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74 ty 250 wont rev out


james n
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Does the TK carb fitted have the main jet screwed into the float bowl like I suspect, if so replace the o-ring that sits on the bottom of the needle jet as if it is damaged it lets fuel bypass the main jet & it will run very rich

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  • 3 weeks later...

Hi again,  this isnt my bike I was trying to help a friend get ready for vintage motorcycle days.He ended up borrowing a friends motor.   here is alist of what was done.

new points ,condenser,coil,air filter and o-ring in carb. and ing side main seal.

we also tried a differant ty carb and exshaust  we could not get any new reeds.to try.

The bike runs great with the air cleaner and the air box lid removed.if you put the air box lid on without the air filter the bike wont rev, if you put the air filter in without the air box lid bike wont rev. take them out and the bike revs fine.

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5 hours ago, james n said:

The bike runs great with the air cleaner and the air box lid removed.if you put the air box lid on without the air filter the bike wont rev, if you put the air filter in without the air box lid bike wont rev. take them out and the bike revs fine.

Based on this info, it seems logical to conclude that the problem is associated with air intake, not the carb or exhaust or timing.   I just looked at my ’74 and confirmed my memory that the air get into the airbox through 2 holes on the left side of the box.  These are behind the oil reservoir.  If they are completely blocked, the bike will not run.  If they are partially blocked, the bike could run at low RPM but not get enough air to run faster. 

 

I suspect that something happened to the airbox or the oil tank leaving too small a space for air to get to those intake holes. 

Good luck.  Let us know what you find out.

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  • 1 year later...

Can't be sure from your description, but if the engine is blubbering instead of reving when you open the throttle you might have the air bleed screw out too far or have a bad main crank seal leaking which causes a two way vaccum/pressure leak which causes the mixture from the carb to be mixed with too much air which will misfire or just not make much power.  That is assuming that the carb is all clean and good with  new NON-Ethanol premium fuel and 50-1 synthetic oil mix or equivalent.

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Sound like it is running too rich at full throttle.  You haven't done something daft like clean the main jet out with a twist drill have you?  Or perhaps you have put the needle back in the wrong notch.   Either way you should have a sooty plug, to confirm it is running rich.   Maybe the cure is to reduce the main jet size until it gives the correct performance.

edit....  Just had another thought...  Is the Cold-Start mechanism working correctly, with no rotting of the rubber seals.?

Edited by scifi
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