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Mercury 3.5 hp 4-stroke idles but won’t run long under load

N

nalsalam

New member
I’m at my wits end.

I bought the motor at auction. It looked almost brand new.

I took the carb off and cleaned it. (The rebuild kit is $40 and only gives you a new float and float needle valve.) Found debris in the bowl so I installed an in-line fuel filter. Got fresh non-E gas.

Starts and idles. The transition between idle and part throttle is not smooth. In gear it runs a short while and then stalls as if it is running out of gas. (I am careful to turn on the fuel petcock and open up the vent.)

Took the carb off again and was careful to make sure the pilot jet and its passage was clean because I read somewhere that the pilot circuit is key for up to 25 percent load and is a common problem for engines that start and then die after a bit.

Maybe it started and idled a bit better but it behaved the same when I put it in gear and tried to go somewhere. It dies. I need to let it rest a bit. I restart. Try to go. It dies. And the cycle repeats.

I noticed that the carb has no mixture screw and my lake is at 2600 feet so I decided to buy a new carb that has a mixture screw. It's $70 which doesn’t seem too bad. I installed the new carb. The fuel mixture screw came 2-turns out and I left it there. When installed the mixture screw faces inward.

Same problem. Starts and idles well but will not run under load for more than a minute or so. I need to wait a minute or so before restarting.

While it was idling I felt around the thermostat housing and it heated up and I think I felt the pee go from cold to warm.

What should be my next steps? Thank you for any suggestions.

 
Your statement about "under load" caught my attention... For an inexpensive attempt, try a can of Mercury Powertune, (about $12), use as directed This sounds like a possible carbon build up problem....according to Mercury tech, a "fairly common" problem with their 4 strokes
 
The motor is running now. The last thing I did was change the spark plug. But the symptoms screamed fuel starvation to me so I find it hard to believe that was the solution.

I did discover a filter tube in the incoming fuel line that was supposed to be attached to the petcock but was loose. I remove it because I had previously installed a normal inline filter. The stalling under load problem did not go away.

Trying to be logical and wanting to test the flow of gas from the tank to the float bowl, I removed the fuel tank and carb and the float bowl off the carb. Then I open to petcock to let the fuel flow through all the tubing, the petcock, the inline-filter, the carb body, and the float needle valve. The flow was good. Put everything back together and replaced the spark plug.

The motor runs under load! Mirabile dictu (it's wonderful to relate) as my dear departed mother used to say when I picked up my toys or did something equally unlikely as a kid.

I guess the lesson I learned is that although it pays to be logical sometimes you just need to cover the bases.
 
Went through a similar experience here. Mercury 3.5hp idling fine, dying under load. When it lost power, the only way to prevent it from dying completely was to put it in neutral in a timely fashion, rev slightly, and back in forward, without pushing it too much. Rinse repeat until reaching the destination.

Disassembled and cleaned the carby, checked the float, jets,... Same problem.

Checked for fishing line around the propeller axis. Nothing.

Dismantled the gearbox, took this occasion to replace the oil in there (never did before, was very milky). Still no change.

Replace the petrol with clean 98 out of the service station (in case whatever I put in before came from an old and possibly stale jerrycan I use for my lawnmower). No change.

And, yup, the spark plug it was. I replaced it and it is now running better than ever. There was no sign that the previous one was fouled. But it was definitely the issue.
 
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