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Part 120: Solution to the Idle Problem with the Summit Carburetor

The annular booster cluster holds both the accelerator pump nozzles and idle restrictors.

Back in December of 2022 I installed a brand new 500 cfm Summit carburetor. That post can be read here: Summit Carburetor. Initially, this carb was only going to be temporary while I was waiting on the build of my C2AE block and a 2x4 Holley 4000 setup. The 292 Y Block has really responded well to the Summit unit sitting on top of the Mummert intake manifold - swapping gears, going wide open throttle, reading the spark plugs, and all the rest have gone extremely well. However, as of late the three digit heat here in the Antelope Valley of Southern California has exposed an idle problem.

Although I do not have an AFR gauge (Air-Fuel Ratio), I believe the carb is running lean when at idle. When the engine has run at a hot operating temperature for a lengthy amount of time, the heat soak seems to play havoc with my idle mixture screw settings. It is almost like having a carburetor with two personalities. Once I adjust the idle mixture screws to a richer setting to compensate then the mixture is way too rich when the engine starts back up after cooling off. The fuel is not boiling out. (I do not have ethanol in the fuel, and the sight glasses for both bowls are normal.)

I did some research on Summit's website and disovered that they sell an idle feed restrictor kit that gives several options to either increase or decrease fuel delivery. So, I went ahead and ordered both the kit and a couple of new gaskets for the fuel bowl.

I had to make labels for each space in the trays after identifying all of the restrictor sizes.

The idea on the kit being that for the conditions being experienced, the current idle feed restrictors in the carburetor are too small. In addition, from what I understand this Summit carburetor design depends heavily on the amount of fuel coming from the idle passageways and their connection to the intermediate transfer slots.

On the bench I double-checked the butterflies position against the transfer slots.

There was not a need to take the carburetor off the intake to replace the idle feed restrictors, but for the sake of the video I made and to have access to the transfer slots I put the carb on the bench. All of the settings were brought back to a baseline, and I removed the annular booster assembly to gain access to the idle restrictors.

The screwdriver is pointing to a weight and check ball - be careful if you flip the carb over!

The idle feed restrictors are located underneath the annular booster cluster.

The restrictors are extremely small and stampings are hard to read.

I had to use a magnifying glass to read the stampings indicated on the restrictors. They are really quite small. The kit ranges from .043 to .059 and offers six separate sizes. I removed the restrictors that the carburetor shipped with and they were .046. I did not remove the secondary annular booster since this was a primary problem anyway. I chose the .052 size (two sizes up from .046 in the kit) and installed the restrictors into the cluster assembly. The carburetor is pretty simple to disassemble and reassemble, but I did show a few of the nuances in the video below. Before removing the lid on the bowl you will need to remove the clip from the rod that connects the choke assembly to the choke plate. (There is a close up in the video.)

After reinstalling the carburetor, I ran the electric fuel pump to refill the bowls and hit the key. The Y Block fired right up and idled fine just like it normally does when cold. (no choke needed - the plate wide open) In about 15 minutes the engine was up to 190 degrees or so and idling well and running smooth. I took it out for a spin and noticed a definitive improvement right away. Another 20 minutes of driving, stopping, and idling at various times gave me confidence to make a final adjustment on the idle mixture screws using a vacuum gauge. I was amazed at how much a difference a small change (the difference between .046 and .052 is only .006!) that swapping out idle restrictors made in the idle characteristics of this carburetor. During the entire time, the engine never leaned out enough to cut off on me at a stop light and the idle remained otherwise steady.

Summit Carburetor Idle Feed Solution

As I write this post, we are in the middle of Memorial Day weekend here in the United States. If you are a US citizen I hope you take the time to pay your respects to the men and women who have lost their lives while providing for our freedom. We have much for which to be thankful for in this land, and during this weekend may we be especially grateful for those who have paid the ultimate sacrifice for that which we enjoy. May God bless America.

The Hot Rod Reverend


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