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]

i disagree with the "you should not be adjusting the linkage to balance the carbs" comment. by balance i presume you mean same rate of flow. you need common initial settings across the board. air bypass (closed or open), idle air mixture screws...the angularity of the linkage arms and drop links should be as similar as possible. if you have both of the idle adj screws making contact with the stops, and the linkage is out of whack, you'll prob notice difficulty sustaining a stable idle with an equal indicated flow rate. back one of the idle screws of its stop. this will allow you to match the 'free' carb flow to the other by simply turning the thread linkage rod in or out while monitoring the most forward venturis on either carb. you can increase the idle speed with the single screw. now check all 4 vents. this is where the air bypass screws come into play if there is a discrepancy. turn the idle back down. adj the idle air mixture screws. you need to be patient. a little adj can take a long moment to have full effect.

you need to inspect your linkage if the flow rate is constantly changing thru the range of motion. worn heim joints that rattle around and just plain slop makes setting the carbs up a hopeless and frustrating task.

k

[type11969]

This is what I mean. Linkage disconnected, pull out the unisyn or preferably the scroll type balancing tool. Match the flow rates of both sides by adjusting the idle speed screw. Match the flow rates of all the barells with the idle air bypasses (if you really want to be sync'ed). Put the sync tool away, the linkage cannot and should not be set up such that it will mess with the syncing of the carbs, it should be set up such that it maintains the initial sync. Connect the linkage such that the butterflys open and shut at the same time and the same rate. You will need to mess with the length of the linkage and may need to shim it to get the angles to match from side to side. Now, with the flow rates the same through the rev range, you can mess with the mixture screws to dial in the fuel for each barell.

The linkage does not come into play when the idle screws are touching the stops. If you synced the carbs properly, the flow rates of the carbs will be the same at any throttle position if both carbs are open to that same position. The catch is whether or not the linkage is opening the carbs up to the same position at the same time.

i disagree with the "you should not be adjusting the linkage to balance the carbs" comment. by balance i presume you mean same rate of flow. you need common initial settings across the board. air bypass (closed or open), idle air mixture screws...the angularity of the linkage arms and drop links should be as similar as possible. if you have both of the idle adj screws making contact with the stops, and the linkage is out of whack, you'll prob notice difficulty sustaining a stable idle with an equal indicated flow rate. back one of the idle screws of its stop. this will allow you to match the 'free' carb flow to the other by simply turning the thread linkage rod in or out while monitoring the most forward venturis on either carb. you can increase the idle speed with the single screw. now check all 4 vents. this is where the air bypass screws come into play if there is a discrepancy. turn the idle back down. adj the idle air mixture screws. you need to be patient. a little adj can take a long moment to have full effect.

you need to inspect your linkage if the flow rate is constantly changing thru the range of motion. worn heim joints that rattle around and just plain slop makes setting the carbs up a hopeless and frustrating task.

k