I have a Mikuni TM45 flatside carburetor that is used on harley davidson motorcycles with twist throttles. I use a thumb throttle on my ATV. The throttle return spring is too stiff. Where can I get a different spring for the carb? If there isn't one that fits, how can I modify it so that it isn't so stiff? Also, I need some kind of very, very small tool to remove the throttle lever lock pin to get the spring out.

 

I have not done this but I have heard of people cutting a loop of the spring off and reinstalling the carb top
Paul

 

I don't think I would cut any coils off the present spring.. shortening the spring would only make it stiffer. There are some after market attachments for the thumb lever that claim to lessen the effort required, but they look kind of stupid to me. I'm sure you have thought of the real solution, which is a twister. I use them on both of my quads, even the KQ! TOTAL CONTROL!!

 

I have heard that cutting loops out will actually make the throttle stiffer, but that wasn't the case on my ATC250R. I cut a couple of loops out and it's action was noticably lighter. That doesn't mean that all react that way though....

Personally I don't like a twist throttle on an ATV. I don't mind them on a dirtbike, heck, that's what I started on years ago... But it's just different on an ATV, I guess it's the body english used....

 

Maybe I'll try to make my own spring out of wire or I could have a custom spring made. Or I guess I could switch from a torsion spring to a tension spring and come up with a way to mount it on the carb and the throttle - the distance the throttle rotates is probably less than 1/2 a turn.

 

Keep in mind that the spring also keeps the throttle closed against opening from air velocity. Like when you are jamming down the trail and suddenly have to let off on the throttle and jam on the brakes -- the engine is still trying to suck fuel thru the carb, and if the spring isn't strong enough, the slide will open and the engine will continue to rev.

Could be a dangerous situation.

Just something to think about before you cut too much.

 

If you want to try something to reduce thumb fatigue, go to this website.
www.yakkeyracing.com and check out the Silver BATT. It is a piece of aluminum that slides over the thumb lever. The added weight of the aluminum gives more leverage to your thumb, and it makes the thumb throttle much easier to pull. I have one of these, and I absolutely swear by it.

 

Here's what works for me.

Take the spring and put a phillips screw drive down the center. Go to the grinder and push the spring with the screw driver to the SIDE of the grinder. While the spring is rotating put your thumb on the spring (wear gloves). This slows the spring speed. Now your grinding the outside of the spring down and it will make the spring wire diameter smaller. Only do a little bit at a time.

Don't cut and loops off the end it could cause some major throttle problems. Mostly throttle not closing completly.

I hope you could understand what I wrote, a if not email me and I'll draw a diagram and fax it to you.

Waymod

 

Both methods require some common sense.

My spring on the 250R was way long, and I could easily take some loops out with the spring still being compressed at probably a 2:1 ratio when mounted and fully extended, still way more than necessary to close the throttle securely.

I am kinda nervous about grinding on the spring, if you screw up just a little, you have a thin spot in the spring and are asking for a broken spring. Also you risk losing the temper in the spring further weakening it.

I think the best solution would be to find a comparable length, lighter weight spring.

 

you guys are forgetting the possiblity of the cable binding and/or dirt inside it. i changed my clutch cable and noticed a big difference in resistance. dirt/corrosion had gotten into the cable sleeve and caused it to slide improperly. i would check that before cutting up your springs....