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Old 07-12-2018, 12:28 AM   #1
Junior Member
Join Date: Feb 2018
Posts: 23
Default 39 Merc hesitating under load...driving me crazy


This one has me pulling my hair is such a sweetheart otherwise.

Background: Stock ‘39 Merc. Original 33k mile car, never had an engine rebuild that I’m aware of. Fires up easy, idles smooth, shuts off cleanly, doesn’t run hot, even 90 psi compression on all cylinders, fully cleaned/replaced/rebuilt fuel system, new carb, rebuilt diver’s helmet distributor from All Ford, new coil, new plugs, generator puts out 6.7 volts, battery always reads 6.4+ volts.

Symptoms: has a hesitation under load in all gears. Not bad in first, worse in 2nd, worse still in 3rd. When motor revs out of gear there is no hesitation. Vacuum reads at 22 at idle (although it jumps around very slightly). Under load it drops considerably and is in the 5-10 range in 3rd. Can not identify any vacuum leaks and believe me when I say, I HAVE CHECKED.

I rebuilt the stock 94 carb and feared I may be having issues with the power valve. I temporarily installed a brand new change, of course. Also, it seems to get slightly worse as the car heats up. When it’s cold the hesitation is barely perceptible, as it reaches full operating temperature, it gets pretty bad. Could it be a valve issue?


Any help appreciated. Thanks.

(I included a pic of the Merc in it’s current most enjoyable state: parked)
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