Using a Tri-Z thumb throttle with 38mm carb

[El Camexican]

Soon after my first ride on my 250 I realized that a twist throttle in the tight stuff was going to get me hurt, so I picked up a stock throttle assembly with the intention of switching systems.

Not long after finding a nice used one I read that the stock assembly would not open a 38mm carb all the way, so it went into the parts bin with the rest of my good intentions.

Fast forward to this past weekend when the trike tried (and succeeded) to show me who was in charge a few times and I’ve made a vow to never ride it on a trail or mx track again with a twist throttle.

Started looking at aftermarket units and while there is some nice stuff out there I thought I’d take a look at the stocker one more time before I dropped the coin on a new one.

So here’s what I found. The arm swings in an arc, but because the cable pulls straight (there is no cam in the housing) the distance of cable travel can be measured in a straight line. I concluded that distance to be 32.51mm.


Inside the housing there are cast protrusions on either side that act as stops for the lever. My thought is that I can remove material from these stops as well as the unstressed side of the lever to increase the range of movement the cable lever has.


The first attempt added 2.98mm to the range, so I took a little more off all three contact points. This netted another mm for a total of 36.49mm, about a 4mm gain in total.


Given that the carb slide is slightly open to start with this might be enough, but if not I think I can easily get another 2mm of range if need be. When time permits I’ll check if the current cable will fit the stock assembly. I may even be able to use the fangled stock throttle cable assembly.

Has anyone else here messed with one of these before? Did it work?

[yaegerb]

This may be dumb, but could you use a 250r thumb throttle and put an aftermarket cover on the housing so people won't know it's a Honda thumbie? I only say this cause my 38mm works perfectly on my R with thumb throttle.

[El Camexican]

This may be dumb, but could you use a 250r thumb throttle and put an aftermarket cover on the housing so people won't know it's a Honda thumbie? I only say this cause my 38mm works perfectly on my R with thumb throttle.

Could be an option if this doesn't work. Thanks!

[yaegerb]

Could be an option if this doesn't work. Thanks!

No worries. I feel your pain with a twisty...after a few accidents me and my motion-pro parted ways.

[3 Wheel Drive]

I'm using a banshee thumb throttle with a stainless banshee cable on my 39pwk. That would be cool if you can get the stock tri z thumb throttle to open up your 38mm carb.

[El Camexican]

No worries. I feel your pain with a twisty...after a few accidents me and my motion-pro parted ways.

Tell me about it. I scared the out of myself last weekend. Got the nerve up to try and get a little air time on the warm up/kiddy track. Landed near the end of a table top, twisted the throttle against my wishes, launched off the top, landed when I should have been starting to turn and again unwillingly hit the gas. It was ugly to say the least, but I managed to stop before I launched off the berm into the bushes. A guy came over later and said "That thing sounds angry and it looks like it would like to kill you". I'm a trike advocate all day long, but the guy had a valid point. Power band is like a light switch and it's impossible to work that throttle tube and exert the body language the thing demands all at the same time.

[Billy Golightly]

The only way to get the stock Z thumb to open a carb that size or bigger is to actually cut a hole in the housing for the arm to pass through and stick out from ( crazy I know, but have seen it done before). Our resident Tri-Z guru that has since passed on, Yamahondaman, always ran and suggested everyone to run 250R throttles because of their cam design and the ability to fully open big carbs.

[YTZ drew]

If you end up wishing to try a 250R thumb throttle let me know, I have one I'm not using (sadly I don't own an R). It has a linkage inside that I haven't studied much, but it appears to slow the initial throttle application.

[yaegerb]

The only way to get the stock Z thumb to open a carb that size or bigger is to actually cut a hole in the housing for the arm to pass through and stick out from ( crazy I know, but have seen it done before). Our resident Tri-Z guru that has since passed on, Yamahondaman, always ran and suggested everyone to run 250R throttles because of their cam design and the ability to fully open big carbs.

Sweet, glad to know it was an ok idea

[torker]

If your experiment does not work, a banshee or HONDA throttle and a blaster cable (non TORS) will do the trick. I use Honda throttles and the non TORS blaster cables. Works for me.
Good luck man.

TORKER.

[El Camexican]

The only way to get the stock Z thumb to open a carb that size or bigger is to actually cut a hole in the housing for the arm to pass through and stick out from ( crazy I know, but have seen it done before). Our resident Tri-Z guru that has since passed on, Yamahondaman, always ran and suggested everyone to run 250R throttles because of their cam design and the ability to fully open big carbs.

Awe crud Well that's good to know, but I'm still going to try it before it goes back in the Rubbermaid box. I gained about a 2mm head start by removing material from the closing post as well. Not sure if the person who cut the hole used that area as well YTZ drew, I may take you up on that once I give up on this attempt.

[onformula1]

Before you go to the "Dark Side" & since you ride a KTM motorcycle, maybe try a adjustable twist throttle and set it up on the enduro setting, they work great, last forever and you will not need a box of bandaids!....LOL- old school trick we used to use for kids on a ATC70. (wrap them around the thumb part)

They have used Magura's on (KTM's) for years. Like your trike.

http://www.amazon.com/Magura-Motocro...agura+throttle

They do make a big difference.

[NeverLift]

I have your same carb on my Z and I'm using a 400ex throttle(same as 250r). I had to do the same grinding your doing and I can open my carb all the way with a mm to spare.

[250rRoostmaster]

TRX450r thumb throttles work as well, just have to grind out the cast portions kinda like what you described earlier. I have a thumb throttle from a 07 trx450r on my bike. Nice new throttles feel great.

[El Camexican]

Took a few more measurements last night and I must concede that it can't be done. I figure 38mm of pull is the max and that would require extending the arm a couple mm. On top of that the cable, when slack is considered needs a full 40mm of travel to work on the 38mm carb.

Thanks to all