On my 2 stroke bikes I have the slow black throttle tubes - just because I am a pokey guy.

I just got a 08 Beta 4T and the throttle is a bit too sensitive for me. Since I had a spare slow throttle tube in the parts bin, I thought I would swap the two. But when I pulled the 4T apart, the throttle tube is quite different (see the picture).

Can I swap them?

If not can I trim the 4T one down to make it slower?

Looks like the 4 stroke has a non-linear cable pull ratio. This is usually done to smooth out the action of the throttle when it is near closed. Trimming the ramp to make the action more linear will not help you with off-idle throttle sensitivity because the ramp is the bit that affects throttle opening at the other end of the throttle action

You need a tube that has the cable end holder following a smaller diameter circle - like the other one in your photo

Edited December 4, 201114 yr by feetupfun

you could get use to this with practice but if you ride 2t aswell as 4t then you will struggle ,you could use the full 2t throttle and cable and use a solder on type of end to fit the carb

I had a YZ 250 that I "modded" a ramp style throttle for like the one you show. It work really well. For the cost of a throttle tube i'd give it a go? I used a hacksaw to carefully lower the ramp height so the tube ket the same diameter. It won't change the inital cracking of the throttle feel but it will stop more wollop coming in so fast imediatley after.

My YZ was a pussycat over tree root covered side slopes after this mod. Previously it was a nightmare.

Feetupfun - I get exactly what you mean - smaller diameter, but keep the taper.

Pindie - I am with you - whats a throttle tube.

I am going to use both these ideas. Smaller diameter, still maintain some ramp though. I might need to drill another hole to move the cable nut (??) close to the center of the tube. I seem to recall that when I got a slower tube for my enduro race bike, a small change can make a big difference.

Well, it didn't work. Putting a deeper channel in the throttle tube broke through part of the structure. You cannot see if from the picture, but that ramp on the throttle tube is not solid all the way through. The channel I was cutting ended up cutting through the ramp. This probably would have resulted in binding - not a good thing.

But not to worry! I took my old 2 stroke fast throttle assembly from the parts bin, and rigged it to work. Even though its the fast throttle, it is a lot smaller than the 4 stroke throttle tube. I had to machine an adjusting barrel though (with a metal lathe) - so this mod may not be for everyone.

Works well in the shop - I will do a woods test tomorrow.

Edited December 4, 201114 yr by TrialTrial

Oh my! Never really thought about this, but it makes sense, as on the Rev 4t when you openthe throttle, you do not really open the throttle. You are only opening the throttle valve which must move near a full 90 deg I think, and the change in opening is proportionatly less at increased openings, thus the ramp style to accomodate.

With a liniar tube, you will have smaller changes at higher openings, yet not sure how much difference it will make at smaller openings, or just how much you will have to twist to get full throttle?

I did this. Machined the ramp off and made it non progressive. It is much better!

I also tried a "Fast trials (black) throttle" tube and it was a bit too slow but the bike was very controllable.

I'd think you could find a good no ramp throttle from the shop.

Oh my! Never really thought about this, but it makes sense, as on the Rev 4t when you openthe throttle, you do not really open the throttle. You are only opening the throttle valve which must move near a full 90 deg I think, and the change in opening is proportionatly less at increased openings, thus the ramp style to accomodate.

With a liniar tube, you will have smaller changes at higher openings, yet not sure how much difference it will make at smaller openings, or just how much you will have to twist to get full throttle?

Copey, it's been some time since I messed with the CV type carb, but would a slightly stronger slide spring soften the initial throttle response to a degree?

Jon

I also tried a "Fast trials (black) throttle" tube...

I thought white was faster (larger diameter), and black was slower (smaller diameter)

I thought white was faster (larger diameter), and black was slower (smaller diameter)

For the Domino type, thats true.

Jon

Copey, it's been some time since I messed with the CV type carb, but would a slightly stronger slide spring soften the initial throttle response to a degree?

Jon

I have not tried it, but it sounds logical, Jon. Do you think we should tell him?

I thought white was faster (larger diameter), and black was slower (smaller diameter)

Bugger! quite right I've got it the wrong way around. Sorry.

I've fitted slow tube throttle on my 4t with cable spacer. Nice improvement but considering alloy throttle tube from trial enduro direct to speed up the latter half of throttle travel. Anybody used one?