93 Yamaha 4 Stroke Runs Good But Slows Down

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tdunedun

New Member
I have a 93 4 stroke. Stock engine with governor disconnected and carb mildly jetted. Starts right away and idles good. When I take off it runs great and will run wot then after short period it will only take half throttle. If you try giving more it wont take it. It almost seems like its running out of fuel in the bowl. I checked the fuel pump by pulling line loose will running and it seems to be pumping. It fills up a bottle does not seem to have a lot of pressure which is probably because of vacuum style pump. I even swapped fuel pump for another I had same problem. I tried hooking both up to increase volume and it ran wot longer than with one before it started back not taking much more than half throttle.

I took the pedal start off and tried a push button start switch it cranked right up but the died as soon as you let off button. Does the starter supposed to run constant when it is running?
 
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tdunedun

New Member
I also went up a couple of sizes on main jet.

It runs great and has good throttle response when its cold for maybe 2 to 3 minutes and then it will only take half throttle. If you give it more it dies and backfires.

I think I worded it wrong I know the starter always spins, but is supposed to be energizing from the solenoid all the time (like it does when it first starts).

Thanks for the feedback.
 
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tdunedun

New Member
Yeah it seems to be sealed but I do not have a foam filter over the paper one. I have seen the response check the airbox on several other questions. Why is the air box so important on these engines? Any other yamaha engine will run without any filter at all.

Has any one tried any other carburetors on these engines with any sucess like tillotson or off a street bike?
 
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