1991 Yamaha 85 HP

[butterdeez]

I have a 1991 85 Yamaha Outboard motor on my boat.. I am having a big problem that i cant seem to solve and everyone i ask keeps telling me the same resolution but nothing works.....

My motor starts up with no problems it runs great up to about 4200 RPM but if I take the throttle up any further than that it runs sweet for about 30 seconds and then it sounds like the motor misfires or bogs down ( i really dont know how to describe it) so i have to let the motor sit for a good 20-30 seconds before i can get back to it....The mechanic at the local marina told me to diagnose for a fuel restriction so i started at the pick-up tube and worked my way up to the anti-syphon valve:replaced it: next to the Fuel filter: replaced it; onto the primer ball: replaced it; next the fuel connector: replaced it; fuel pump: replaced; took all of the carbs off and soaked them for a couple of days each and reassembled...still the same problem....the motor only does this when it is in gear in the water...it does not do it when i have the ears on it in the driveway....i have a 17 pitch prop on it...Someone told me that the prop could be the problem which makes absolutely no sense...the motor will run at 5300-5500 rpm for the few seconds before it dies out ....If there is anything else that anyone one can think of to try please help....very frustrated

 

[sc_shane]

Re: 1991 Yamaha 85 HP

Just for shoots and ladders - have you replaced the plugs since all of this happened? My 1989 90HP did something similar to this, then I cleaned the carbs, replaced filter (entire housing, not just element), replaced some hose while I was in there and replaced the plugs with the NGK's gapped exactly the way it calls for. After I did all this myself, I did take it to a Yamaha master mechanic for an hour of "tuning" it in (getting the carbs set right). My problem went away. I don't think the mechanic did anything buy adjust the carbs right (he charged me an hour labor @ $80, but it definitly ran better after) but maybe he did. I dunno. Just curious if you've tried new plugs yet.

 

[JB]

Re: 1991 Yamaha 85 HP

"took all of the carbs off and soaked them for a couple of days each and reassembled"

What did you do about clearing the high speed circuits? Simply soaking will not clear gummed jets and orifices. You have to apply pressurized air or mechanical clearing. Setting the float levels?

The timing also seems to indicate that one or more cylinders are quitting. . .maybe there is a heat failure in the ignition for one or more cylinders. Try running it without the cowl on it. If the trouble fails to appear you have a heating failure.

To isolate a heat failing part put her on the muffs and heat ignition parts with a hair dryer. When idle drops you have just heated the heat sensitive part.

Good luck.

 

[rodbolt]

Re: 1991 Yamaha 85 HP

so far I dont think you have trouble shot anything, just tossed parts at it.
to find a fuel restriction you have to use a vacum guage teed into the fuel pump intake. maximum vacum at WOT is 4"Hg at the pump intake. less and your ok more and there is a problem, keep backing the t up until you find it.
the ign system is simple to diagnose with a peak reading meter on the CDI output to the ign coils.
total time to test both fuel and ign should be less than 1 hour. 30 minutes if you use the test wheel.