Q&A / Tech

Mailbag: Carb Jetting for Atmospheric Conditions

Got questions?

We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re making a few jetting changes to accommodate for high-humidity racing.                                

D.D. • Agana, Guam
Q: I recently bought a 1970 Plymouth GTX. It came equipped with a Chrysler 440, a 750 cfm carb on an Edelbrock manifold, a stock Chrysler marine distributor, a chrome ignition module, and an ACCEL coil. The COMP Cams camshaft has .650 inches of lift, 290 degrees of duration, and 108 degrees of lobe separation. It also has TRW dome pistons, a Dana 60 drivetrain, 4.56 gears, a Chrysler 727 transmission, and tire tubs. The compression ratio is about 12.5:1.

The carb was jetted for a cool climate, but here in the tropics we have 86 percent humidity. Currently, the car runs in the 11-second range at 120 miles-per-hour. Should I jet the carb up or down? How do I find the right jet level?

Also, I’m reconfiguring the ignition to an MSD system. Should I bump up the advance? What should the max advance be?

Finally, how do I determine the size of the torque converter, and what should the stall rating for this car be?

 

A: With your cam and compression ratio, you could really use about 100 cfm more carburetion. If you haven’t run the car yet, get a spark plug magnifier and few sets of spark plugs, and make a few test passes. Check the color on the plugs afterwards: black means too rich, white is too lean, and tan is just right.

We’d also recommend a fast timing advance rate for your combination. Maximum advance should be right in the 36-38 degree range. As far as the torque converter, the stall rating is usually recommend by the maker of the cam or the converter. In this case, COMP Cams recommends a 5,000 rpm stall-speed converter.

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