Yamaha 50HP 4 stroke won't rev past 4200 RPM's

[Arcticwildman]

Buddy of mine has an Alumaweld with motor issues that has me stumped. 2001 Yamaha 50 HP 4 stroke (carbs) with approx 230 hrs on it. Motor won't rev past 4200 99% of the time and misfires like crazy when it's running at that RPM. Idle is smooth with no miss or anything. Occasionally will run up to 5400 or so but as soon as you come off step and try to go again it just stops at 4200 and starts misfiring. We've replaced the plugs and all filters (gas and fuel/water). All I can think is that he has old gas with a bunch of water in it. Added HEET and Seafoam but it doesn't seem to make a difference. Anybody got any ideas? I think he needs to drain the tank entirely but figured I'd pick the collective brains of folks here for something else that may be causing it.

 

[Dupont Spinner]

Buddy of mine has an Alumaweld with motor issues that has me stumped. 2001 Yamaha 50 HP 4 stroke (carbs) with approx 230 hrs on it. Motor won't rev past 4200 99% of the time and misfires like crazy when it's running at that RPM. Idle is smooth with no miss or anything. Occasionally will run up to 5400 or so but as soon as you come off step and try to go again it just stops at 4200 and starts misfiring. We've replaced the plugs and all filters (gas and fuel/water). All I can think is that he has old gas with a bunch of water in it. Added HEET and Seafoam but it doesn't seem to make a difference. Anybody got any ideas? I think he needs to drain the tank entirely but figured I'd pick the collective brains of folks here for something else that may be causing it.

Look down into the tank and see if you can see the water. Raise the opposite end of the tank.

How about the water pump? You have good water coming out at all speeds.

Fuel pump is another thing to check. May have a spot of water on the diaphragm. Also the diaphragm may have a pin hole in it. Many times there will be a water droplet on the diaphragm and when it freezes over winter it will put a small hole in pump.

 

[NORTH2AK]

Check fuel water separator (plastic screw on jar)to see if the indicator float is towards the top (indicates water). Drain each carburetor (screw on the bowl). Be careful with the "heat". He might also have a fouled plug.

 

[Arcticwildman]

Good stuff!! Thanks guys, I'll pass it on to my buddy.

 

[copperlake]

Test resistance on coils with a good VOM. Should find proper reading on the interwebs or call Dewey's. Also make sure ground to coils is not funky. I would eliminate the fuel tank by sticking the disconnected supply line and sticking it into a fresh can of gas or use a portable OB tank if he has one. If he hasn't done so, as has been mentioned, remove the internal filter and clean the parts and blow the screen with compressed air. The symptom of bad fuel pumps is that the bulb won't get hard when squeezing. Oh, don't forget the plug wires. Lots of other things too: bad stator, broken rings, cracked piston on and on....