Silly tuning question

[davreeve]

I currently have a pair of 40 dcnf,s fitted to my engine with 36mm chokes.The inlet manifold starts off plenty large enough where the carbs bolt on but at the join to the head the diameter is down to around 30mm.My question is should the manifold be opened out or is it like this to increase air speed?

[Muttley MK2]

What size is the gasket? What size is the port in the head? Your chokes are there to speed up air flow through the throat of the carbs and to help atomize the air/fuel mix. The fuel charge then needs to be kept up in velocity so that the fuel and air stay mixed. If you make the trip from the carb to the cylinder too slow the the fuel will drop from the air. Too quick and it will lean off. The most you should do is to match the port and manifold to the gasket. If you do not have a paricularily hairy cam then this is sufficient.

[davreeve]

The ports in the head are matched to a standard gasket which is roughly(the manifolds still fitted so cant measure at the moment)5mm larger in diameter than the manifold ports.

[Muttley MK2]

With mild tuning whats important is keeping a smooth and constant flow of mixture. Gasket matching is a simple and very effective way of doing this.If you have a sudden change in shape or size then you will spoil this flow. If you have a step up or down in size this is a bad thing. Smooth these out and the overall efficiency will be improved.

[davreeve]

Cheers for that