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TT carburettor throttle slide return spring

Submitted by david_cooper on Fri, 02/04/2021
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After maybe 1000 miles of use over a few years, my throttle slide started becoming reluctant to drop cleanly when closing the throttle. It turns out that the return spring is not the current type. Also the top of the slide does not have the circle recess. I have a new spring coming, but does anyone know if the one I have was the earliest design? The carb has been professionally re-sleeved by a well known specialist.

I think perhaps the narrow spring has buckled a bit. But I still don't know why the slide seems stickier than normal. The only change I made recently was to fill up with Shell V-power.

The picture shows the spring. (Air slide temporarily removed when I took this).

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Submitted by robert_tuck on Fri, 02/04/2021

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That spring does look too puny to do the job, I run a Dellorto SSI  which is a  virtually identicle carb (which came first??)  and it has a better set up.   I do lightly lube the slide with a smear of mollibdimen disolphide  (CV) grease , that does tend to richen the mixture a shade.I also wonder if a few microns of chrome on the slide would be a  better combination than brass on brass ?.

Submitted by david_cooper on Fri, 02/04/2021

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Thanks chaps. I just found a picture of an RN which, oddly, looks similar. I have a 'copy chrome' brush plating kit...maybe that's a good idea? I don't like two metals the same type rubbing. I'll find out shortly if the later large spring will fit on top of the slide. Also some moly grease...

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Submitted by robert_tuck on Fri, 02/04/2021

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With the SSI carb that slim  spring is only used on the small carbs  up to about 20 mm. Then there is a switch to a  big spring as  in Ians post.

Submitted by nicholas_clark on Fri, 02/04/2021

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As purchased my 1936 Inter's TT carb had the slim parallel slide spring as David has described. The spring was most likely original as the bike had been last used in 1960 before I purchased it in 1986. Some years later I changed the spring for the later conical spring as David is contemplating. The conical spring should be more efficient and not buckle like the slim parallel one. As you say the picture of the Rn spring is the same as the TT one. That picture is from the manual for Internationals up to 1953 and Manx to 1951. My 1936 parts book also shows a slim parallel spring.

Regards, Nick 

Submitted by david_cooper on Fri, 02/04/2021

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I look forward to receiving the spring I ordered. But meanwhile I've stripped it all down, cleaned the cable and twist grip, and the slide is now dropping properly.  At least I now know that it does have the correct internals. It seems to me that they stiffened the spring to make it more reliable, although the original design worked, as long as everything was clean and well maintained! Thanks for the information.

 


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