XBR 500 - brake in acceleration

[guest1512]

Hi guys. I have the unsolved problem with my XBR. While accelerating I encounter something like an opposition when the frequency of a rotation is between 4500 and 5000 rpm. If I am  stubborn enough I can force it and above 5000-5500 rpm acceleration is normal.  This  problem occures mostly on 5 gear.  Air filter and a spark plug are new, also this year a camshaft has been changed and valves regulated according manual.
Did anyone encountered such problem or have any suggestions?

[guest295]

Have you cleaned the carbutetor? If it stood for a longish time the jets may have become gummed up with brown 'varnish' due to evaporation of fuel. Butane lighter fuel is an excellent solvent under pressure; squirt it through all the jets and orifices.

[guest1512]

Problem has been fixed. When a new sport camshaft with a longer times of opening valves has been mounted, the fuel support condition required another main jet. I knew about it, however I went wrong direction changing jets with wider and wider. Consumption was rising and power got down. At the end I drew attention to a black spark plug.
Final solution was a SMALER jet then original one.   

[guest1512]

After converting from XBR 500 to XBR 600 (with Honda NX 650 cylinder) a problem returned. However I have no break in acceleration but my engine don't react on operating with a carburator's screw. Normally screwing in should low RPM. Now I can screw it to the limit and there is no reaction. And a spark plug is black. I think it is time to replace the main jet with a smaller one. But I already have only a #142 size. Has it any sense? Voodoo or something?

[guest564]

I didn't run the stock carb with a modified cam but on the Amal I had to run a bigger throttle cutaway to compensate for the much richer mixture when initially opening the throttle. On a CV I would suspect that you need to change the spring or alter the air passage way, the main jet will probably still need to be bigger than standard.

[johnr]

if it was stuttering at 4000-5000  revs, then i suspect that the needle in the carb was at fault rather than the main jet. the main jet regulates the ammount of fuel in the fuel air mix at high revs, its the needle that regulates the mix from 1/3rd to 2/3 revs. try lifting the needle a slot, that is, move the circlip on the needle down by one slot.