View Full Version : 351W Compression?

07-09-2008, 10:19 PM
What is the correct compression readings for a Pleasurecraft 351W in a 79 19 Skier? I was told to do a leak down test if possible. Excuse my ignorance, but what exactly is that and how is it done?

07-10-2008, 01:03 AM
Actual compression numbers depend on SO many things, so the hard numbers aren't as important as the consistency between cylinders; you don't want to see more than 10% variation. Make sure to disconnect your lanyard so you have no spark, remove all the plugs at the same time, and run the test w/the throttle open. Crank the same number of revolutions for each cylinder (at least 5 IMO). Any "number" above 100 lbs is fine (bet that motor will turn about 150 psi) but again, it's the consistency that is really important.

The leak down test is a good test, but not necessary if compression tests turn good results. Generally, I run a leak down test when:
1. One cylinder's compression is out of whack w/the others
2. I have one cylinder that is clearly dead, and I have spark and fuel.

Anyway, the point of the leak down is to identify where the leakage is occurring...and how much. Since I don't own a proper leak down gauge set, I do what I call, a "poor man's leak down test". I take my compression tester, and remove the schrader valve from the tube. Screw the tube into the first cylinder and get that cylinder to TDC and charge teh cylinder w/about 20-30 PSI of regulated air. If you have an issue w/cylinder sealing, you will promptly here air leaking. The sound will come from one of four places, and each of those places tells you where your problem is (if you have one). So go around and list for air flowing:
Carburetor -leaking intake valve
Exhaust -leaking exhaust valve
Oil fill -Bad rings/worn cylinder/hole in piston(!)
Radiator -Blown head gasket (in the boat, you'd have to pull a coolant hose and listen for bubbling water)

That's it. Pretty simple and very clear results. Again, unless you have the gauge set that can measure actual leak down percentages, I'd just do the basic compression test, the move forward based on those results.

07-10-2008, 07:39 AM
the hard numbers aren't as important as the consistency between cylindersPerfect advice.

Only move on to a leakdown if you find a large variation between cylinders.

07-10-2008, 09:51 AM
Thanks guys. I appreciate the help.