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1988 Mercury Black Max Compression and Leakdown Readings. What now?

Jonas0815

New member
Hello All,

I have a 1988 Mercury Black Max 150, Serial Number B251261.
The engine is running, but it is rough at low RPM (below ~ 3000 RPM),it isrunning good at high RPM and I think it has full power at top end. The roughoperation in the low range is bothering me.

Lots done already: Carbs cleaned, link&sync, new coils, wires, plugs,stator, trigger, switchboxes, new crank seals, new cylinder head gasket, ...

When I had the heads off, I could see some scouring in the cylinder walls, butnot sure how serious it is and if that is the reason for running rough at lowRPM.

Here are the compression and leakdown readings.
Note higher leakage rate on cylinder 4-6, but compression is good on allcylinders.


Is it really worth to re-build the engine? Or is myproblem somewhere else?
 

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Thank you for the reply.
That is a good point, the blow-by affects the suction through the carbs.

Can somebody comment if the compression and leak-down readings I have posted support this?
I am looking for convincing advise if I should invest the time and money to rebuild the engine...
 
Rebuilt a V-6 that had compression readings well over 100 psi.---4 of 6 were scored.----Time for this motor to come apart to decide on the path forward.----Scoring indicates that rings may be jammed in the grooves and do not seal properly.----And there are those who say a leakdown test can be done on a 2 stroke , not sure why that is !
 
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What people forget is compression test is just that....if cylinder has enough compression to support combustion. It is not a good indication of mechanical condition as cylinders can be worn/damaged and still have good compression.
 
If there is leakage past rings during a compression test , then there will be more leakage to the crankcase under higher pressures when motor is running.----I see many folks say---" compression is over 100 but motor won't idle "---
 
That's because the crankcase air/ fuel mixture is polluted by exhaust gas. If one can get the motor started and in gear--and sometimes it's not possible--it will make respectable power at high rpm since the pollution is offset by a mass of fresh air.

Jeff
 
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