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1978 yamaha DT175 air coming out air intake problem

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i have a 1978 dt175 I have been rebuilding it for the past few months now I have one last issue    air is coming out of the air intake I just bought new reed valve assembly put in new crank seals and cleaned exhaust pipes I cant find the problem..... does anyone have any advice or know maybe what the problem is?

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If you installed the valve correctly then there shouldn't be any blowback from the engine.

Did you check to see that all the reeds are completely resting on the frame of the valve so that they are sealing the passageway at rest?

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Jaguar57  

I bought the whole reed assembly should I have put oil on the edge of reeds to use as a seal? Or would that even help at all? 

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when running there is a tad bit of oil there from the oil in the gasoline. yes it helps.

If your air filter is dirty and a bit of sand got lodged between the valve and the reed then that could prevent it from closing.

Take it off and seal your mouth to its intake side and blow into it and then suck air from it. You should not be able to suck any air.

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Jaguar57

Okay sweet thanks I'll try it tomorrow ! 

If that does not work do you know anything els it could be? 

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It's a brand new reed valve assembly straight from the box I oiled it up still not working

 

could it be timing? 

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actually in the microsecond it takes the reeds to close some air can be blown back.

if it runs good then don't worry about it.

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+1, air has momentum in, when the reeds slam shut there is pressure then reversion. That's where you see spit back through the carb.  If your seeing signs of actual combustion blowing back, carbon in the transfers, burnt reed tips, it probably has the wrong piston in it or was over ported. Either can short circuit the ex port to the transfers.

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