CB72 Carb - Throttle Slide not fully open

[Phil-UK]

Hi

Found something odd with L/H carb on my CB72 which may or may not be making a difference to full throttle performance and wondered if anyone else has ever seen it.

Basically the throttle slide does not fully open and the engine side of the slide sticks out slightly into the throat of the carb ( 1mm-2mm ), and it is not down to cables or adjustment.

If I assemble the slide ( with needle ) leaving out the cable, return spring etc., into the carb body I can position the slide so it is fully open and clear of the top of the throat. However, if I then fit the carb top, rubber washer and screw on ring as I tighten up the ring I can see the slide gradually being pushed down until it just sticks out on the engine side.

I have measured the length of the slide and compared with the other carb and spares and all are the same height ( 33mm on the engine side ). Looking at the rubber washer I can see marks where the top of the slide has compressed it, it does not fit inside the hole - the R/H carb is the same,

It seems that the slide should almost be flush with the carb throat, the R/H carb slide is like this when fully open- unlike say an Amal where the slide dissapears quite a way above the carb throat.

If this having an impact it may affect the Power Jet system by disturbing the air flow as well as not allowing the needle to be as far out of the main jet as it should.

My thoughts at the moment are to see if I can find a new rubber washer to see if that helps or to relieve the top of the slide so allowing it to move further up.

Any comments or ideas ?

thanks

Phil

[mike in idaho]

Can you get the slide all the way up by pulling the cable up at the carb top? Perhaps the slide inside the cable junction box(where the two carb cables meet the upper cable) is bottoming out at full throttle. Run the cable adjustments out on the carb tops a little bit, then readjust the upper cable (at the throttle) to get back the free play needed. Reading this over , it occured to Me that maybe your right hand carb able may be adjusted to lift that slide farther, causing it to hit the carb top before the other one is fully open. Back off both idle stop screws an equal number of turns (so you can return them to their settings later) so both slides bottom out in the carb bodies. Adjust the cables until both slides lift at exactly the same time when you open the throttle.

[Phil-UK]

Hi Mike

Thanks for your reply which gave me some more ideas.

I changed the rubber washer under the carb top and that got the slide just about flush with the throat when pushed up. Then thiinking about what you had written I opened up the throttle cable junction box and found that the nipples on both cables from the carbs stuck out beyond the white plastic "block" that all 3 cables connect to - effectively limiting the amount of movement.
I filed the nipples down so that they were level with the block and that gave me another mm or so, now when the twistgrip is fully open both carb slides are clear of their respective throats by at least 1mm.

cheers

Phil