Motorcycle Repair: throttle linkage / throttle cables 1989 ZX-7R, throttle cables, throttle linkage

Question
Chris,

I own a 1989 ZX-750r, the H1 model, not the 750F, its the racing Ninja.  I have a problem with my throttle grip when I roll it forward from the spot that it stops at on its own, the idle drops and the carb balance actually changes, I put my balance gauge on it to see it, what happens is when I balance the carbs at idle, then if you roll the throttle forward, the 3&4 carbs go up about 2CM Hg, the 1&2 stay the same, and if you rev it up to 3000 rpm, the 3&4 draw in about 4-6 CM Hg more than the 1&2, now if I adjust the balance at 3000 so its even, and let it go, without rolling the throttle forward, the sitting position is 1&2 is 2 CM Hg higher than 3&4, but if you roll the throttle forward, they go up 2 CM Hg and all balance together, so my question is what the heck is causing this forward roll?  Is it just dirty or bad throttle cables?  Could the twist grip itself be bad?  In the same experiment I did loosen both throttle cables all the way up and it still does it, so one is not pulling the other I don't think.  this bike does have the two throttle cable set up.  Have you ever encountered this before?  Is it that the throttle linkage at the carbs is messed up? or is it strictly in the throttle cables?  I am pretty sure that the cables haven't been cleaned or lubed in a few years, so I am going to try that, other wise the only way to make it not do this is to make the decelerator cable so tight that it pulls it all the way forward, but that is so very very tight that if I turn the wheel it drops the idle too, so I am very confused as to what the problem is.  So any help or thoughts would be very appreciated.

Thanks for your time,
Jeff Lundell

Answer
Hi Jeff.
 On a push-pull cable setup, when the throttle is all the way foreward (idle), the carbs should all be pulling together.  What they are pulling is only important to performance.  They should not change their comparative pressures because the throttle is applied or released.  It should be the same for all four carbs at whatever setting the throttle is at.

 There should be some slack on the push cable as well as the pull cable at idle.  This will give you some freeplay on the throttle (approx. 1/4 inch) at idle with the grip all the way foreward.

Since you mentioned that the cables have not likely been lubed properly, as is recommended, then part of the problem could be the cables.  However, the problem with the cables may have caused the carbs to be further desynchronised at the last synchronisation.

Check that out and see what you find.

Good luck.
FALCON