Mercruiser - why wont it stay running?

by Pamela
(Billings Montana)

Help! I bought a 1974 mericruiser 4cyl. inboard boat and it runs great until you are on the water for about thirty minutes. It then starts cutting out and eventually wont start. we have to get towed back to shore. It has new wires, plugs, points, and oil change. Seems like it is when there is a load on the motor it bogs out . could this be a head gasket problem? Thanks in advance for any advice!

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Jul 20, 2016
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Check ignition sensor
by: Liss

Had a similar problem after replacing plugs,rotors,coil,ignition switch, water&oil separator it ended up being the ignition sensor or so far it appears that way.

Jun 25, 2014
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Four-cylinder MerCruiser won't run on the lake
by: Bruce

Check your fuel filter and see if it is plugged up. Try a different set of plugs if you haven't already but only if the original plugs are starting to look dirty namely black soot. Check that the gaps on the plugs are all set correctly and that you have all four plugs with the same heat range going by the number on your plug. Check with a marina to make sure that those are the right plugs for that motor.
It is easy for a motor to run out of the water because there is no load which would demand that the motor burn more gas.
The plugs may be getting fouled to the point where they can't handle the gas they are expected to burn when running the boat on the lake.
Check your fuel lines make sure all the hose clamps are tight and that there are no holes in them because if there are holes you will be sucking air which will mean that the motor will be getting Less gas when it is demanding more when running with a load on the motor. Don't tighten the hose clamps so tight that you crush the hose that will be defeating the purpose of the clamp and eventually will create a leak.
The worst problem may be low compression. The engine is demanding to burn more gas but it doesn't have the compression to get the gas burnt. Have your marina repairman do a compression check. Or get person knowledgeable on how to do a compression check and it would be a lot cheaper for you to do it. If you don't know how you can find out on the Internet and they will tell you.
Hope I have helped you to some degree.

Jun 25, 2014
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running
by: Don

The only way to check to see if's a head gasket is bad is to pull a compression check on each cylinder,and it should be hot(up to temperature)when you check it ,and if one is low well it could be a valve,and if they are all low the motor probably need rebuilt.
Sorry Don



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