I have a brand new NOS carb. I also have a fresh rebuilt carb. I also have a good condition older carb. I also have a really old pretty worn out carb.

They all do exactly the same thing - and I have no idea why. The power cuts out for a split second when you get past half way on the pedal. This is when the second throttle opens. If you slowly depress the pedal through the transition to the second barrel it will hesitate slightly at the same point but wont cut out completely.

I have a theory that when that second barrel opens quickly the vacuume in the manifold drops and the mix leans out momentarliy - but I still don't know what to do about it.

I have a fresh A14, brand NEW carb and this is the last thing I expected!

Does anyone know what causes this?

Check your timing. 5 degrees initial is the factory specification. I don't know if that's the cause or not, but it's the first thing I would check.

It's an A14 - is that correct for A14 as well? I see conflicting information online.. some places say 10 degrees some say 5.
-david

Some new information.. I had it running too rich. I leaned it out today and got it closer to proper mixture, the problem got a little better but is still there.

Also, with this brand new NOS carb, under power on the first half of the pedal the power is uneven. Lots of slight increases and decreases in power.. not harsh but definitely noticeable. I'm about to go buy a wideband O2 sensor so i can tell whats going on!

Perhap your A14 is not using the stock cam or exhaust???

If so you may need to richen the middle, perhaps by changing the pump.

Well if you changed the carbs and still have the issue?, it's not the carb.
Might have a bad fuel pump?, or clogged filter? Sounds like it's not getting quite enough fuel under load?, and it starves when the secondary kicks in.
IDK but if ya change the carb and it still dose it, should maybe stop looking at the that and try something else.
Good luck , hope you get it sorted.

I know this isn't a suggestion but I thought I'd pop in & make it an exclusive NW thread!

we've got Portland, Seattle, Edmonds, & Everett, pretty amazing.

do you have a 1200 NSR?

My one thought was fuel starvation though...the ignitions not cutting out is it?

I'd check the distributor is advancing enough. As for static timing, I have noticed US models run 5

did you use a new gasket and or some kind of sealant each time you put the carby on?. and check for air leaks around the base of the carby and manifold.

I had this hesitation problem with an A14 B310 Coupe I used to have some years back. It only arose after I fitted extractors and 2" exhaust (Otherwise Std engine).

I tried all sorts of things including slightly drilling out jets (I thought maybe the exhaust was more efficient and causing lean out). I think now it was probably the exhaust system was too big for the engine/carb combo.