I have read and re-read PB Anders' epic work. Bought a Henry meter and everything , but there is one thing I am puzzled by. It looks like from idle to part throttle the pulse length is controlled by the contraction of the aneroid bellow. This produces a pretty straight vacuum- inductance slope. Which means that if you set one end of the curve for A/F ratio, say at idle, or part throttle, all of the other points on the line are fixed, based on the bellows and the spring rate. If you move a line with constant slope all of the points move by the same amount.

Nonetheless, there is some talk about setting mid throttle as though it can be adjusted independent of closed throttle. The reason I am asking is I have been chasing a partial throttle dead spot in the 2,000-3,000 rpm range under partial load. Idle is too rich (11-12) and at the dead spot the mixture goes way lean (16-17 or higher).

I can move the diaphragm stop (outer screw), but that doesn't change the range or slope of the aneroid, it seems to be mostly involved with WOT control (which works fine for me). So I cannot understand how that could affect partial throttle ranges.

Is there a way of adjusting mid-throttle independently from closed throttle?

I should add that it is a 2056 with 8.5 static compression and a Web cam 73 with so called Raby grind (can't blame Jake here, I ordered it without his help). I rather think it would work better as stock. But, I get really close.

You have three basic conditions:

1. Vacuum is high. The internal canisters are expanded and the armature is limited by the outer diaphragm screw. This is your idle mixture.
2. Vacuum is low. The internal canisters are... compressed isn't the right word... neutral? and the armature is limited in the opposite direction by the nose stop screw (the one with epoxy on it). This is your full throttle mixture.
3. In between the stop screws aren't touching anything, and the varying expansion of the canisters controls mixture. You can adjust the fuel ratio in this zone by adjusting the center diaphragm screw. This is your part load -- everything in between idle and full throttle.

If you lack idle vacuum, the outer diaphragm screw isn't pulled against the stop. And if it's not pulled against the stop, you're missing part of the adjustment. You need more vacuum to get to the 'idle zone'. In this case you may have to compromise your idle mixture to get your part-load mixture correct.

Thanks Mark. I have fine vacuum, and it is easy to get the mixture right at idle. I assume that the bellows are expanded, the diaphragm is at the stop and the inner screw is locating one end of the bellows.

Full throttle is fine too, and I have used the adjustable outer screw to tune it.

At mid throttle I also assume that the inner screw is locating one end of the bellows, and they are partly expanded, hopefully bringing the armature to a point between that of high vacuum and low vacuum. However, I think I cannot adjust that point with the inner screw - the only one touching the bellows - without also affecting where the armature is at idle. Sort of what you said.


I bench calibrated the MPS at the beginning, and in stock form it didn't really make my engine happy. One thing I am not sure of is at what level of load the diaphragm starts to pull off of the stop (outer screw), but I suspect it does that when I push the peddle in fast, as I get past the dead spot easily then. Lacks a little finesse though driving all on or off.

I'm confused by the unconventional terminology being used. On the diaphragm there is an inner screw (idle), and outer screw (part load) and then on the MPS case is the WOT 'outer' screw .

What is 'mid throttle?'
Does 'part throttle' = 'part load'?
Which 'outer' screw are you referring to the WOT screw or the part load screw?
Is 'closed throttle' = idle?

If you adjust the inner and outer diaphragm screws at the same time, you can adjust when the stop lifts off the plate.

If the MPS passes the vacuum test I would leave it alone. Guess I didn't read why you are adjusting it. Don't forget to check fuel pressure. You may be blaming the MPS for an unrelated problem.

Different cams have different vacuum signatures.
Attach a T-Piece between the MPS and Manifold, route a hose to your steering rack and attach a vacuum gauge. Run the engine in 4th gear under load with constant throttle at different RPM. Once you understand what vacuum is present at which RPM level you understand what you adjust. Part load mixture is going high up in RPM under constant load and if you tune this for not running too lean you will run too rich below 2000 RPM since you probably have less vacuum below 2000 RPM.
Adjustment is a compromise if you go away from stock configuration.