mechanical error with 1978 Honda TL-125; need expert advice?

Question

I have recently purchased a 78 Honda TL 125 dirt bike that needed to be restored to be ridden. I have done everything within my power to make it run as best as I can and everyone tells me that I did a pretty damn good job. However; I have a problem with the throttle. Inside the top of the carburator where the slide, needle and throttle return spring are located, somehow it is dysfunctional. This confuses me because these parts are precision machined to fit perfectly, anyway, the problem is that if I open the throttle up all the way, the slide will stick in that position, effectivley making the bike "out of control". I can almost open it all the way but if I go any further it will stick, I know though that it is made so that I can open all the way. If anyone can tell me why it sticks and how I can fix it, I shall be eternally greatful. lol

I have operated the carburator without the cable it is definitely the slide that is sticking. I have found all existing burrs on the slide and I have sanded them smooth with fine grit sandpaper. My neighbor who is a long time biker came over and looked at it, he said it is possible that the slide is misshapen due to some kind of bending via pressure on the sides of the slide causing the gap for the guide inside the slide housing in the carburator to be tweaked closer together, effectively making the slide no longer perfectly rounded. He told me that if that be the case then my best bet is to go to my nearest motorcycle salvage yard and do some wrench twisting there to see what I turn up.

Answer 1

Paint the throttle slide with "dychem blue" (machine shops use it, or search the web).
Insert the slide into the carb, then remove it.
Where the dye wares off, is the contact point that is restricting the slide.
Then clean up the slide and carb bore with 600 wet/dry sand paper and buff smooth with "semi - chrome" (fine compound).
If you can't get dychem blue, paint the slide with a wide tipped permanent marker (magic marker).

Edit
Check the groove cut the length of the slide.
And check the pin in the carb, it rides on.
Sometimes "backyard" mechanics will jamb the slide in, without lining everything up and damage the pin.

Answer 2

is the slide sticking or the cable??

what I would do is remove the cable & run petrol or degreaser through it to clean it & then apply some light machine oil..( check the twist grip isnt sticking to while the cable is off)

I would check the operation of the carbie with the cable removed.. If the slide is sticking in the carbie I would pull it apart & see if its binding somewhere ,, clean it all with degreaser & reassemble (I think the carbie should be put back together without oil or grease) if it needs some lube in there to help it work I would use spray teflon OR the lightest oil I can find (spray you get for cooking) is pretty light..

hope this helps. if you can find what is sticking it would be easier to answer

Answer 3

CV or slide carb ? Oh well, either way it should have a spring to close it. Does the slide have a tight spot or burr on it that could be sanded off ?

Answer 4

Contact the vintage japanese motorcycle club(vjmcc.com)ask on there. Your neighbour seems to know what he's talking about .Personally I,d ditch the carb and fit an equivalent modern Mikuni one--end of problems!

Answer 5

Could it be that the needle is coming out of the main jet and propping the slide open? check needle length and position. maybe somebody raised the needle - or put the wrong needle in it. Take the main jet out and see if it still sticks.

Answer 6

It's probably a frayed throttle cable wire,get a new throttle cable.