Carb Accelerator Pump

After doing lots of research I found some interesting information about Edelbrock carbs and the lean bog that they seem to suffer from when mashing down full throttle. My car suffers from this, if I press the throttle gently then there's no bog or stumble but if you stamp on the throttle it momentarily dies and takes a moment to pick up. The problem is more pronounced when flooring it from lower revs - something I haven't been able to do until I solved my poor low speed running problems by changing to a smaller carb.

So now I needed to find a way of covering the "hole" in fuel delivery between low speed cruise and WOT. The WOT mixture is good at around 12.7:1 but there is a nasty spike up to 16:1 or even 17:1 where it stumbles and dies before picking up again if I floor it sharply. First port of call was the accelerator pump. On the Eddy carbs you can adjust the linkage to give more or less pump volume. The picture below shows the linkage attached to the plunger in black with 3 holes to choose from. Stock, the linkage is in the middle hole. For more pump shot move to the top hole, for less, the bottom hole.

This didn't have much effect so I started messing around with shooter sizes. Edelbrock do an accelerator pump nozzle kit part no. 1475 which has a variety of sizes. You can just see the nozzles below above the venturi throat with "31" stamped on. This is the nozzle size in thousandths of an inch. The nozzle kit has several sizes up to 0.043" (43 stamped on top).

I swapped to the largest nozzle size which also failed to solve the stumble. The extra fuel just doesn't get flowing soon enough. What I think I need is a more active pump shot. I noticed that I have to move the throttle quite a bit to get anything out of the pump nozzles. The spring on the pump plunger is quite weak so this gets compressed first before pushing fuel out. Luckily, Edelbrock do an uprated accelerator pump plunger with a much stiffer spring, part no. 1468.

With the new plunger fitted, the nozzles squirt fuel with the tiniest movement of the throttle so hopefully that might improve things. Won't get a chance to drive the car until next weekend to find out though.

Update - this did nothing to cure the lean spike which I have now identified is to do with the secondary throttles opening quickly. Thankfully there is a way to tune this out but information is a bit lacking in this respect. See the next post for the solution.