Holley carb needle & seat question for grumpyvette

[bigbore468]

Am looking for some information on Holley carb needle & seats; hopefully grumpyvette or any other carb tech guru can shed some light on this subject.

The specific application for the needle & seats are for Holley 4150 DP carbs with center hung bowls running pump gas with 10-15% alcohol, not e85.

Also .097" seats, non adjustable captured viton tip needle; like Holley P/N 6-506

 

I've noticed there are two different size diameter needle stems that Holley uses.

See the pictures for reference.

 

My questions:

Are these supposed to be interchangable? I've recently purchased a few genuine Holley 6-506 needle & seats and they have the fatter stem. NOS same part number needle & seats I have from years ago have the smaller diameter stem.

 

Is there any advantage to using one or the other with a particular float; either duracon or nitrophyl?

 

I know the alcohol in the pump gas will tend to swell the viton tip a bit, and could cause sealing issues; which is exactly the problem I'm trying to solve.

So assuming the fuel system is clean, float levels set properly, proper steady fuel pressure, etc., is there a needle & seat / float combination you would recommend?

 

Appreciate your advice.