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> It's the flow that gets split, not the pressure. Yes you're right Jim, but the flow is double when both valves are open therefore the pump needs higher pressure to have the same response > A dual carb 1600 uses only slightly more fuel than a single carb 1600, because > the only difference is that there is SLIGHTLY less air flow resistance in the > dual carb setup, so the dual carb car can suck just a bit more air. It's > nothing close to a factor of 2. Of course you're right in that the twin carb engines don't use twice as much fuel per mile (though mine seems to try ;-)) I didn't mean that. What I'm getting at is that there must be some histeresis in the needle valve- a response time between the level in the chamber going down to a level that the valve opens and lets in more fuel. This will give rise to changing characteristics in the carb as the level changes which needs to be minimised. It is possible and will happen that both valves will be open at the same time. To keep the same characteristics as a single carb engine, the pump will have to deliver twice as much fuel in this period (only when the valves are open) as a T1 pump. Now each individual carb will get through half as much fuel/time as it only supplies 2 cylinders, so in theory the valve should open 1/2 as often and fuel consumption will not suffer. I still think this makes sense- but I might well be wrong. Its just strange to me that my car should have a (just) noticeably different character with the right pump. Might be psychsomatic though- the new pump sure looks nice ;-) > Here something I've been wondering about in this thread: Don't the mixture > screws control JUST the idle mixture, while the main jet controls the mixture > across the open throttle range? So I don't see how misadjusting the mixture > screw could make much difference unless the engine spend all its time at idle. The idle jet doesn't stop working just because you are no longer idling- it still sources fuel and it really does seem to make a difference (on my car at least) to the running of the engine throughout its range. > The higher pressure will certainly raise the fuel level in the float bowl > somewhat, but someone will have to send me a carb before I can tell how much > this might change: I suspect not much. This might well be the answer- but surely this means that the effective pump pressure has been diminished by feeding 2 carbs as otherwise a single carb pump would do, and too high a float chamber level would be a bad thing so VW wouldn't have spec'd a higher pressure pump?!? > > Didn't someone suggest that the change might have been to compensate for the > pressure drop in that special little valve some of these cars had--the one with > the crossover connection between the intake and output lines from the pump. > That's the best idea I've heard so far. No idea what this valve is- I know the earlier pumps had an antisyphoning valve built in- is that what you mean? I don't know, I thought I had it sussed in my mind, but now I'm wondering again Mark Seaton '73 1600TA London ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/