good compression

rogerrush

Cadet
Joined
Aug 17, 2008
Messages
23
I have a merc inline 6 about an 85 did a compression test it read 90 to 100 is that to low.
 

sschefer

Rear Admiral
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Nov 13, 2008
Messages
4,530
Re: good compression

If it runs good then run it until it won't run no more and then rebuild it. If it's not running, rebuild it, it's too low.

Not too technical, not over zealous, just good ole common sense.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,275
Re: good compression

Roger, If all the cylinders are within 10-15% of each other, the motor is fine, regardless of what your gauge says...
 

rogerrush

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Aug 17, 2008
Messages
23
Re: good compression

My motor gets up to about 3700 bogs and dies . Started about 1200 and died but, rebuilt carbs got to about 2200 put new switch boxes now 3700 and dies????other ten rebulding the powerhead anything I can do to bring up compression
 

sschefer

Rear Admiral
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Nov 13, 2008
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4,530
Re: good compression

How long have you had the motor and when was the last time you decarb'd it. A SeaFoam decarb might clear some of the carbon out of the ring lands and give it a little more life.

The compression is low. These engines like to be in the 130-135 psi range. You're off more than I would like to see in a motor. The idea of even within 10% of each other is fine up to a point but if you've lost 30lbs it's going to cause a pretty significant power loss.
 

asm_

Petty Officer 1st Class
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Apr 11, 2010
Messages
245
Re: good compression

Unless the compression gauge you are using has been calibrated. I wouldn't take the 90psi value at its face value. All it is saying is all cylinders have about the same compression.
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: good compression

IF...........the compression was low it would only LIMIT power output, it would NOT die.

You still have a fuel or ignition problem that needs to be diagnosed and treated.

Have you done a spark test with a spark tester set to a 7/16 gap?

Has the fuel pump been rebuilt?
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: good compression

Your compression is fine.

The probability that 6 cylinders would fail to the same degree at the same time is zero.
 

rogerrush

Cadet
Joined
Aug 17, 2008
Messages
23
Re: good compression

The carbs have been rebuilt once this year by me when I first took it out. That did not help then I put two cans of seafoam in the tank ran it for about 3hrs before I took it out. only helped that it would run up to about 2200 RPMs then die. took it to a marine machanic he rebuilt them himself and fould that they did have some clogging.After he rebuilt them now it will get to about 3700 then die. then fual pump was new last year.Will the diaphamin the pump streitch and not pump right??
 

sschefer

Rear Admiral
Joined
Nov 13, 2008
Messages
4,530
Re: good compression

If the compression is low because of normal wear and tear like it sounds then it's likely that your also not building adequate crankcase pressure. This will result in a weak pulse signal to the fuel pump and you'll simply not have enough fuel to get past the point your at now.

You can do a leak down test to check this as well as a primary compression test of the crankcase to see if this is actually the cause. If it's not, then you can eliminate that as the problem and move on with the trouble shooting. I'd highly recommend this as the next step. The procedures are in the merc service manual or you can take it to a good marine mechanic and have it done.
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: good compression

Two test to conduct while running at 3700

Squeeze the primer bulb which will 'assist' the fuel pump. If the fuel pump is limiting fuel flow then your hand should make a difference.

'Bump' the Choke to artificially richen the fuel mixture. If the fuel system is limited then adding fuel should make an improvement.

These test should prove if your problem is fuel related.

What is the WOT Ignition Timing set at?

Take along a timing light and verify WOT Timing.

Also, check that the timing light FUNCTION at WOT on EVERY CYL.

A weakening stator can limit WOT RPM as it may not be generating adequate voltage to fire the ignition at that load, yet seem to run just fine at lighter throttle settings.

Attach a DVA Meter to the stator wires and test run at speed, observe voltage raising from idle to your max RPM. What is it?
 

sschefer

Rear Admiral
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Nov 13, 2008
Messages
4,530
Re: good compression

Good point Charlie, I was concentrating on the effects of low compression but a stator could also be at the root of the problem. He may just be correcting other problems as he goes and discovering new ones along the way.
 
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