Vintage Dirt Bike Q & A

Hello. I recently bought a barn fresh 1974 MT 250 to restore with my son. We spent a good deal of time to get a spark (had stator rewound, put in new condensor, etc.) For the first time today we were able to get it to start. The problem is, it runs at probably 3000 - 4000 rpm without touching the twist grip. I have cleaned the carb thoroughly and have installed a new old stock throttle cable from Western Hills Honda. It appears that the throttle itself in the carb doesn't completely close, thus the high rpms. Could I possibly have the wrong cable? Or is there some other problem here? I have played with the cable adjustments and can't find a way to get more "length" in the cable to allow the throttle to close further but can't find any way to do this.

If you are absolutely sure you have used all the adjustments.

What you do is pull the little end fitting cap off to expose the plastic covered cable. Cut and peel off ~1/4" of plastic. Unwind 1/4" of the coiled housing. Carefully use a Dremel or hack saw to cut the coiled metal housing. You don't want to cut all the way through as you may damage the cable. Just cut it ~80% and fold it back and forth until it breaks.

You didn't say if you tried adjusting the idle adjusting screw. The idle adjusting screw is the biggest screw on the side of the carb. It adjusts the idle speed by raising or lowering the throttle slide in the carb. Back this screw out a bit to see if the slide lowers and causes the idle speed to drop.

Before you do any cutting, why don't you remove the cable from twist grip. Then one of you can start it and the other can pull on the cable directly. Since you removed it from the housing you will gain some extra length and you will see if it is the cable length or the carb slide not returning causing your problem.

Thanks for the suggestions. What I ended up doing is trying the old cable (a previous owner had cut the end off of the oil pump cable to "disable" it) which actually had about .45" of extra length on the throttle end. It worked like a charm....bike runs great, no lights working and both speedo and tach inoperable but I guess those problems are for other posts....thanks again for the help...

one last thing....if I do the cable mod suggested for my new cable, which end of which cable to I do the cutting on?

You may need to heat the nipple to get it off, but I would work on the carb end. Straight pull there so if you wound the cable it would be less likely to fray.

I posted this message once before in this forum and it happened to be his problem. Maybe it applies in your case also?

When my MT does this it is usually because I have pulled the cable out of the Y where the Oil Pump cable splits off. Make sure the cable is seated properly on all 3 ends of the Y. Also if the cable adjuster jamb nut on the top of the carb isn't tight the adjuster has a tendency to move out while you screw slide cap back on. The slack adjuster is typically hidden under a rubber cap

_________________
74 MT-250
06 Husky TE-450