Small Engine Guys!! Mikuni Carb won't rev

[TerryD]

We have a second hand Yamaha 80cc Raptor 4 wheeler and it sat for 4-5 years till Alex (oldest daughter) was old enough to start riding it. Trying to get it running again, we pulled the carb and cleaned it out, blew out all the passages (only like 3?) and it ran for a day or two, though not really well, then we got busy with other things last year.

Yesterday evening I tossed the battery in it, started cranking and no good, won't start. So I pulled the carb again and I guess the shut off valve was leaking and filled the carb with gas in the last year. So I cleaned it out, cleared the jets, blew it out again, and dropped the float a little since it had been over flowing while idling last year. Started right up after install, and adjusted it till it would idle well. Took it out in the yard and the engine won't rev without the choke on. Put the choke on, hit the gas and it'll scream, but take the choke off and it's like it's just not drawing fuel.

Pulled the float bowl again and dropped the float thinking it was uncovering the jet when pulling hard. No real help after doing that, so Alex and I just swapped the fuel filter and still no change. Put the choke on and it'll rev fine, but with it off still nothing.

I can get decent pull at low RPM, low throttle, but open the throttle up or rev it and it starts cutting out. Any suggestions on what to check from here?

 

[sweetk30]

these small engine carbs I have found can have jets with super tiny ports up the center and make it real hard to clean out or check .

pull the jets and flash light test them to look up the center . lots of times just 1 tiny chunk of scum got me . I use to play with atv's a lot back in the day and old dry gas in a carb was the worst to get cleaned out . sometimes took 2-3 times to get it out . then good to go .

 

[TerryD]

I used a tip cleaner on it yesterday and cleared the idle fuel jet (guessing at name), the power jet was clear but I'll pull it again this evening if I get a chance and give it another run through. I should be able to drop the jets without removing the whole carb again, the throttle valve is annoying to get loose.

 

[sweetk30]

just watch the tip cleaner . only use the smooth end of the tip or you will bore out the size and mess it up . . . . .

 

[diesel4me]

Ethanol causes a lot of small engine carbs to get deposits in the jet passages in the casting where no tools can get at it,and it forms a shellac so tough I've had a hard time pushing a tip cleaner or other tool through jets that had that crud harden in them...

It might help if the tip cleaner hogs the jets out a tad--most small engine carbs are set up too dam lean for emissions and one speck of anything in the wrong spot will cause bogging on acceleration or dying out when your pulling a hill,etc..

I have had limited success cleaning a carb that had ethanol gas go sour in it--only some cleaners will touch the grain based goop ,petroleum solvents like carb cleaner doesn't always work.

Some methods I have used to salvage severely gummed up carbs is to boil them in pure lemon juice,its pretty cheap at the grocery store--white vinegar also works--on carbs I had that still refused to work good after those treatments got dunked in a plastic bottle filled with battery electrolyte ,it'll fizz like root beer ,but leave it in only 10 minutes,or else the brass parts and possibly the casting can dissapear..then rinse it out well with baking soda & water to neutralize the acid..

Some guys I know tell me Sani-Flush or Vanish toilet bowl cleaner also works well...both of the carbs I cleaned with battery electrolyte worked like new,after the lemon juice and Gumout type stuff failed..

When you look up into the venturi,where the throttle butterflt is,right next to where the butterfly closes on the venturi there is usually a few teeny holes drilled there--one is for idle feed,the other(s) are "transition" ports that dump extra fuel when the throttle is snapped open quickly--works like an accelerator pump on car carbs--if those block up it'll probably want to only idle good,and want to stall out or "flatten" when you try revving it up..

 

[TerryD]

I didn't go back out there this evening. Might pull it off again Tuesday and give it another good going through. Iv'e got a bucket of carb cleaner that does some really good work so I may give it an evenings soak in there to see if that helps.

Thanks for the advice guys! I'll check back when I get some more time to throw at it.

 

[JoshHefnerX]

Is that one of the carbs w/ the vacuum operated sliders in it? If so, make sure the slide is moving properly.

 

[TerryD]

I didn't notice anything that looked vacuum operated in the carb. The needle is lifted by the throttle valve and the only other pieces I saw that moved were the adjustment screws and needle/float.