At this year's Vintage Motorsports Festival at Thompson Speedway Motorsports Park I had an issue with the carburetor on the Quantum 2.

photo credit Chris Raia

Me in the Quantum 2 chasing Dad in the Quantum 1 (Photo credit: Chris Raia)

This is nothing new for me. I seem to have a particular problem with Solex carburetors and the Solex 40AI was acting very similarly to the Solex P11's that I have on the Formula S and the Sonett. In hard right turns, the engine stumbles and won't pull. It is made worse the harder I corner, which is obviously not optimal.

While I haven't yet come to a conclusion on the P11 carbs, the issue with the 40AI seems to be related to the atmospheric pressure vent on the top cover of the carb.

Carb diagram

It is fortunate that on the Quantum 2, I can see the carburetor from the driver's seat. As a result, I had a clear view of what was going on when I entered the right-hand corners. Fuel was POURING out of someplace on the carburetor! It was like a waterfall of fuel!

Looking at the carburetor, there is only one place that this waterfall could be coming from. There is a vent on the top, near the left-front corner. That vent provides atmospheric pressure to the float bowl. Naturally, entering a right turn under braking the fuel in the float bowl sloshes to the left-front corner. In this case, right up and out of the vent!

This does three things:

  1. It reduces the amount of fuel in the float chamber. This is bad because the float on this carb is very large in relation to the volume of the float chamber. In a racing situation it is possible to run the float chamber dry of fuel if there isn't enough in there and the fuel pump can't deliver it fast enough.
  2. The sloshing fuel moves away from the pickup ports, which are located at the two bottom-back corners of the float chamber.
  3. The sloshing fuel exiting the atmospheric pressure vent blocks air (atmospheric pressure) from being able to enter the float chamber. If the atmospheric pressure is not correct in the float chamber, the fuel will not flow to the jets properly and the engine will be starved for fuel.

I talked to some friends online and the general consensus was that a modification of the vent was the solution. Their instructions were to extend the vent with a fitting and hose to a filter. However, I saw a problem with that. Sure, it will prevent the fuel from pouring out of the carb but no matter how far I extend the vent it will still be blocked by fuel once it sloshes over the hole inside the float chamber. It won't allow atmospheric pressure into the chamber. So I took a close look at the carb cover and formulated a plan.

Un-modified carb cover

Un-modified carb cover

It appears that the cover was designed so that the fuel input could be either at the side (as it is in my case) or at the front simply by drilling and threading the appropriate cast port. I figured I could thread the un-used port and drill a hole from the inside of the cover into that port. This would give me a second atmospheric pressure vent at the opposite side from the stock vent. So even if the fuel sloshed up and blocked one vent, the other would still be open!

I put together a few pieces of brass plumbing from the local hardware store in order to fabricate a filter housing. I soldered on a mounting bracket. Inside the drilled cylinder is a piece of filter mesh stolen from a fuel filter.

modified carb cover

The hole drilled next to the float valve is the new second atmospheric pressure vent.

modified cover

Here are the two new ports. I would later cut them shorter.

installed showing bracket

Installed, showing the bracket


Installed and ready to race again (link to gallery)

Since this modification was made, I have raced the car at the Pittsburgh Vintage Grand Prix and I experienced no problems at all with the engine cutting out in right turns!