Concentric idle and pickup "Truckle Slide"

[David Taylor]

The Concentric "Truckle Slide"

It is possible to get a stable idle (truckle) and perfect pickup every time with a very simple modification to the slide. After much experimentation on my Triumph 250 Trail Blazer I found that filing a 1.7mm deep notch in the slide as shown in the photo produces a reliable 1000rpm idle, good pickup and smooth relaxing riding in top gear at low rpm. Mid range and top end performance remain the same. The notch should be of the same radius as the carb throat so it blends in at full throttle.
The slide shown in the photo is very worn and in unmodified form produced an unreliable idle, fluffy pickup and the bike could not be run at low rpm. This simple modification completely cured those problems even on this worn old slide!

It is an easy mod to try. Start by filing a 1mm notch, test it then file some more off until you like the result. If you file too deep you will get a too fast idle. To fix it, shave a little off the slide skirt.

I hope this helps!

Screenshot_20181017-162645_3.png (212.96 KiB) Viewed 8588 times

[beat]

hi David

I do something similar on my Mikunis TM Flat slide throttles .

I say in principal, it is nothing else than giving some additional air and fuel in to the engine in a throttle position where commonly only the Pilot System is function.
and I do also made very good experience with this modification !

a bit trikky not to give too much, - means - not to mess up the idling.

left the one with the mod.

beat

[eebtr7]

Yes, I saw this trick in an early 1970's issue of 'Big Bike' magazine wherein a 1/8" square notch was filed into the base of the slide. This applied to both the Concentric as well as the Monobloc carburetors.

But, as the world was turning away from British machines as the companies went out of business, the trick was quickly forgotten with the Brit bikes and their AMAL carburetors being tossed into the junk piles.