venturi I.D. .25 FP?

A friend of mine is just learning to make Delrin venturi's on his lathe. We need to know the approximate I.D. for a .25 FP venturi using a .156 (?) diameter ST.51 NVA. (Maybe we should be looking at a smaller diameter NVA for this smaller engine as well?) We're shooting for a stunt run, we aren't racing. Any suggestions?
Thanks,
Nils

>A friend of mine is just
>learning to make Delrin venturi's
>on his lathe. We need
>to know the approximate I.D.
>for a .25 FP venturi
>using a .156 (?) diameter
>ST.51 NVA. (Maybe we should
>be looking at a smaller
>diameter NVA for this smaller
>engine as well?) We're shooting
>for a stunt run, we
>aren't racing. Any suggestions?
>Thanks,
>Nils

Box stock, the CL version of this engine comes with a .257 or .260 venturi and uses a .135 diameter OS needle. This is really too big for the stock engine...but...with the ST needle (.155 diameter) the .260 will come out about right. What you are looking at is the choke area, so you need a larger venturi with a larger needle going through the middle to give the same amount of choke. If you have the ability to make your own venturis, you may want to make several, one a little larger and one a little smaller than this, and try them out to see what works best for you.

If you need O-rings, which are a metric size and a bit hard to get, I have some.

Thank you Leonard. I will relay this info to the guy running the spinny thing. If we need O rings I'll email you.
Nils