Mailbag: Recommending a Carburetor and Gearing Combo to Fix 700R-4 Shifting Problem

You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re recommending a carburetor and gearing combo to combat an improperly shifting 700R-4 automatic transmission.

chevy pickup rear end

(Image/YouTube – davesgarage512)

Q: I have a 1986 ½-ton Chevrolet pickup with full time four-wheel drive. It has a 350 crate motor and a rebuilt transmission. The truck has 31 x 11.5-15 tires. 3.08 rear axle gears, 700R-4 automatic transmission with overdrive, and a four-barrel carburetor. My problem is that it goes in and out of overdrive all the time.

What would be a good combination of carburetor and gears that would help this problem? Would lockout on the front hubs help?

A: We recommend a 600 cfm carburetor. An Edelbrock Performer would be a good choice because it’s calibrated for maximum mileage. We would also recommend a 4:10 rear gear axle ratio. Doing so will put your engine back into its street-friendly torque band.

And yes, locking hubs would be a good choice also. Your transmission is shifting improperly because of the tire size and gear ratio.

Make sure the TV cable is adjusted properly because 700R-4 transmissions are extremely sensitive to adjustments.

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  1. Full time 4 wheel drive? If you change the rear gears, you have to change the front gears too. That should take a bite out of your budget!

  2. Might be worth noting the type of torque converter. The 700r4 is the 4l60, and when it went to electronic control became the 4l60e. On the e variant you have the ability to see and adjust the converter lockup speed and amount, which can feel very much like another shift. On my vortec 4200 I’m in overdrive by 50km/h (about 30mph) but the converter doesn’t lock up until about 60-65km/h. It’s possible that the in and out of overdrive feel is actual the converter lock up threshold and that you are in 4th gear the whole time.

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