to justify purchase for a one-time use, but hey, ya never know.

Ask around with local buds and check DECENT shops.


26th May 2006, 19:07
Aaron (ASWRacing) provides some good info in this thread:


26th May 2006, 19:11
I read some of your post from before. I won't say you don't have a valve problem. But I think I would get a set of the plug lights, that you put between your plug and plug wire. People use them for decor...
Anyway, if you watch the bike run in the dark you should be able to tell if there is a problem with the plug firing out of time? If one is only firing every other time it should be easy to spot.

Moved On / My Own Choice
26th May 2006, 19:21
GOTWA - good thread, how'd I miss that. Oh yeah, I didn't look...

26th May 2006, 19:27
GOTWA - good thread, how'd I miss that. Oh yeah, I didn't look...


It was a decent thread made into a good thread by those much more knowledgable than I. ;):)

26th May 2006, 20:01
Here's something that might help. All airplane mechanics have to do leakdown tests on engine annuals, so if there is a small airport nearby maybe one of them can help.

26th May 2006, 20:14
That makes sence about leakdown...
they presurize cyl and test for leaks, and place of leak

i cant do readings...
but i could do a homebrew leak test....
my compresion tester has a quick connect like on an air hose....
why cant i just hook it to my compressor....
pump it up and see if it holds air or if it comes blowing out exhaust or carb
which would tell me what valve is bad
it isnt accurate...
but it would tell me IF or not

i mean if i fill her with 100psi....

and i get nothing, or almost nothing out the exhaust or out the carb... then it proves my indy is full of it.

if i get a draft out both or either... then i know he is right.

at this point im not so concearned about How much... as i am about IF or NOT

what ya think?

26th May 2006, 20:35
I've used the "home brewed" test before. It works. Like you said, not accurate but we're not interested in how much but, where our leakage is. Go for it.

26th May 2006, 20:43
Just make sure you're at TDC on the compression stroke. I was at TDC on overlap once and it really freaked me out. Had air coming out both ends. My buddies got a good laugh out of it though.:roflblack

26th May 2006, 21:59
You will always get some leakage. How can you tell how much it is without measuring with the gauges? As aswracing (I think) said, a fresh, new motor might have around 6% leakdown, and acceptable is 12% leakdown. You can have 12% leakdown, and you'll hear it coming out all over the place. Will you jump to the conclusion then that it's leaking (wherever) because you heard it coming out?


27th May 2006, 02:11
i was told that you should have 0 leakage thru the valves, and some in the crank is normal. im just worried about valves.

27th May 2006, 15:31
On any motor that has been run, there will always be some leakage past the valves. Somebody correct me if they know different. That's been my experience.


27th May 2006, 16:27
I posted my way of doing it on your other post "just got back from indy"

27th May 2006, 16:58
I'm not sure about air cooled engines but with cars (yuck) at normal operating temps a dead hole that has less than 20% leak and ok compression is caused by ignition or air leak problems. A 4-gas and lab scope helps immensely. if leakage is 80% or more double check for TDC on compression stroke; both valves closed. High leakage intake valve=air out of carb. Exh valve=air out pipes. Rings=air out oil filler. Matt ASE L1

28th May 2006, 03:30
In Canada you can get a leak down tester at Princess Auto for something like $40.00. Works real good. Of course you can get ones that cost hugely more but don't let anyone tell you the cheap ones do not work. They do work. Good for the home mechanic. Look around at various auto supply shops.


Moved On / My Own Choice
28th May 2006, 03:47
Yo Mick!

For the record, IF you ever make it down to the U.S., please feel free to come this way down to Philly, you got a place to stay or at least to stop over!