Carbs are notoriously bad when not set up correctly.
A quick list of things to check before you go any further.
1/ Take a look at all of the joints for the carb set up. Depending upon who installed it and which carb set you are using they could have installed thick card or noeprne (spelling?) seals. The former is okay, but does get choked afer a while and degrades.
The latter gets affected by heat and solidifies, espcially on the single carb type 4 set up. When this happens the seal can be broken by the joint loosening.
From what you describe it sounds like you are pulling air in from somewhere - at low air volocities the car runs fine, however when you increase the pull on the air flow you might be dragging air in from somewhere else.
The second thing is to look into what type of carb you have. I dont have an answer for you on what you have, however in twin barrel weber and Dellotro carbs, they are names are clearly marked.
First of all look at the shape of the carb. how many venturie (Spelling again - how many barrels does it have) and what shape are they?
If it has twin barrels which are circular, with very defined barrels in the main body of the carb, then you most likey have single - twin barrel Weber or Dellotro.
If, on the other hand you have a square single barrel with no defined barrels within the body of the carb you most likey have a single progressive Weber (not good).
Not being that much of an expert in carbs if anyone want to make me look silly, feel free to do so!
To find out if you are running lean or wet, take a spark plug out - if there is chocks of carbon build up - you are running wet (too much fuel)
If the plug is whiteish and possibly corroded you are running lean (not enough fuel)
Another good tip to check if you are running wet is ask yourself how many times it backfires? With too much fuel going in to the engine, the over excess of fuel will tend to leak out into the exhaust system and then ignite when the exhaust valve opens again, causing a nice bang!
The overall diagnosis comes down to this.
If you are running wet, not enough air is getting into the system compared to the amount of fuel going in. This is normally caused by poor Carb set up and the wrong jets in the carb(s).
If you are running dry (check your plugs) this can be down to either poor jetting again, or air being dragged in through somewhere else in the system (ie the Venturie (spelling again) is not being fed the correct information on air flow is is not dumping enough fuel in to compensate.
There is a last factor which affects the set up of the car. The Cam on the 914 is specifically set up to run with FI. Cant remember off the top of my head but I think the valves close earlier that they do with carbed cars - hence while the fuel is still trying to wind its way down from the middle of the engine, the valves can close, letting the fuel seperate (de-emulsify) and when it does enter the cylinder, you dont get a clean burn of the fuel.
Okay - please feel free to rip this answer to bits
LOL!
Chris G