74 GT550 Compression #'s and good news...

[imquattro]

Well, it's been a busy morning w/ my students.
We pulled the motor out of the '74's frame (which we're not using -refer to this thread for details viewtopic.php?f=2&t=5351.)
I showed the students how to do a compression test.
Carbs, exhausts, and plugs removed:
We made slick use of the starter too.
The results were:
137psi(L)dry
140psi(C)dry
140psi(R)dry
I have no #'s to go by, but from my skimming the forum, am I correct to say these #'s are acceptable?

We then did a leak down test of each cylinder.
We blocked the intake & exhaust ports with rubber expanding freeze plugs.
(NAPA Part #600-4003 1"-1.125" for intake)
(NAPA Part #600-4007 1.5"-1.625" for exhaust)
Set the piston TDC and supplied shop air (regulated to 100psi by tester) using a Moroso #710-89600 leakdown tester.
Got worried for .10 second when I heard air escaping from the R cylinder when testing the L. Then rememered about the crossover hoses....
After carefully pinching the oil scavaging crossover hoses off, we found around 5% or less leakage across all cylinders! On a COLD engine.
So WOO HOOO! No crank rebuild for me!
Kids learned a lot of impromptu lessons today.
Quickly complaining that the 2 hr class was way toooo short.

[Suzukidave]

On the leak down test you posted that the air supply was regulated to 100psi .. i think it should only take like 6psi to do the test ?

[imquattro]

I guess that's the car side of my background testing.
100psi is an easy # to use to because any amount not being held by the cylinder is an easy % of that 100.
If I put in 100psi and the cyclinder holds 95psi = 5% leakage.

[Suzukidave]

yeah .. 6~8 psi is all thats really needed and the tested cylinder should hold the pressure for around 10 min without a major amount of pressure drop .. any more than that and you are really risking blowing out the seals .

[Suzukidave]

http://www.youtube.com/watch?v=zdq7NRcUIp8" onclick="window.open(this.href);return false;

[imquattro]

Sucky thing is, I went through the steps to replicate that test and found the center cylinder to leak into the right cylinder. Not much, leaked down over a minute or two, and when I did it w/ our smoke machine it was definite. Smoke out all the openings of the adjacent cylinder. Only one way (Center into Right) and not back, 'cause I tested that cylinder next (well, last in line that is, working left to right). Both outer cylinder's hold pressure.
As I'm thinking about this, why would they say that anything over 10psi would/could blow out the crank seals? The compression & combustion process FAR exceeds 10psi.
Also, I'm wondering why the compression #'s are so good?

[Suzukidave]

The primary case compression is different from the piston compression , the higher numbers you got are testing the rings and those numbers are created after the rings passes the exhaust port on the way up and those numbers are good . The primary compression just involves the pressure developed during the transfer of the gas / air mixtures being forced up the transfers as the piston travels down the cylinder and that remains fairly low so the crank and end seal arnt really subjected to that much pressure .

[tz375]

Dave is correct. You cannot check crankcase leaks on a 2 stroke at 100psi, but you can ruin the seals.

100PSI is perfect for 4 strokes as you mentioned. On a 2 stroke, primary compression is around 1.2:1 or a maximum pressure of say 18psi or a pressure differential across the seals of 3 PSI.

6 is the general rule of thumb for testing because high pressure has two effects. It just presses the seals hard against the shafts so they appear to seal and it tends to blow the seals.

If the pressure is leaking between cylinders at 6psi the seals are leaking and the crank needs to be rebuilt. With all due respect, if you are teaching students then is is important to be sure that you know what you are doing. Please check with other sources and then repeat the test with your students and use it as a teaching moment to talk about 2 stroke bottom ends versus 4 stroke top end pressure testing.

Do not use a 4 stroke leakdown tester. Just hand pressurize the bottom end to 6psi and see that it maintains that for 6 seconds.

If the cylinder you tested was fixed at TDC when you added that pressure via a plug hole, you have tested ring sealing and that's OK. What we need to do on a 2 stroke is to test the bottom end low pressure seals. That's where you use 6 psi.

In fact testing at TDC you don't need to seal anything else off because all it's doing is testing the rings, but it's not a good test on a 2 stroke.

[imquattro]

so thanks for the info.
A shop manual would help too....
This one is only good for so much....
http://www.3cyl.com/mraxl/gt/manuals/gt ... ntents.htm