low compression head gasket?

bquick8

Seaman
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Sep 7, 2002
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what is the purpose of these,to boost compression on on a cylinder?They must be thinner than the stock gasket to decrease the volume of air/fuel being compressed,right?I've got a v4 crosser with starboard bank compression readings of 90(#1) and 97(#2) and am willing to try anything other than a rebuild in the middle of fishing season.Any Ideas on potential gain in psi possible?
 

Dennisanoka

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Jun 10, 2007
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Re: low compression head gasket?

Actually outboards are low compression by nature, therefore the gaskets are referred to as low compression.

So- please test the other side and report back the readings for all 4. Why do you suspect you are having a problem? What led you to run the test to begin with?

At this stage 90 psi is not great but it doesn't necessarily require a re-build either. Your other readings may indicate the next step.
 

Dhadley

Supreme Mariner
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Feb 4, 2001
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16,978
Re: low compression head gasket?

When you say "bank readings" we'll assume you mean both starboard cylinders are 90 and both port cylinders are 97. I wouldn't be too concerned about that at the moment. A good decarb job may be in order but if it idles good then I doubt you have any real issues.
 

wilde1j

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Apr 15, 2002
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5,964
Re: low compression head gasket?

Low compression head gasket is thicker and reduces not increases compression and is used for poor quality fuels that you might get in Central or south America.
 

Dennisanoka

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Re: low compression head gasket?

Wow-very interesting. I'll add this to the long list of things I thought I knew, but didn't-but do now. Thank you-
 

bquick8

Seaman
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Sep 7, 2002
Messages
59
Re: low compression head gasket?

sorry guys,the details are as follows: motor is a 1991 88 spl(j88msleim).Performed a compression test and results are-top port110,bp107,ts90,bs97.These readings haven't changed since I bought the boat 6 years ago.While idling on muffs,when I pull the plug wires one at a time,the top starboard(#1)cyl barely makes a noticeable drop in rpm.Checked the coils,spark tested ,just rebuilt the carbs(clean as a whistle).The running symptoms have been a slight miss on cold start(is there such a thing in Hawaii?)trolls smooth for 3 hrs.then when I throttle down,it starts to miss then sounds like it's running on 3.It'll restart a couple of times,but won't idle and stalls in gear.This motor also seems to be consuming ALOT more fuel lately ,to the tune of 7+gph at trolling speed(8k).One thing I do know that needs replacing is the rectifier.Running on a fully charged brand new marine battery.Any ideas? Thanks for all your help!
 

Benny1963

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Sep 17, 2006
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1,476
Re: low compression head gasket?

may want to spark check all cyls with tester should be blue and jump 7/16 gap or you can hook timeing light to cyls one at a time and look at lite with engine running you will be able to see what cyl is misfireing by cosistency of strobe
 

Dennisanoka

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Jun 10, 2007
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Re: low compression head gasket?

"trolls smooth for 3 hrs.then when I throttle down it starts to miss"

Just for clarification when you say it trolls smooth for 3 hrs. what's your trolling RPM? I have basically the same engine and I troll at idle or maybe 1000rpm. I know we are fishing for different kinds of fish but I am curious.

Dhadley suggested a good decarb job. If you do a search for decarb you will find step by step procedure using Seafoam. There a couple versions in my opinion in your case I would tend to like the one where you use a separate fuel container filled with 3/4 gal "mixed" gas and 1 can Seafoam and combine that with Seafoam sprayed directly into cylinders after removing the spark plugs and then spraying it directly into the carb throats while running. You'll find the details when you do the search.

I would definitely do this before pulling the heads off. It has saved alot of $ and time in the past.
 
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bquick8

Seaman
Joined
Sep 7, 2002
Messages
59
Re: low compression head gasket?

I did do a decarb about 3 trips back,I'd say10 hours running time since.I don't really know the rpm while trolling since my tach isn't working (that'll be another post someday!)but I'm clipping along at least 8 knots.
 

Dennisanoka

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Jun 10, 2007
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252
Re: low compression head gasket?

In that case my next step would be to compare the spark plugs to see if the ts90 cylinders' plug does look fouled which to me would indicate that it is not firing either at all or at low speed. If that proves out to be the case I'd go back to Benny1963's suggestion and check the spark. If the spark proves out to be acceptable I would begin to think it is an issue with compression on that TS cylinder.
 

bquick8

Seaman
Joined
Sep 7, 2002
Messages
59
Re: low compression head gasket?

I don't believe the defective rectifier would casue it to sneeze at low rpm ,right?
 
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