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

Rollin Thunder · #7 07-26-2010, 10:09 PM

the max rpm at full throttle is about 41-4300 rpm depending on trim. Is this where it should be? I always thought it was normal, But honestly have never had anything to compare it to. But no it ran fine otherwise. I will have the guy doing the compression test re align the alternator. I dropped the boat for compression this afternoon after I looked at the alignment of the belt and it seemed about a hair off, but enough to cause some issues. He works on my other boat and seems to do great work. I have good faith he will get the rest done. The guy interested in it has some interesting plans for it. I will recommend this site to him if he does go through with buying it. He owns a fiberglass company and wants to redo the cockpit and cabin, then add a higher windshield and hard top with side curtains. HE said he wants a boat in excellent mechanical condition so he can put his money in to cosmetic. Which My boat IMO fits the bill quite well being I have replaced every mechanical part on this boat from the steering all the way to the cooling. So im hoping and praying it goes through. Thanks again for the help.

THEFERMANATOR · #8 07-26-2010, 11:32 PM

Quote:

Originally Posted by step up here

jasper has a very good name in automotive and marine

JASPER has a horrible reputation in the industry if your on the side of it I am. The only good thing they have going is there warranty. I couldn't tell you how many of there engines and trans that I have heard of being DOA at the dealer, then the dealer gets heat from the customer for it taking so long whne it is actually JASPERS piss poor work that was the problem. I've seen inside some of there remans, and I sure wasn't impressed. Then again I am known as a bit of a perfectionist(spent most of the day doing head gaskets on a 25HP KOHLER because I didn't think the block was flat enough, so I got out a flat block and some 320 and went to work resurfacing the block on the mower).

Rollin Thunder · #9 07-27-2010, 07:51 PM

Quote:

Originally Posted by THEFERMANATOR

JASPER has a horrible reputation in the industry if your on the side of it I am. The only good thing they have going is there warranty. I couldn't tell you how many of there engines and trans that I have heard of being DOA at the dealer, then the dealer gets heat from the customer for it taking so long whne it is actually JASPERS piss poor work that was the problem. I've seen inside some of there remans, and I sure wasn't impressed. Then again I am known as a bit of a perfectionist(spent most of the day doing head gaskets on a 25HP KOHLER because I didn't think the block was flat enough, so I got out a flat block and some 320 and went to work resurfacing the block on the mower).

I agree, but The boat needed a new engine and I wasn't willing to pony up 7-8k for a new crate motor when I new I was going to sell it. Took 3 engines from them to get it right.\\

Hey just curious. How did you get your tahoe to burn rubber like that. I have an 07 and for the life of me (even if I turn the TC off) She wont do it. I have been told by my buddy its all the torque management GM puts on their trannies. Neat picture though!

RWilson2526 · #10 07-27-2010, 08:18 PM

Quote:

Originally Posted by Rollin Thunder

Hey just curious. How did you get your tahoe to burn rubber like that. I have an 07 and for the life of me (even if I turn the TC off) She wont do it. I have been told by my buddy its all the torque management GM puts on their trannies. Neat picture though!

Oh that aint no regular truck...I think he added a flux capacitor!