Yamaha DT400 1975 Motorcycle?

A

Allan

New member
I have a 1975 Yamaha DT-400. I have rebuilt the engine, and put in first oversized piston, new carburetor, new CDI. The motorcycle can sit for a month and you can kick it one time and it will start. Idles perfectly, then as soon as you take off stutters and stumbles; I have run it up to 5000 RPM and this is the way it runs.
I pressure tested the crank case, it has good compression and I have re-jetted the carburetor. I can set the timing anywhere on the magneto and the motorcycle will crank. I have made it run rich, and have made it run lean and it makes no difference. I also took the silencer off the muffler, have a new K&N air filter and a new ignition coil.

My question is, why will the motorcycle not take throttle smoothly and accelerate?
 
Loose or defective pulse coil - behind the flywheel.
Did you check the timing with a timing light? - if the timing is not set perfectly, you're going to have running problems.
Is the CDI brand new from Yamaha, or off another bike from a salvage yard? - it may be defective.
Worn crank bearings will screw up the piston timing (the spark plug can fire at different piston heights).
Leaking spark plug cap (shorting to ground).
Spark plug cap not making a good connection to the high tension lead (spark plug wire).
Jet needle height set wrong or loose (it's on the throttle slide).
Water in the gas - water is heavier than gas and will sit at the bottom of the float bowl.
When the engine RPMs increase, the strong vacuum sucks the water up just enough to clog the jets - but won't enter the engine.
When the RPMs drop to idle the weak vacuum can't hold the water up and it drops back down to the bottom of the float bowl.

What does the spark plug look like when you take it out - fouled?
Oil pump not adjusted.
The ports might not have been chamfered when the cylinder was bored.
Leaking reed valves, or reed block.
Leaking crankshaft seal - left side.
 
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