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View Full Version : Compression..........JSRE? Anyone?



04-19-2001, 06:42 PM
Buddy of mine is trying to up the performance on his outboard. (isn't everyone?) Dont laugh now, but he's got a '1998 Force 40 by Mercury' on an aluminum fishing rig. Two cylinder, power trim. He's already bought the carbon fiber reeds, the 50 hp carb, and it runs great. Gained a few hundred rpm. I told him to check the compression and send the head to JSRE, and when he checked it he said it has 145-150 lbs of compression! I know factory V6 mercs have about 125 psi, so I told him his gauge is bad. We took it out yesterday and ran around a bunch, then tested his motor with my newer tester, and I got the same reading within 2 psi. What's the deal with that? He can't have the head shaved if he's already way up there. HELP!

Jay Smith
04-19-2001, 09:42 PM
JW
140 to 150 is alot of compression for a stock motor are you sure the heads haven't been skinned?
Jay @ JSRE

04-20-2001, 05:11 AM
Pretty sure they haven't been shaved, he bought it used (but it was barely used) and the head bolts are still un-marred by a wrench. Sounds like you are as suprised as I am with those psi numbers. The motor was on a pontoon boat before he got it. Is it possible low-performance motors like that use high compression but less timing advance??

Greg Moss
04-20-2001, 06:36 AM
Those motors are not much different than the old Chrysler. I get some of them in my shop to work on every once in a while. They are a crossflow motor the compression on them will give a high reading, some of them will read as high as 170lbs. Cuttting the heads will not do any good. those motors worked good on pontoons I don't know about them for performance. Greg

04-20-2001, 07:18 AM
In 1996 your motor started being produced as a looper.Your fourty isn't going to yeild more than about 50 hp with out major mods and loss of reliability/useability. With the 50 carbs you can get away with about .020 off the head then you must run 93 octane. Any more than that and your asking for burnt pistons. Forged wiesco pistons are about $70.00 each including the rings. You can bore up to .040 over. They are the same as the 50 looper. If reliability is not a factor shave the head, .060 bore it .030, pop in a set o wiesco's, bore the carb .150 up your jets by 60 % disconect the rev limiter, raise the exhaust port .200, buy a drum of alcohol,and a speed master lowerunit, and at 7200 rpm you should see about 75-80 hp. Then find a good used 70 hp 2 cyl looper to put on the fishing boat for half the cost of all these mods.

04-20-2001, 04:09 PM
Thanks everyone! I'm thinking he'd better stay with the stock head and play around some more with the prop. He does want to keep it as reliable as when it was a stock 40. His fishing boat is rated for a 75, and I cant talk him into selling the Force and getting a good used 75. I double checked the year, and it's a 1997, so going by that it must be a looper. Thanks again!!

RichS
04-20-2001, 05:02 PM
Not to say that I'm getting old!!!!!!!!!!!!!! But. If ya wana work on it work on it, if ya wana run it run it! Give me another beer! Anything ya do to the MOTOR part is gona cost ya, if ya can get it with setup (without revs) it MAY not. But what do I know! (I like that one)

04-20-2001, 06:13 PM
Since we are only a few minutes away, and I'm having such a sucky time finding a perfect Grand Sport or V-King for low $$$, tell me what you got for sale! Any go-fast toys turn-key you trying to unload??

ODDD1
04-21-2001, 12:03 AM
the 2 cyl force is a crossflow motor.....140 to 150 psi is normal and healthy.....dont get too crazy with the rpm on these motors...they dont flow air well enuf to make alot of power and they like to break cranks across #1 rod journal if you spin em too hard......

MtDoraGary
09-12-2006, 01:44 PM
JW
140 to 150 is alot of compression for a stock motor are you sure the heads haven't been skinned?
Jay @ JSRENot THAT unusual. My Chrysler 60 would regularly read 175 lbs per cylinder.

Jay Smith
09-12-2006, 05:32 PM
Shows ya how versed I am on Chryslers HUH ?

Scream And Fly
09-12-2006, 05:41 PM
What's a Chrysler?:confused:

They made outboards? ;)

MtDoraGary
09-12-2006, 05:54 PM
What's a Chrysler?:confused:

They made outboards? ;)
Good ones! Even racers, albeit too late...