I am trying to work out a flat spot rich condition between 3600 and 4600 RPM on some race prepped single shaft 40-PII carbs. The carbs have the following jetting: 160 main 160 air, 57.5 idle with 1.1 idle air, 25 Emulsion tubes, no enrichment tubes with enrichment jets soldered shut. 37.5mm venturi, float height has been reduced 2mm below the low setting. Large needle valve, and I have the accelerator pumps with just a dribble coming out with a full stroke and 34 degrees of total advance. Tried larger jets with larger air correctors and smaller venturi's, and both produce less torque and horsepower and still have this same dip.
Looking for ideas on what to try to work out this flat spot.

-chris

Larger jets affect mixture across the entire range in which they operate (idle versus higher rpms and load for mains). The air correctors are for the uppermost rpms where the engine is breathing large amounts of air.

I know very little about Solexes and even less about the differences in emulsion tubes, but those tend to be the main solution for controlling the middle of the transition between idle to main circuits. What you're seeing is a lean condition more toward the bottom end of the RPM range - so, try to find an emulsion tube with fewer holes at the TOP of the tube than what you currently have - this effectively applies to the bottom end of the rpm range. Or if you're adventurous, you can experiment with a spare set of ETs and plug some of them up using JB weld or other epoxy.


What kind of AFR gauge and software is that you're using?

This is from a superflow dyno run with their AFM. I clipped the Hp and torque numbers to focus on the air fuel mixture. increasing air correctors made the afm go to 13.5-13.75 range, not safe. Increase main and airs 170 190 airs fuel curve in not very flat, less power and lean at 6800 up yet 9:1 at 4500 rpm. Went down on airs made the lower end wore but improved the top end. dropped to 160 mains to lean out the entire range but as not able to eliminate the dead spot. Have not tried the Emulsion tubes not sure which hole impacts the 3600-4600 RPM range. Would like to know which holes impact what range of rpm/flow.

Doh! Sorry, I read both your post and the graph wrong. So you've got a pig rich condition at 3600.

In that case, adding a hole at the appropriate location in the tube will help. Again, the bottom of the tube as installed affects the top of the RPM range as I understand it with Webers.

Might be worth buying a set of spare emulsion tubes of the same design to experiment with. Start by drilling an extra hole toward the TOP of the emulsion tube as installed. Be sure there are no metal shavings leftover, and try them out. If still rich, drill another hole.

A type 4 engine with that rpm range will always run rich just below where the power comes on.
Its a result of double carburation from reversion induced by cam overlap, runner length and rpm.
Long v stacks have been used to contain the fuel cloud which forms above the carb inlets but this lowers the peak output.
Exhaust system design can have as big an effect as intake length on the amplitude of the reversion.

The best way to deal with that is to only use part throttle through that rpm range.
The engine will run much better there with the throttle around 50%.

The man knows of what he speaks - I'm just now kind of wrapping my head around reversion. Thanks Chris!

Even with the "double carburation", 9:1 is still pretty durn rich, is it not. Do you think he can do anything tuning wise to improve that valley?

I like Chris's suggestion though - Just keep your foot in it and don't go below 4,500 RPM

I assume this is a race car?

I would work on exhaust and gearing
On the track you can avoid that rpm with gearing

This^^^. If you never see that rpm on the track who cares what the AFR is.