How much leak down can a motor have and still run good?

[nelsoncat]

I did a compression and leak down test aon a good running motor and got some shocking numbers. Compression all within 2 to4 psi but all around 1oopsi. leak down the best cyl was 12% and there were 2 over 50% on the BALT how can it even run or did I do something wrong. All were done cold.
Craig

[Jay Smith]

Craig,

I tear down and rering race motors at 6-8% and will allow a lake motor to go to 10% before we start thinking about a rering job !Normal "healthy" motors will have a leakdown in the range of 4-6%.........
Sometimes with a bad leakdown number the motor will still
"run" but will allow allot of the fuel vapor and subsiquent power be lost by reason of blow by as the ring cannot contain the compression under a load !

Sounds like yours is WAY in need of a rebuild if those numbers are accurate!

Good luck !

[Balzy]

for port and starboard like the instructions tell you? I have found some motors to give you high readings if not done correctly. Were the bad #'s all off the same side?

You need to turn it over counter clockwise for the starboard side and clockwise for the port side. Damn, I hope I got that right. Check your instruction. The ones on my website don't explain so I need to update that I guess.

Is this motor gonna abe at BTTW? We can redo the test at my place one evening if you want.

[baja200merk]

compression from what i remember is 73,57,48(sbrd) 42,10,68(port) still runs 6100 rpm but idles like chit!

[quickonstep]

talk about low compression lol i thought mine was bad :)

now, can a merc run with those kinda compression numbers
hahahaha

just razzin you merc guys

Jesse

[nelsoncat]

As good as this motor (78 1750 merc)runs I was quite shocked to see these readings!!!!!!!!
Its on the STV now but tonight I'm going to put Brads (Procomp)
old 2.4 on. I'm just waiting on a few parts that should be here today. I was trying to do it from memory but I'll redo just for the hell of it. Mike, do you have the inline adapters. I need one if you think of it throw one in the Blazer next week.
Thanks

Craig

[Balzy]

The reason you need to rotate CCW (Counter Clockwise) on the starboard side is so the crank is swinging up from the center of the motor. With air pressure in the cylinder pushing down on the piston the rod will tend to push the piston tword the outside of the motor because of the angle of the rod pushing on it. The outside of the piston is where the end gap of the rings are. The piston pushing against the outside of the bore will close and seal up the end gap some. You are really not trying to read leaks at the end gaps. You want to read air leaking past the ring itself. You do the port side CW (Clockwise) for the same reason.

What I do is put 10 to 20 PSI on the tester, turn the motor in the correct rotation with a long ratchet on the flywheel nut until I feel it getting close to TDC. Then I increase the pressure to 100PSI. You will be able to feel as you get close to TDC, it gets real eassy to hold. If you go too far the motor will turn over TDC and spin. That's why I use a ratchet, NOT A BREAKER BAR. The ratchet can spin free in that direction. A breaker bar will want to tear your arm off.

Craig, I have 90 degree adapters in stock. Are you gonna be there Saturday? We will be going to the house anyway after the presentation.

[ProComp]

That motor loved a 29 SRX on my ProComp. If someone has one give it a try. A 28 ET or Mazco might work well too. Remember that motor has been sitting for a long time. I'd put some oil in the cylinders and crank it over by hand slowly before firing it up.
Good luck. Hope it runs as well for you as it did for me. Make sure there is oil in that gearcase too as I can't remember if MERCMAN shipped it full or not to Kevin.

Run good fuel too as the heads are cut 60 thou from stock.
I always ran 32:1 Merc Premium Plus.

Good luck making it to BTTW.

ProComp

[thumper]

Dunno what you guys are worried about.
Go to my compression numbers and dont worry about it uness there's a rod sticking out the side and even then, it'll getcha home :D :D :D

http://forums.screamandfly.com/forums/showthread.php?threadid=61543&highlight=thumper

[nelsoncat]

I've had a case of S@F withdrawl. The phone line had a lightning strike and fried my modem. Just slapped one in from Radio Shack 1:10 AM Sunday morning. Actually I just got in from swapping motors, not quite done yet. Mike, I will be at BTTW Thursday and Friday. Saturday my daughter is in a 3 on 3 basketball tournament that I must attend in exchange for my 2 days away in paradise(BTTW) with my boat friends. I have a few boat goodies to donate for auction. T man what were your numbers, I remember seeing them somewhere!!!

Later

Craig

[Balzy]

that I have your 90 degree adapter the glove compartment for ya. When things at BTTW get wound up I usually don't remeber very well. Guess maybe it's a good thing we doing this for AD, ha? LOL