I have a Tom Lay reworked ST51.  Today, for no apparent reason, the needle broke off so I had to take the NVA out.  It so happened I have a new ST51 NVA (for a G51 which my engine is) from Super Tiger.

When I compared the NV that came out, to the new NV (looking only at the orifice location) I saw that the old NV had the orifice "significantly" offset from the center of the venturi.  The new Super Tiger NV had the orifice "significantly" offset from center as well, but the opposite direction as the original NVA.  I thought all needle valves would ideally have the orifice in the center of the venturi?  Is that true or not?  Are all ST(G)51 orifices offset?  Any idea why?  (Obviously I have more questions than answers.)

Mystery (sorta) solved.  Took a look at a stock G51 today.  The "stock" venturi and NVA mount through the motor casing behind the venturi which is a much narrower area.  Tom Lay puts a different venturi in and apparently drills the motor casing through the venturi (as do many manufacturers) to capture and hold the venturi.  The result is the NVA now goes through a much wider part of the motor casing resulting in the orifice "off center" affect.

The remaining unsolved part of the question is...how much, if any difference, the offset orifice actually makes.  Any thoughts?

It is all "up in the air" (pardon the expression) until we find out a date.  My brother cannot go on on the weekend of the 15/16 because of a prior commitment.  If that is the case, I will probably not go as well.  You know, I lose my free room and board.  My wife cannot go if it is the weekend of the 22.  That would only change my leaving and returning home date but I would still go.  As soon as we get dates, I will let you know.

Mike

Considering that the fuel metering system used in clpa is about as crude as it can get and not changed in over 75 years. I think that the hoizontal offset makes little or no difference so long as the hole is not obstructed. I do know that if I had a setup like that it would bug me to no end. Kind of distrust those remote needles for similar reasons.

Peter's answer is pretty close to my view. I have put plastic washers on both ends of the spraybar to center the hole in the venturi bore, and found that to be a bad plot. The whole spraybar turned. Aluminum or brass spacers would work ok, if you get a good seal. Adding more places for air leaks doesn't seem like the best plan. I'd prefer the hole be centered, but if it works, it's good, and then wouldn't bug me at all. Steve 

Hi Steve,
I have a MVVS 40 (and yes the factory did indeed make a stunt version!) that came standard with a peripheral jet carby insert and a side mounted spray bar which looked great but was rejected because the engine ran too fast and needed to be choked down. And remaking a peripheral insert is a real pain.

A friend of mine made an insert for me and we tried to reuse the standard spray bar in a 'restrictor' fashion but the jet was way off centre - it needed to be a much longer spray bar in order to shift the hole over. So I swapped it out for an Enya spraybar and the jet is now perfectly central.

Can't help but think that the jet's central placement would best avoid the boundary layer issues that I wanted to avoid with the standard peripheral setup.

Correct, a Aeroproducts etc etc or V 60 spraybar should get it central for drilled case , bar accross venturie .
Initialy Id thought wording was hole tangetally of for c/l of needle, giveing eclipse opening @ neddle point .

The suction should be stronger at the centre of the venturie, the 10 % at the walls is decelerated from drag .