compression test, and couple other questions...

Rollin Thunder · #1 07-25-2010, 08:34 PM

Hi, I have somebody intrested in my boat and he would like a compression test done. I have a rebuilt by jasper 3.7 liter 4 cylinder mercruser engine. it was rebuilt in 06. What numbers should I be looking out for? also there was some oil leaking around the back of the valve cover. Its not much, but something he questioned, is that a gasket most likely, or am I looking at something serious? One other little thing is the alternator belt seems to like to pop off if we go over 3500 rpm, its not serious since you can just pop it back on, but something I'm confused as to how to fix, it seems to be at the right tension, and the bracket and all seem tight, any reason this would happen? Im worried as I need this boat to be sold yesterday, and dont want to scare a potentially serious buyer off. We took it out for a sea test Friday and spent about 2 hours on the water with him just driving around, and the boat ran very well, everything working as intended, Just some problems that seem more preventive than actually something wrong, But I have been very wrong before. Thanks for the help guys, I REALLY appreciate it.

spareparts · #2 07-25-2010, 09:25 PM

the actual compresion numbers will vary depending on the guage used. I look for consistancy between the cylinders more so than the actual number itself. 10% difference from the high cylinder to the low cylinder is the normal max allowed. you will probably find numbers around the 135 psi range. The oil leak is probably just a gasket issue. Teh altenator belt issue could be more of a problem. mercruiser didn't use a belt driven altenator on those engines, they were built int eh ba;lancer on teh front of the engine, they gave a lot of problems, so the aftermarket came up with various solutions to fix it by mounting a belt driven altenator on teh engine. You need to look real close to the belt/pulley aligment, aslo look for run out on the crank pulley, check to make sure you have a correct belt for your application, gennerally autmotive belts won't stay on a marine application

Destroyer · #3 07-25-2010, 11:18 PM

As far as the belt issue goes, take the edge of a metal yardstick, and put it across the entire flat face of the crankshaft pulley and the altenator pully. The two faces should be in line with each other with no gap present between the yardstick or the pulleys. If there is a gap of more than .010 (ten thousandths of an inch) the pulleys are considered to be out of line with each other and need to be aligned.

Also, like Spareparts said, from a side view, CAREFULLY (don't get too close) look closely at both pulleys while the engine is running. Does the front of the pulley look like a solid line or is it a little blurry? If it's blurry that means the shaft it's rotating on is bent or the hole in the pulley is not 100% perpendicular to the pulley.

spareparts · #4 07-26-2010, 08:49 AM

I remember from an altenator kit I installed years ago, that you had to set the run out on the drive pulley, then drill and pin it in place to the balancer once you got it centered

Rollin Thunder · #5 07-26-2010, 12:42 PM

ok thanks for the help guys. I will go out and check everything out. Then take it for its compression test.

step up here · #6 07-26-2010, 06:18 PM

your compression test should be good since the boat ran good did not over heat did not stall and reached almost max rpm . before the belt launched. alternator could be realigned without a major problem. rebuilt engines are only as good as the mechanic. jasper has a very good name in automotive and marine . hope you make out well. along with the new owner