Intake diameter vs Carb size question

[hardcastlephil]

Good evening,

This might be a bit technical but I'm hoping someone knows the answer....

Doing some work on the 3 wheeler this winter and No.1 Job is to change the twin bike carbs for a single SU.

It's 2 cylinder 600cc (2cv engine) and has one carb for each cylinder.

I'm building a manifold for a HS2 (1.25"/32mm) carb. I was going to make it from 33mm internal diameter tube (so one 33mm tube to each cylinder), but it would be much easier for a variety of reasons to make it from 26mm internal diameter tube.

I'm wondering that as the carb is only 32mm - which is then divided between two cylinders, will using the 26mm make much difference? I was wondering if the would only need equivalent of 16mm per cylinder to run before it was restricted by the actual carb size and therefore the inlet size about approx 16-20mm really wouldn't make the difference?

Thanks!

Phil

P.s. it's quite a long inlet too - about 30cm in all. I used to use the same length when running twin carbs and it was okay.

[TooMany2cvs]

hardcastlephil said:

Good evening,

This might be a bit technical but I'm hoping someone knows the answer....

Doing some work on the 3 wheeler this winter and No.1 Job is to change the twin bike carbs for a single SU.

It's 2 cylinder 600cc (2cv engine) and has one carb for each cylinder.

I'm building a manifold for a HS2 (1.25"/32mm) carb. I was going to make it from 33mm internal diameter tube (so one 33mm tube to each cylinder), but it would be much easier for a variety of reasons to make it from 26mm internal diameter tube.

I'm wondering that as the carb is only 32mm - which is then divided between two cylinders, will using the 26mm make much difference? I was wondering if the would only need equivalent of 16mm per cylinder to run before it was restricted by the actual carb size and therefore the inlet size about approx 16-20mm really wouldn't make the difference?

Thanks!

Phil

P.s. it's quite a long inlet too - about 30cm in all. I used to use the same length when running twin carbs and it was okay.

If you're going back to single carb, why not use the standard manifold and Solex...? Alternatively, the racing boys are using Weber 32/34 DTML on modified standard manifolds.
Unless you've done some serious head work, the standard manifold diameter's not a million miles from the port size, so you're going to have a restriction there anyway if you go much bigger.

[hardcastlephil]

Evening,

Thanks for the advice. Didn't realise the racers used Webbers. I don't mind the solex, but i've just always had a soft spot for the SU. I'll have a measure of the port size tomorrow. I'm not really too bothered about speed, but I want it to run to its full potential.

Phil