Yamaha 150 loss of power

[dgh]

Hello,
I have a '90 Yam. 150 hp w/some issues:

1. Idles fine, but as of late has a loss if power. I used to get around 5500 rpms, now about 46-47. Also seems slower out of the hole.

2. I have been fighting an overheat issue, only @ 5000+ rpms (or atleast the warnings come on). With my new loss of power issue, it does not overheat, because it does not reach higher RPMS.

Prior to the loss or power issue, I removed and cleaned carbs and pumped out some bad fuel,which needed to be done, boat had sat up for a while. Replaced spark plugs, thermosats, did a comp. test.
What do you think? Thanks!! dgh

[jb123]

what did comp test show-
have you checked every link in fuel system,from tank/pickup,pumps to carbs?
i would first start off by running on a hose and one at a time pulling plug wires while running and ensure a falter/change to isolate your cylinder issue. sometimes you may have to pull off 2 at a time to hear a definate sound change.you can spray gas out of a bottle into the clogged carb to HEAR the difference or use your finger as a choke to richen gas mixture. most probable a clogged carb.

as for the overheat, whens the last time you replaces impeller and housing?

[Ken]

dgh,
Did you completely tear down the carbs? Removed all jets and cleaned/inspected? Any o-rings on ends of jets that are split or distorted need to be replaced. Did you install a carb kit? Did you syncronize the linkage on the carbs?
You are only able to get 4700 rpm now - do you notice any missing or surging of the motor, or does it run smooth at 4700? If it has been at least five years since last fuel pump install, I would replace with a new one.
You might check timing - maybe ignition advance has retarded a bit. Gonna need a Yamaha shop manual for your particular model.
On the overheating, if it has been at least three years since last water pump repair, I would do it. Also, need to check the pressure relief valve (PRV) to make sure it is not stuck partially open and dumping cooling water out the relief holes.
Good luck [img]smile.gif[/img] ,
Ken K

[dgh]

thanks!
ok, i took off plug wires and found one cylinder not working. i then pulled the related carbs off, including all jets and cleaned and replaced.
That helped, got back up to around 5200...then the hot buzzer went off again. once it cooled, i could not get over 4700-4800, again!

as for the impeller, not sure last time it was replaced, but i it does pump nicely through the "pee hole" at all rpm ranges.
im not sure about the fuel pump and i dont know where the pressure relief valvenis? or what it does!
thanks again! david

[Ken]

david,
Since you're obviously doing your own work on your Yammie, you need a Yamaha shop manual to guide you along. Go to www.yamahapubs.com to order online - they will ship to your door.
The manual will show you exactly where the PRV is and how to clean and inspect it (it is located on the lower part of the exhaust plate - has a rubber hose going to it). It's function is to relieve over-pressure of the cooling system - has a spring that holds a valve in place - when cooling system water pressure exceeds 20 pounds, the valve opens and dumps water out the relief holes that are located toward the top and rear of the upper casing. The valve and seat and spring can get clogged with mineral deposits and keep the valve from closing all the way, and thus, allowing cooling water to dump before going thru the water jackets.
Good luck [img]smile.gif[/img] ,
Ken K