Motor ran fine then started to break up at 4500 RPM, sounded like a fuel problem. I took off the carbs and cleaned them blowing out the passages and soaking in parts cleaner. These are my current symptoms: From idle when I go to full throttle it will go up to mabey 5000 Rpm then quickly start to slow down to 4500RPM and will not go faster. If I back off the throttle then slowly advance it it will run at 5000 RPM. Try to advance it further and the surging starts and it will go back down to 4500RPM. Now then before I cleaned the carbs I could only get 4500 RPM now I can get 5000 RPM. I checked the fuel line by switching lines and even gas tanks, cleaning the tank delivery tubes which were clean, the filters are clean also , the fuel connectors are not leaking air and the bulb does not seem to leak. I can pump the bulb but it is not flat when it starts its surging. Should it be hard? Can the idle adjustment screws have anything to do with my high speed RPM issue?(should I back off them for a richer mixture?) Plugs are new and gapped and appear clean. Idles great all the way down to about 500 RPM. Motor ran great all of last year and has not sat for any significant time. The fuel is fresh. I have not messed with the link and sync as these carbs came off without having to affect it. Going out tomorrow any help is appreciated. Thanks .
Last edited by w2much : August 11th, 2007 at 01:51 PM.
Reason: Spelling
your describing a lean running condition. run it lean long and you will be showing us detonated piston pictures .
test the fuel pump output pressure, ifthe pump is over 3 years old replace it for general maintenance.
you may not have gotten all the passages clean. sometimes it takes twice,especially if you did not clean the fuel lines and dissasemble and clean any debris from the fuel pump.
its why I usually replace lift pumps anytime I do carbs.
but it wont be the first time me,you or anyother tech has had to do a set of carbs twice.
Still seems fuel starved. I replaced the fuel pump with a new OEM pump. Thought I found the problem when I again checked the fuel filter, although it was still clean there was a hairline crack along the top where the bolt protrudes. So yesterday I tried to go WOT same results as before I installed the new fuel pump. ( spent $41.00 hate whenb that happens). I then bypassed the fuel filter with the hairline line crack. I optimistically pushed the throttle forward for WOT ( no happy ending here) and the same problem. So I guess its time for card rebuild kit. I was being tight and only cleaned the carbs the first time, reusing the same parts and gaskets as they all appeared to be in new condition. I will order the new parts and have it together for next weeks fishing. I will post results thereafter. Thank you for your replies, know that they are appreciated.
Have a manual somewhere I say she lost it, she says I lost it, I ordered one on Monday, should have it in a day or two. Also I ordered an OEM carb rebuild kit. When I pick it up I will also get a couple of feet of fuel line and eliminate it as an issue. Will post results when available. Thanks for your post, always appreciated.
Yamaha 50 HP 1986 Seems fuel starved?
.
HUMAN BUILT = HUMAN FIXED
Linear Mode
