2002 Yamaha 60 lost power under load

coderunner

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I have a 2002 Yamaha 60hp with carbs. It ran well for years until recently when it lost power under load. It will start great and rev normally in neutral. When under load / in gear, it will not rev up and does not have the power to get fully onto plane. It is not an intermittent problem. It will not go onto plane now.
I assumed this was a fuel issue but have had no success curing the issue. Here is the order of things that have been tried / done so far:

1: Replaced the fuel / water separator and the bulb and hose from the tank to the engine. Cleaned the fuel filter inside the engine cover too.
2: Had the shop clean the carbs. (They cleaned them twice, since the first time didn't fix the problem.)
3: Shop replaced the fuel pump.
4: Shop replaced the thermostat (they noted it ran hot while in the tank with them. Likely nothing to do with the issue.)
5: Checked compression. (180-185 in all cylinders)
6: Checked spark (Inline spark tester flashes the same with all cylinders at idle and high RPM)
7: Changed plugs
8: Ran on a portable tank with new hose and clean fuel
9: Checked the prime start "electrothermal valve" on the #1 cylinder. (It moves as Clymer manual says it should. I didn't check the prime start on number 3 cylinder as I didn't want to pull the carbs off to get to it after the shop was in there twice already.)

I'm looking for ideas of where to go from here. I'm $1500 into shop payments and hate to let them shotgun it further.

I'm curious about the accelerator pump. I can move the shaft to the plunger in the accerator pump in and out by hand and feel resistance. It moves in and out but feels as if it's not moving smoothly. It makes a noise when moved as expected with the plunger moving (maybe a fart like noise). When I move the throttle, it seems to not move the shaft to the plunger much, even when moved to wide open throttle. It seems like maybe its sticking a little in the accelerator pump.
 

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99yam40

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you must be talking about a F60 which is a 4 stroke motor.
Not a 60 which is a 2 stroke motor.
posting the motors model number helps others know what motor you are dealing with.

I would be checking that the timing belt did not jump.
valves need to open and close in the proper time with the pistons.

also did you check the spark timing with a light?
 

coderunner

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Cleaned the accelerator pump. It had some deposits in it. The plunger moves in with full throttle now. Didn't change anything though.
 

coderunner

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I did not check the timing / use light. Thanks for the reply. I'll look into that next.

Motor is a F60TLRB
 

coderunner

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No timing light yet.
Haven't cleaned the carbs for a third time, but have been testing the ignition system.

I checked the ignition coil resistances. The primary winding is very low resistance as expected from my specs table. The secondary winding spec table shows expected resistance of 3500-4700ohms. I'm measuring 14.1k ohms (14100ohms). I get the same resistance for both ignition coils which would make me think they are both ok. I wouldn't expect both to go out at the same time with same resistance. I'm thinking more likely user error in measuring. Pretty easy to measure resistance though. Maybe table value wrong?

I measured CDI output and got 60 mV DC (0.06V) on both wires going to ignition coils (was expecting 105 V per table). Not sure if this is my measuring error too.

Then, for kicks, I ran the motor on three cylinders (in the yard at idle and slightly higher) by pulling individual spark plug wires one at a time. It ran fine on three cylinders with each try. I couldn't tell a difference. I wonder if this same test under load might help me identify a cylinder that is not firing (for whatever reason).
 

coderunner

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I ran in the water under load with each spark plug disconnected one at a time. There was a noticeable difference when each plug was unplugged except for when cylinder two was unplugged. When cylinder two was unplugged, it ran about the same as when it was plugged in. This leads me to believe that cylinder or carb2 is the problem. I went back and looked at the pic I took of the original spark plugs when I pulled them. Plug 2 was rather black (see pic). Four had a rather brown appearance. Not sure what to make of that. Going to pull carbs and clean carb 2 myself soon. Is there an adjustment that will make carb 2 run too rich with high speed / under load?
 

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