Dual carb question, Dellorto 40s on a 1.8L to be exact Options

[terrymason]

The problem - The car will go very lean around 2500 rpm, as if the pump jets were too small.

History:
New fuel pump (3.5 PSI to each carb)
55 idle jets
55 pump jets
140 main jets
175 air correctors
These carbs are new to the car, and I'm trying to dial them in.

The car is lean when I first start it (16 / 17 on my air / fuel gauge), but then levels out to a nice 14.7 idle when warmed up. While driving, I can give the car 20% or so of the pedal, and drive up to highway speeds nicely, but when I goose it, it will go very lean (18-22 on my gauge) followed by a series of backfires and loss of power. I started off with 35 pump jets, and have moved up to 55s, which appear to be the most popular around here. I've also adjusted the pump jet linkage, from the nut fully loosened, and moved it in as far as it would go before affecting the main throttle shaft (like 60 turns or something).

What do you guys think, do I need 75 pump jets?

[rhodyguy]

the linkage ABOSOLUTELY comes into play when the idle speed adj screws are resting on the stops. when sitting static, a misadj linkage can push/hold one side open further than the other. just one of the screws resting against its stop will control the idle speed. they do cease to have any effect when the carbs are open to the point where they are both off their stops. if the throttle plate openings are unmatched whether at idle or wot your car isn't going to run right. you want to make sure everything is connected when the side to side flow is matched at the base, common step up.

k

[type11969]

Wait, if both idle speed adjusting screws are resting on their stops, how can the linkage hold one side more open than the other? Both screws are resting on their stops. When sitting static, the linkage should not be applying a twisting force to either butterfly! If you are suggesting that the linkage is holding one screw off the stop while the other is on, well, your linkage is out of adjustment and, obviously, both screws are not on their stops. If you spin the one screw that is not on the stop to meet the linkage then the flows are no longer balanced.

If you match the flow with the linkage disconnected (idle speed screws fully seated on the stops), then you connect the linkage such that it matches the butterfly positions of both carbs throughout the rev range, the flow will be matched throughout the rev range. I don't know how else to explain it. The linkage's only function is to make sure that both butterflies are in the same place at the same time, it should not be used to match the flow of the two carbs at any one point, because at any other point it may no longer be matching the flow.

the linkage ABOSOLUTELY comes into play when the idle speed adj screws are resting on the stops. when sitting static, a misadj linkage can push/hold one side open further than the other. just one of the screws resting against its stop will control the idle speed. they do cease to have any effect when the carbs are open to the point where they are both off their stops. if the throttle plate openings are unmatched whether at idle or wot your car isn't going to run right. you want to make sure everything connected when the flow is matched.

k