1975 yamaha DT400 runs fast when choke is off and then dies or runs up in redline and can't slow down, and have to shut off key and choke to slow down. runs better with choke on. please help with any ideas. checked magneto seal,made gaskets for reed housing and checked reeds. cleaned carburetor and set float in several positions. Thanks

What about needle height ? and slide . are the located properly, the cable, disconect it , set carb to idle , just aliminate every thing,, pilot jet, are u shure its clear ,,

I always suspect crank seal in this type of post, you say you checked it though. did you try spraying it with WD40 or similar when the bike was running...to see if it affected the engine speed / tone ?

Thanks, no oil around or not wet around crank seal. not tried slide, but will try. Thanks for your replies, I sorta think maybe a bad carburetor. Still can't find any parts, new or used.

Reminds me of when I run the fuel out of 2-stroke outboards. Runs fast then stops. Will run a bit longer with choke out. Too much air one way or the other? No idea other than sounds like carby.

Gordo

bad carburetor. Still can't find any parts, new or used.

I have a slide here. & sold a complete carb , you still get bits for mikuni . yeh new parts.

Thanks for everybody's help. The problems was, coated gas tank several years ago and this came loose and clogged carburetor. I pulled and tugged and finally got this stuff out and cleaned and washed gas tank after putting chains in tank to looses rust. soaked carb in acetone 2 hours without any rubber on it (this will eat rubber up) installed inline clear filter. This machine runs like new. Thanks again for your help.