Solex, converting from split shaft

[Martin Benade]

I just got that kit (from alfa 1750 on Ebay) to put solid shafts in place of split shafts in a pair of 40 P11s. I notice the new accelerator pump lever on the shaft is much longer than the original setup. Will that matter, or does the spring that does the actual pushing take care of everything regardless of the lever length?

[Dave Erickson]

Martin,
the design of the accelerator pump linkage changed from solid shaft to split shaft. The main difference is that the stroke is larger for solid shaft than split shaft. The hole for the accelerator pump rod in the solid shaft pump lever is more offset from the centerline of the throttle shaft than for the split shaft carburetor. I think the intention was to reduce the volume of gas pumped. Alfa1750's kit replaces the split shaft parts with solid shaft parts (except that the angles of the flats on the throttle shaft are match the angles used for split shaft, so that you can use the split shaft throttle levers).

However, you should still be able to adjust the pump volume to .3cc per stroke.

Dave

[Doug McDonnell]

per 2 strokes isn't it Dave?

[Martin Benade]

It appears that new linkage would allow for a much greater pump volume, but I think what you are saying is that the parts are correct and can still be adjusted to the original pump volume. Is that correct?

[Vic Skirmants]

per 2 strokes isn't it Dave?

Correct.

[Dick Weiss]

Ah--slight confusion here,

Solex carb injection ranges between 0,45cc/2-strokes for summer an 0,65cc/2-strokes for winter.
The Zenith + is 0,3cc/2-strokes max.

Dick

[Felix Macaluso]

I would have thought that by now the carbs would show signs of wear in the butterfly area and elongated shaft holes that allow air leakage. What condition are yours in?
Felix

[Martin Benade]

The ones on the car are certainly worn that way, but the pair I am working on seem pretty good. They are newer, only about 45 years old.

[Mike Horton]

The ones on the car are certainly worn that way, but the pair I am working on seem pretty good. They are newer, only about 45 years old.

...it depends on how soon they were removed, and how long they were on the shelf. I have two sets, for my engine, and learned the early black split shaft blocks, were a poor material, which didn't hold up over time, and the later, lighter space age material, has so far, held up much better. Both my sets, have otherwise good shaft fit in the brass bushings, and the split shaft adapters, fit tightly to the original shafts. The set on my 912 engine now, have been completely excellent, for the 10 years since I overhauled them...and with David's wise ethanol fuel jetting council, have smooth power application, up to 32 mpg, and better richer than original, have helped even here in hot TX simmer weather...best service I've had, since my Zeniths. I've been happy, and with mine, the typical Soles myths, have proven not true,

[Mike Horton]

The ones on the car are certainly worn that way, but the pair I am working on seem pretty good. They are newer, only about 45 years old.

...it depends on how soon they were removed, and how long they were on the shelf. I have two sets, for my engine, and learned the early black split shaft blocks, were a poor material, which didn't hold up over time, and the later, lighter space age material, has so far, held up much better. Both my sets, have otherwise good shaft fit in the brass bushings, and the split shaft adapters, fit tightly to the original shafts. The set on my 912 engine now, have been completely excellent, for the 10 years since I overhauled them...and with David's wise ethanol fuel jetting council, have smooth power application, up to 32 mpg, and jetted richer than original, have helped even here in hot TX summer weather, controlled my oil temperatures to normal...best service I've had, since my Zeniths. I've been happy, and with mine, the typical Solex myths, have proven not true,

[Mike Horton]

The ones on the car are certainly worn that way, but the pair I am working on seem pretty good. They are newer, only about 45 years old.

...it depends on how soon they were removed, and how long they were on the shelf. I have two sets, for my engine, and learned the early black split shaft blocks, were a poor material, which didn't hold up over time, and the later, lighter space age material, has so far, held up much better. Both my sets, have otherwise good shaft fit in the brass bushings, and the split shaft adapters, fit tightly to the original shafts. The set on my 912 engine now, have been completely excellent, for the 10 years since I overhauled them...and with David's wise ethanol fuel jetting council, have smooth power application, up to 32 mpg, and better richer than original, have helped even here in hot TX simmer weather...best service I've had, since my Zeniths. I've been happy, and with mine, the typical Soles myths, have proven not true,

...corrected below,

[Dave Erickson]

It appears that new linkage would allow for a much greater pump volume, but I think what you are saying is that the parts are correct and can still be adjusted to the original pump volume. Is that correct?

