Power drop at high rpm

EngstromNK · #1 07-13-2006, 12:22 PM

The last time I went out I had been running at 4000 rpm for about 10 minutes when the rpm's droped to 3800. The boat felt like I had stepped on the brakes. I throttled up to 4400 rpm and it ran fine for a minute and then dropped again. I throttled back and it cleared itself again. This went on for about five minutes until I arrived at my destination. The ride back was uneventful. Should I start with the carb or fuel pumps? I have a '91 Yamaha 150.

Another piece of info. When I was pumping the bulb before starting it never really got solid. Then I noticed a sheen in the water around the engine.

Thanks,

Keith

Mac_Attack · #2 07-13-2006, 12:34 PM

To me it sounds like a fuel problem. *Good place to start. I noticed in your bio you are on the Cape. *Please be advised that there may be a lot of joyful screaming coming from West Yarmouth on Friday. *MJ is hangin his motor!
There are some good mechanics on this site. *Stay tuned they'll be of better help than I. *Billy Mac *;D

Mulv80 · #3 07-13-2006, 12:56 PM

Sounds like crud in the carb to me. I had a similar problem where my rpm's were dropping from 5300 to 4500. I came home and drained the carbs and saw some debris come out. I ran the boat again and it ran great for a minute than dropped rpm's again. Finally I removed the carbs and cleaned them with carb cleaner and compressed air and it seems to have fixed the problem.

If you suspect it may be your fuel pump, have someone pump the primer bulb while loosing rpm's and see if it makes a difference, but I would almost guarantee its the carbs.

Blue_Runner · #4 07-13-2006, 02:15 PM

Don't rule out an obstruction in the fuel line and or filters. I had the same thing happen with my racor water separator. Unhooked the line from the sep to the tank, blew trash out, hooked back up and problem solved.

steplift20 · #5 07-13-2006, 03:10 PM

as you know the gas [ thanx to that m fkr pres bush] mis screwed up htey add ethenol to it and its messing up the boat motors just like your having its happening to me also
runs good than it bogs down
do you have a water seperator
iof you dont put one on
they say it should be 10 mirons but im not sure if thats a good idea
good luck

EngstromNK · #6 07-13-2006, 03:35 PM

I forgot about my racor. I will drain that first and go for a run.

Thanks guys.

steplift20 · #7 07-13-2006, 08:39 PM

hey let us know what comes out of the racor
just curious
but found out mine wasnt the bad gas/its was a problem but also have another problem
after the motor would warn up about 5 minutes it would lose power and the rpms would cut down to about half what it should be so i brought it to the shop today because its a problem i cant figure out