Stumble with rebuilt Thermoquad /340

Is the fuel in your area E10 or similar with ethenol mixed in with the gas? There are several things that I had to do to help out a similar problem with my 78 318 truck.

The biggest difference came when I read that with ethanol you must increase the primary jet size by two. In other words, if your carb normally came with #98 jets, go to #100 or maybe #101 jets. Jets and #1966 metering rods are still available from places like Quadrajetparts.com. (Yes I know, the name, but the site also sells parts for other carbs as well.) Pick up a spare junker TQ and remove the accelerator squirter assembly and enlarge the holes to .032 if they have not already been enlarged. Install this onto your TQ.

Different metering rod step up piston springs are very difficult to come by. Junker TQ carbs are pretty much the only source for these. A longer stronger spring will lift the metering rods out of the jets quicker, but will also really affect your fuel mileage.

Another part that is often overlooked is the electric choke assist module. If it is bad, you will have a hard time getting things right. The older ones are sometimes available on ebay. The newer ones still seem to be available at O'Reilly's auto parts as a special order. This choke assist will help to gradually open the choke. Mine was found to be shorted and so my choke would go fully open just a minute after I'd turn the engine on.

The off idle stumble drove me nuts until I did these changes to my 318 truck. I'm currently running a Frankensteined 1973 vintage TQ that I bought at an auto swap meet 20 years ago. It is the 1 1/2 primary bore with #100 jets and #1997 metering rods and a drilled out accelerator pump squirter. Because I live in California, my truck has all the smog equipment installed and working.