86 Yamaha 150HP wont run above 2g's

[Parkhil2]

I started this motor for the first time yesterday, was excited it started but it ran like hell. The motor idled a little rough, but as throttle is increased the motor runs rougher and then will not turn above 2000 RPMS. I started to pull wires off plugs one cylinder at a time, the top cylinder on each side made a big difference in idle quality, but the other 2 on each side did not make much of a difference. This pointed me to fuel delivery, either the carbs or fuel pump. Am I correct that the top cylinder on each side is fed by one fuel pump, and the other 4 by another fuel pump. Also after leaving the motor sit overnight I went out and noticed fuel sitting in the bottom housing of the motor, and the fuel filter cantister was half empty. I re-primed the bulb, and it refiled filter. Any opinions or advise on this would be greatly appreciated. The motor hasnt run in maybe 2-3 yrs prior to yesterday. <br /><br />Thanks<br /><br />Bill

 

[Stevenhj]

Re: 86 Yamaha 150HP wont run above 2g's

There is a rev limiter if the oil system is not working properly OR on some models if the engine is tilted more than 30 degrees it will derate to 2000 rpm's. You might check the trim sender. If it is broken or malfunctioning it could be sending a 30+ reading to the rev limiter. I had this same problem. The plastic lever on the trim sender was broken. I replaced it and the problem went away.

 

[Parkhil2]

Re: 86 Yamaha 150HP wont run above 2g's

2 good points I should have mentioned originally. I have been using a remote tank iwth pre-mixed fuel as a precautionary measure. When I did get everything powered up there is a oil warning buzzer sounding with the key in the on position. Also the tilt on the motor works, but the trim does not, so it only goes down till it hits the trim cylinders, but this isnt more than 30 degrees I dont think. Due to the age of the motor I was contemplating disconnectting the oil injection system and just premixing. Is this the first step I should take? How can I figure out why the Oil warning buzzer is going off? Both tanks are more than half full. <br /><br />Thanks<br /><br />Bill

 

[Stevenhj]

Re: 86 Yamaha 150HP wont run above 2g's

The low oil warning system is probably what is causing the motor to only run 2000 rpm. You may want to check the oil level sender in the main tank which is on the engine. If it is bad it could be sending a signal to the warning system that the oil is too low. One of the wires to the oil level sender could be grounded causing the warning system to engage.

 

[rodbolt]

Re: 86 Yamaha 150HP wont run above 2g's

were it me I would test compression first. buy a service manual second, then if the compression and spark test were ok rebuild 3 carbs and replace the fuel pumps. never heard of the trim sender affecting RPM other than if tilted the old units had a mercury switch that would inhibit oil transfer. replace the pressure relief valve grommet t-ststs and water pump kit. then it should run a while. the filter on the engine will stay about 1/2 full in normal operation.after sitting 3 years the carbs are nasty.

 

[Ray Neudecker]

Re: 86 Yamaha 150HP wont run above 2g's

You can remove the RPM limiter for testing by disconnecting the white wire from the control unit mounted below the CDI to the CDI. Lower left hand side of the CDI. First I would take Rodbolts reccomendations on the testing and fuel restriction removal. Some PB Blaster or similar product sprayed on the trim rams may loosen them up. Check the level of the trim resivoir, since I can't see any electrical reason for the trim not to work on that model.