Yes, the design of the 40PII accelerator pump circuit is quite strange really. There are four springs involved: the throttle spring on the side of the carburetor, the throttle spring in the center (solid shaft and conversions only), the spring on the accelerator pump rod and the spring inside the pump chamber.

throttle spring

center throttle spring & pump lever

accelerator pump rod & spring

accelerator pump body, spring and diaphragm

There are three levers involved: throttle lever, throttle shaft pump lever and accelerator pump lever, along with the rod shown above that connects the throttle shaft pump lever and the accelerator pump lever:

throttle lever

center throttle spring & pump lever

accelerator pump cover & lever

When the driver pushes the gas pedal, the throttle shaft rotates pushing the pump rod. The lever on the accelerator pump has a hole that allows the rod to go through, so what actuates the accelerator pump is the spring on the pump rod. Since gas is not a compressible fluid, the pump lever lags behind the rod nuts, if the driver really stomps on it, so the pump continues to pump until the gas in the accelerator pump chamber has either squirted out the pump nozzles or passed through the bypass port back into the float bowl. The accelerator pump lever hits a stop, and the pump rod continues to pass through the hole in the lever as the throttle is opened wider, so no additional gas is pumped at mid to wide throttle. Here are some pictures that show this, along with the bypass hole in the check valve:

When the driver lets off the gas, the accelerator pump rod returns, pulling the accelerator pump lever back, and the spring inside the pump body pushes the diaphragm back, refilling the pump chamber to the point where the pump lever stops it.

So the length of the pump rod does not matter as long as you can adjust the nuts to bring the pump lever to the right point to get .3cc of gas per stroke. Note that is the pump output, not the nozzle output. Since there are two nozzles, each gets half that amount (so use two strokes to measure the pump volume). But check both, they need to be equal.

[Mike Horton]

Well done, Dave, and timely, as I'm about to start my spare set of split shafts, and though I've been doing this since the '60s, one can always learn more, Thanks!

[Dave Erickson]

Well done, Dave, and timely, as I'm about to start my spare set of split shafts, and though I've been doing this since the '60s, one can always learn more, Thanks!

Thanks Mike. Most of what I described is for a solid shaft carb, but I'm sure you know that.

[Mike Horton]

Well done, Dave, and timely, as I'm about to start my spare set of split shafts, and though I've been doing this since the '60s, one can always learn more, Thanks!

Thanks Mike. Most of what I described is for a solid shaft carb, but I'm sure you know that.

Yes, but a review of operational theory never hurts... I've really had excellent luck with my split shafts, and the cores I found, still had the aluminum tags on a corner screw of each... and as usual, the split shaft interconnect blocks, were completely missing, having deteriorated. I think, the resulting mismatch of the air flows, put more of these carbs on the shelf, than any other cause, I guess I'll soon find out...

BTW, my '68 had vapor lock issues back when I was commuting in it in the Hot TX summer weather, after the heat soak, of a fuel stop. Last spring, when prepping the car for the TX Hill Country Rally, I happened to set the split shaft adjustment on the right side first, and when I moved to the driver's side, found the heat so high, I couldn't leave my hand in place long enough to get that side adjusted...then, EPIPHANY! When the '68 MY only steel tubes for the air lines into the side tin, to the heads, was (legally) removed, whoever did that job, failed to weld in close out fill plates in the side tin, so...the cooling air of the rt. side, was on the other side, now pre-heated by the oil cooler air outflow, now, a cooling air heater, rather than just an oil cooler, and, Wa-La, the elusive heat source I'd been searching for, causing the vapor lock. Shame on me, for not checking for those holes before, and I had lived with that for 8 years. In moderate weather operation, this phenomenon was not present. If you have a '68 912 side tin set, inspect for this, repair it, and you will be rewarded. The combination of the heated air flow, onto the manifold bases, and driver's side carb base, combined with the PC 10% ethanol, is not well matched in this instance. Even with the fuel stop heat, there was enough "fuel" in the float bowls, to get the engine started, but at about the time I needed to blend into traffic at the end of the on-ramp, the fuel was gone, from the evaporation, and the power faltered, a dangerous situation. An electric "boost" pump, masked the situation, but I'm glad to finally have gotten to the root cause of the issue... just sharing,

David, there is another ethanol tale, for your collection. I never experienced vapor lock, in any of my 356s, on real gas... in the same weather...