1985 Mercury 50 HP 4 cylinder runs very boggy

Kalebe1023

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Nov 16, 2020
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Alright, so I bought this motor not running and replaced most every fuel component. I cleaned the carbs, replaced the fuel lines going to the carbs, replaced the tank fuel line and primer bulb. The inside of the tank is clean. I also replaced the in line fuel filter. This made it idle. However, it still wouldn't run good. Every once in a while it would open up completely and scare me to death. But, it would usually just die once I got up to speed. I found one of the cylinders had very little spark and I traced it back to a rotten wire on the trigger. I replaced the trigger and put it back together. I put muffs on it and started it up. Later, I took it out on the water and it ran great. I drove it for probably 5 minutes with no issues. Yesterday, I took it out again. It ran great down the lake for 2 minutes. I slowed down and turned it off to go look at some public land. Then, I started it back up and ran down the lake again after about 2 minutes it started being boggy again. It slowed down a lot. I thought I lost compression on a cylinder because I couldn't find any rotten wires. It would run without dying but it just wouldn't rev out. I put it back on the trailer and pulled the plugs. They all have good spark. So, then I took it home and did a compression test and every cylinder was a consistent 90 psi.( 90 psi isn't ideal but it should run decent.) I am at a loss. The fuel system is pretty much brand new, it has spark, and it has even compression on all 4 cylinders. Could there be an issue where it loses spark when it heats up? Any help would be appreciated.
 

racerone

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Dec 28, 2013
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Did you install a new water pump impeller ?-----Tested the overheat horn if equipped with that ?
 

roscoe

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Yes, ensure engine is cooling, and next time it happens, check for spark immediately.
Could be a coil heating up, but I doubt that would happen in just 2 minute.

Closely examine the wiring, coils, and ground wires.
 

Kalebe1023

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Yeah It has a brand new impeller in it too. It pees hard out the tell-tale hole. I don't believe it has an overheat horn either. I want to say it is an electrical problem because I know it has decent compression and the fuel system has been redone. I might have run it for a little longer than 2 minutes but it wasnt much more. I let it warm up a little bit before I took off. I know it was running for <5 minutes at a time. Is there any way that I could test the coils?
 

racerone

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Serial # of this motor ?---Does spark jump a gap of 3/8" on all leads ?-----Run with timing light hooked up.----See what happens to spark when motor falters.
 

Kalebe1023

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I'm a college student and I won't be able to see the boat until next weekend. I'll get the model number and troubleshoot some more when I go back home. Thanks for all the help!
 

The Force power

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Feb 3, 2019
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I'm a college student and I won't be able to see the boat until next weekend. I'll get the model number and troubleshoot some more when I go back home. Thanks for all the help!
Verify the vent in the tank works and inspect the fuel-pump
 

Kalebe1023

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Nov 16, 2020
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Serial # of this motor ?---Does spark jump a gap of 3/8" on all leads ?-----Run with timing light hooked up.----See what happens to spark when motor falters.
The serial number is 6542046. I have yet to take it back on the water I will have time later this week though.
 
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