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RPMs go wild


cummins93

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I have a 1994 Yamaha big bear 350 4x4, it sat for maybe a year, so i recently got it running and it sounds good but when im in neutral and i rev it a little and let off the rpms will stay the same or go higher and will eventually come back down slowly, the cable isnt sticking, and i have cleaned the carb as much as i can from the spray to the fuel additive to clean it, any advice will help thanks!

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Mine does that a bit too, I have an 89, I just ordered a carb rebuild kit. Theres really not much you can accomplish by sprays and what not, it really needs to be taken apart and cleaned. If you have never done a carb rebuild before dont be intimidated, take your time, take pics if needed, its pretty easy.

Does your model carb have the single cable? Not sure when it switched as Im new to these, mine has two cables, one for the butterfly and one for the needle.

Possibly a vacuum leak, sucking in air form somewhere, thats one of the reasons Im rebuilding mine, I cant run mine w/o the choke part on. Or maybe your pilot jet improperly adjusted. If this quad is new to you, buy the book for it, Im sure it could use a valve adjustment as well. Thats my next plan when I find a way of getting my fuel tank off thats messed up.

Edited by MrSchaeferPants
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awesome thanks guys! and i forgot to mention that when its idling at normal rpms it makes like a popping noise kind of like a back fire but you dont hear it come out of exhaust it backfires into the air filter box if that makes any sense at all,

Backfiring out of the carb is a typical symptom of a lean condition. Vacuum leak (sucking air from places OTHER than air filter), or your pilot screw is set wrong, or both. Or somethings changed and it needs re-jetting, or someones monkeyd in the carb and changed the jets.

Mine does this as well, or well did, like I said I leave my choke partly on. I know for sure I have a leak at the boot between the carb and engine. Take a can of carb or brake clean and spray it around your carb while its running, if the rpms increase, you have a vacuum leak.

If I had to venture a guess :wink:

Edited by MrSchaeferPants
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i just did the carb spray while its idling and i sprayed the whole thing and it didn't do anything UNTIL i sprayed where the boot bolts up to the motor, when i sprayed that the idle went crazy, that must be the problem, when i replaced the boot i actually think i didn't put a gasket there like a idiot, (i think), so i will put a gasket on tomorrow and hopefully solve this thing, thank you guys for the fast response and all the help!!

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My carb kit was $25. Bought it from some atv company that sells on ebay. Yamaha dealer and places like partspitstop doesnt offer a kit (as far as i know), only each piece individually. Im still learning and finding vendors.

If your carb is like mine there should be two screws located at the base/underneath, the thumb screw on the bottom (one you can turn by hand) is the idle rpms. The one you need a screw driver for controls your pilot jet, or your idle air/fuel mixture. Now depending on carb, its in two locations, either before the throttle plate, or after. If its before the throttle plate, its controlling air, if its after the throttle plate (engine side) youre controlling fuel.

If the screw is closer to the engine side of the carb or past the throttle plate, screw it in all the way in until you feel it bottom out (gently) and then back out 2.5 rotations, and go from there. IN leans the mixture, out enriches it. But until your vacuum leaks are under control, it wont matter, for example mine is about 4.5 turns out right now, and I did that so when I go from idle to part throttle, it doesnt want to kill the engine anymore due to my vacuum leaks. Once those are fixed I should be able to go back to around 2.5

My boot is leaking at the carb, theres an O-ring there, not sure on the engine side. From what Ive read these boots are notorious for these problems. Leaking, cracking, breaking.

Edited by MrSchaeferPants
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