I have a Yamaha RX 135 which is a 2 stroke bike, and I have replaced it's carburetor , Previously it was stock 26mm round slide mikuni carb , and its been replaced with 30mm Flat slide mikuni.

After replacement bike runs very fine and and there a lot of improvement in higher end after replacement, However when I suddenly try to open full throttle, The engine makes a deep booooo sound like no air is sucked in or its being choked like bike is running out of of fuel.

Also when doing high speed , when I keep my throttle completely open, as soon as i reach my top limit, engine again stalls and looses acceleration but when using like 70-80% of throttle bike runs very well.

My main and pilot jets are stock I believe which is #17.5 pilot and #115 on main.

Can anyone please explain my why this is happening?

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    Welcome to Motor Vehicle Maintenance & Repair! May 18, 2018 at 4:21
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    Its call a snap throttle Bog, it is way too lean on snap throttle, I would up the main jet. I assume this carb has no accelerator pump?
    – Moab
    May 18, 2018 at 15:27
  • @Moab thanks for the response , I've also thought of same maybe because of bigger carb updrage I need to upjet my carb to match its specs. And yes , By bike is pretty old its a 1989 make so anything accelerator pump or something similar is not there , Its just carb and the engine. Also can you please suggest me a what jet size i can try next? May 21, 2018 at 1:57
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    You go up 2 numbers at a time until the bog stops.So 115 to 117 to start. Its probably close at 115 so 117 may do the trick. Also if that new carb has a jet slide needle it may be adjustable also, move it up a notch or two before trying a larger jet.
    – Moab
    May 21, 2018 at 22:07

1 Answer 1


As per @Moab suggested, I have tried upjetting my main jet, It helped but at lower-mid rang bike was sill not smooth enough, I tried #20 on pilot jet with #117 Main jet and my bike is running completely fine now. It was because of wrong jet size I was using.

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