hi all, i have got cbr600 bike carbs, with 1.5mm jets and the i have the timing set at 11 BTDC, and it revs very high most of the time such as about 2500rpm or 2800rpm, but then some time is comes down to the about 800rpm, but it takes a bit of time to come down. i dizzy i have is a vacume take off one, and i had a vac/dizzy vaule in it, and i took that out and it worked beter, but if i left all the pipes off, it run rubbish, but the revs came down even quicker.

so if i blank of the vacum take off of the carbs, and leave the dizzy open, what would that do?

as well have other people have had this promblem?

and what has others done to the carbs jets size and timing?

thanks

stuart

Mine used to do that, needed a return spring fitting to close the throttles. ( On the bike they would have had two cables fitted, one to open the throttles the other to close them).

Have the carbs been balancedso that each one is drawing the same amount of air at idle? Do you have any air leaks? Are the flyweights in the distributor free and not sticking in the advanced position?

try another return spring

hi, i will fit a spring on the carbs this week, and see if it helps. i have not the gear to check the balanc, but me and my dad tried to listen to them and they sound that they a drwaing the same air in, and all plugs are the same coulour. so, i would have throught there is no air leaks.

the dizzy i have has no points in it. so not sure about that

stuart

As a temporary solution block the vacuum pipes on your manifold
then advance the timing to 34-36 % that will get it running
you really need a modifide dizzy or a megajolt to do the ignition
Jacko

On my manifold you can see 4 bolts that's where i balance my carbs by using a vacuum gauge just block 3 fit gauge read the gauge then move to the next and block the first

painted manifold

An extra return spring will fix it

Difficult to diagnose without seeing it, but as said already id be blocking the vacuum connections, fitting extra return spring and re-tuning.

Engine should run on std dizzy for now, but perhaps not optimised (hence modded dizzy or MJ later). I found revs didnt fall sometimes and it was due to throttle return, extra spring made pedal a bit heavy but brought revs down as expected - the butterfly's just werent quite closing enough sometimes. You can prove this theory by pressing against the linkage when engine is running to see if the revs drop.

I used a balancer that fits in carb mouth, but ive heard that piece of tube and ear can be surprisingly accurate.


On your dizzy you say you have removed the vacuum advance 'can' off the side. Doesnt this have an arm inside that operates against the advance plate? I would have probs left this all in situ. Vacuum advance only operates at low revs / high vacuum to aid fuel consumption. It higher revs it has little effects. The advance weights then bring in full advance at a certain rpm point (from memory maybe 36degs at 3,800 rom - but i could be wrong).
As long as manifold isnt sucking air in where the dizzy would normally connect to on a std car you should be fine.



Are your needles all at same height and a good fit in the carbs?