92 Mercury 115 2+2 4cyl. No Power after running for a while

[shoemaro]

92 Mercury 115 2+2 4 cyl. Compression good, carbs recently cleaned, filters changed. Engine runs good at idle, Runs good initially, but starts sputtering randomly under a load. Go back to neutral, then will run good again for a little bit then start acting up. Basically will take off fine pulling plenty of RPMs then will not rev over 3500 or 3000. Problem gets worse the longer it runs, but seems to be random? I was thinking stator or switchbox from the research I have gathered, anyone else had any of these problems?

 

[DavidMoore]

Yep, at least similar symptoms.
1st thoughts is a fuel problem, most likely a pin hole in the fuel pump diaphragm.

 

[shoemaro]

I don't think it is fuel related. New carb kits plus new fuel pump. It runs good at idle, but after running 5-10min it loses power will not run under a load.

 

[CharlieB]

You cannot assume the fuel system is good.

A clear hose used for a temporary fuel hose between the fuel pump and carbs will show you if you have air sucking into the fuel system anywhere before the fuel pump. An air leak will cause the carbs to run low on fuel at hgiher throttle and the motor performance gets sick, yet still pumps enough fuel for everything to appear fine at an idle.

Clear hose is NOT rated for fuel. It should ONLY be used as a testing tool. Once confirmed and finished testing be sure to remove it and rep[lace with the correct fuel rated hose.

 

[DavidMoore]

You didn't state the fuel pump was new, in that case Charlies' advise is spot on.
You probably have a leak / blockage before the fuel pump.

When it happens check the primer bulb, if it's collapsed the problem is upstream of the bulb, if not the problem is between the bulb and fuel pump.

​+ how new are the fuel lines, old fuel lines and modern fuel are enemies, affected fuel lines can quickly block carbs.

It could be electrical related (you have 2 + 2 so could just be running on 2 cyl's) but the symptoms are classical fuel delivery issues.

 

[shoemaro]

I am willing to try anything versus gamble on a stator. I have a separate tank and hose for my kicker, I will try that to eliminate anything before the fuel bulb.

 

[shoemaro]

Update-ran motor with external tank, ran the same, poorly. It starts out reving to 4500 and then gets worse, sounds almost like it is hitting the rev limiter. I pumped the bulb and choked it when it wouldn't go, it made no difference. Talking to a Mercury marine mechanic, he thought it sounded like the trigger was going bad. Does anyone a good test for the trigger? Or any other ideas as to why it is running poorly?

 

[dmoriarty51]

Any update to this?

 

[shoemaro]

Update, I replaced the trigger and same problem. The problem ended up being the switch box.