63 replies to this topic

[carbon]

If you can use a Maniflow steel inlet sized for twin HS2 this should give good results.

 

For blending the carb internals avoid any changes to the 'bridge' area as this will upset mixture at low-mid rpm.

[grizzler73]

Unfortunately I can't us a maniflow inlet as the regs don't allow it. Carbon, so you think I should not do the squaring of the bridge area, just thin out the spinal area? Also back to my original question, if I should do the squaring, should it be just the inlet side or the engine side before the bridge as well?

[hhhh]

Just came across this old thread and the last question never got answered regarding squaring the SU's bore at the piston. If you were only going to do one side of the piston, it wouldn't be the upstream side since that would make the obstruction of the other side even more prominent. If you only did the downstream side, it would be like an exhaust pipe bigger than the port at the gasket face which is good. It wouldn't obstruct flow at all but would improve it since the sharp corners are gone where the air exits the region below the piston.

 

As I understand it, the bridge only refers to the floor of the carb under the piston, not the bore above it. Vizard says bore modifications don't affect road manners, but I would expect a richer needle profile would be needed. I'd be interested to see a  comparison between just the downstream part squared vs. both. I doubt it would be significant since the diameter at the mouth of the carb presents the same restriction regardless, even more so without a good 1/4" radiused entry. The improvement is most likely from the sharp corners being removed after the piston bore.

Edited by hhhh, 29 January 2018 - 01:21 PM.

[Wim Fournier]

Remember that the intake of an engine is not with a 'constant vacuüm' in te intake manifold. The air is taken in in pulses at a time. Such a pulse in a bigger throat gets lesser deep than in a narrower throat. So, the re-action of the damper piston in the carburettor dome is quicker, taken in account the greater circumferance (rubbing the wall) and the lesser surface and use of a lighter spring.

"This retardation in the piston rise causes the amount of ‘Pull’ on the fuel leaving the jet to increase and thus more fuel is drawn off and this enriches the mixture during the acceleration phase". 

It's this 'Pull' that you need for acceleration. At lower rpm the quantity of air taken in is still so low that the vacuüm around the needle is still quite low. So, a deeper vacu¨m pulse through a narrower throat will help.