1966 Monza 250: throttle is binding

[cooperplace]

I've just put on a brand new Mikuni VM26, and a new throttle cable, teflon lined. The throttle assembly is a Tommaselli and looks original. Some previous owner has removed the throttle stop screw thingie, which is fine, and the whole throttle assembly seems to work nicely. The carb slide goes up and down beautifully. The return spring feels just right. The cable slides nicely. I've routed it to the right of the central spine and then around the RHS headlight stanchion.

But when I put it all together, and open the throttle, it stays open. On letting go of the throttle, there's just enough friction in the system, somewhere, to stop the throttle from closing by itself. I don't like that: it might kill me.

I've put some talc on the handlebar, still doing it. I'm about to go to the hardware store and get some 500 grade paper and polish the handlebar. Meantime, any suggestions as to how to fix this will be much appreciated.

[cooperplace]

I've decided to replace the handlebars. There's pitting and evidence of rust under the chrome.

Sometimes I wish that I found Hondas, Kawasakis, etc., as fascinating. I'm sure they're much less trouble.

[motoital]

Did you take the throttle apart to r&r the grease? If that hasn't been done in awhile that would help greatly. I'm not sure if they make different spring rates for that carb but that can help too.

[cooperplace]

yep, I took the throttle apart. The throttle itself seems to work beautifully. I'll let you know how it works when the new handlebars are on.

[cooperplace]

well, I've put on new handlebars, and still the throttle sticks.

Time for a new throttle, I think. or a stronger return spring in the carby, which I'm not keen on.

Does anyone have any recommendations for a throttle assembly for a 250? I'd like one where the twist-grip doesn't bear directly on the handlebar, but rather on teflon.

[DewCatTea-Bob]

[quote= cooperplace ...
" and still the throttle sticks.
Time for a new throttle, I think. "

____ Have you either tried it with the fuel-tank off or with the throttle-cable disconnected from the hand-throttle ?
If the throttle still sticks with the tank removed, then how about posting a picture showing your throttle-cable's routing-path from the forks back to the carb ?
And it would be revealing to learn whether or not the cable's throttle-position still refuses to return to the closed-setting, when worked-through it's full action-extremes by means other than with the use of your suspect hand-throttle.
These experimental-trials should be tried-out before consideration of acquiring a replacement-throttle. _ Don't you already have any other hand-throttles to substitute & try-out ?



" Does anyone have any recommendations for a throttle assembly for a 250? "

____ Why specify for a "250", when your installed carb is-not stock for ANY Duke-model ? _ Are you considering reverting back to a 24mm-Dellorto ?


Hopeful-Cheers,
DCT-Bob

[StewartD]

Hi Cooperplace,

You didn't say whether you were now using grease or are you still using talcum powder, in answer to Motoital query. I've always used grease on twist grips. The Mach 1 grip runs metal to metal and gives no problem. Here's a photo of the standard bits. Is yours the same?

DSCF3795.jpg

These are a nice design as the little chain does the bending; the cable close to the nipple is not rolled around the drum as in normal twist grip designs. I'd be reluctant to use non standard items if this is the one you have. I think the problem is either the talc or something else.

Cheers,

Stewart D

[JimF]

I use grease on the twist grip, but a drop or two of motor oil as the cable lubricant.

My Mikuni does not have a strong spring, no where near the stiffness of the Del'Orto carb spring so a non-binding cable is a must.

I have also had the situation where I would twist the hand-grip and the sheath of the throttle cable at the top of the carb body would climb out of the hole in the carb body and catch on the lip of that hole.

This effectively makes the cable shorter. I would release the throttle and the "shorter cable" then would not let the carb slide move down. It seemed as if the cable was binding, but the cable moved freely within the sheath while in reality I had too much slack in the sheath.

Jim

[cooperplace]

Hi everyone,

thanks for that info. Mine is a tommaselli, perhaps not standard. I'll post a phote. It's running nylon on the chrome plate of the handlebar tube. I haven't run it with the tank on, just tank off. Pulling on the cable nipple by hand, all is perfect. The carb is brand new, slides beautifully, but doesn't have a strong return spring. There is definitely friction of the nylon on the chrome, which I think is my problem.

If anyone can recommend a nice replacement throttle, I'd be grateful.

[DewCatTea-Bob]

[quote= JimF ...
" My Mikuni does not have a strong spring, no where near the stiffness of the Del'Orto carb spring "

____ Jim has to be referring rather to a SSI-carb, because there could be no weaker of a return-spring than as that employed within a stock 24mm-Dellorto (as original on 250-Monza) !


Dukaddy-DUKEs,
-Bob