Hitchcocks Forum

HI All,
I have a '03 350 bullet, straight through pipe, no air filter just home made velo stack 4'' long (for looks really! ), bike runs fine but the last 1/4 of throttle doesn't seem to make much difference to acceleration/ power. eg will be opening it up to 3/4 and will accelerate/pull then after 3/4 it makes no difference.
Any recommendations as to if its the carb or engine?
Thanks
Pete

Should add i have tried up a notch on the needle and this runs FAR too rich

Hi guys---------- JACKDAW, I don't know of a set-piece answer but here's a line of thinking to start off with}--------With most "cooking" bikes the last 1/4 throttle usually doesn't make that much of a real difference anyway as I expect the jetting at this stage to be slightly rich for protection against piston seizure..... BUT, if upping the needle by one notch makes it run FAR too rich as you put it then that gives me a clue as to the air mass flowing into the engine being not enough to match the fuelling rate, maybe ??......So perhaps a slightly SMALLER choke carb would increase the air mass velocity ???? ( A larger choke would REDUCE the flow rate ).........

That's quite normal on the 350. The inlet tract is fairly restricted and needs opening up and a larger carb fitted to release any more performance. That's what Mr H does with his "tuned" head though I would think a 28mm carb suggested by him is a touch too much. Tollgate suggest a 26mm carb which sounds more reasonable for everyday use.

Yes I agree with Pete the carb itself is bigger than the inlet tract causing your lack of any increase in the final opening. Alans idea of a smaller carb would make the last quarter useable but would not give any increase in power.

To be honest I think the only way to get any increase would be to fit an oversize valve and seat and gas flow the inlet tract, but in real terms tuning a bike takes far more than this and on it's own would make little difference.

That stack is Too big - smaller stacks boost top end so try hosts small bellmouth. Longer stacks are better for torque.

hi all,
some interesting pointers, it does feel like the engine could give a little more but can't so i will have a play with velo stack length then carb diameter.
Thanks for now, will keep posted.
Pete

Maybe...


Lack of response after 3/4 throttle is often a sign of a main jet that is too small.
A bellmouth of the type you describe may well give a weak mixture at full throttle - too much air for the amount of fuel through the main jet.
If moving the clip down the needle gives a VERY rich result then the needle jet may be too large.

ATB