Idling question after running for an hour

Good morrow. I just have a quick question regarding idling. Do any of you find your vans idling rough after driving consistenly for an hour or so? I drive at about 55 for an hour each way on Saturdays and I find once i get off the highway and onto the side and backstreets, the idle gets progressively rougher when stopped at lights or stop signs (If i drop the automatic into neutral it smoothes out). It doesnt quit on me but it does make the cab/body shake a bit. If i put it into reverse, it does almost make the engine quit. Whiel it rests at work for 6 or 7 hours its fine for the ride home until i get off the highway again and i experience the same issues. Just wondering if this is normal. Thanks

Vacumn leak, bad spark plug or wire?? dist cap? Ill add = likely is a vacumn leak. When cold runs fine -then when hot acvts up - vacumn leak -check that intake & carb gaskets- when hot by spraying carb cleaner or wd-40 type stuff in those gasket areas when idiling - see if smoothes out or makes a diff??

i will check that come saturday as it is easy enough to do - thank you

How's the heat in the engine compartment? Maybe the gas is getting too hot and boiling in the carburetor. Maybe the idle jet is more susceptible to the heat.

Drew is the van that's your avatar yours? It looks like Terry Abbott's low mileage 67 ambulance. I ve seen it in person. It sure is cool if it is. The auto's have a dash pot valve on the carb. that help the idle to return slowly rather than quickly like on a stick shift. If you wind up getting a different carb for any reason just be aware of that so you get one for an auto. Check the mount bolts on the carb. They are famous for coming loose on the 240's.

Yes sire, that is Terry's van. The carb is NOT and Autollite but rather a replacement 1940 model number Holley so there isn't a valve (not that I see anyway) like the one you would see on the Autolites. The mount bolts are good as I was the one who rebuilt the crab after getting it and put it back on the block. I backed the richener screw out a little and that smoothed things out and I also - this sounds goofy - step on the brakes harder to keep the van from shuttering. When putting the van into gear when either at initial start for the day or after its been running, it wants to clunk into gears with a lurching motion. Pressing the brakes firmly seems to smooth out the engine at stops and when putting it into gear. The van has a 170 and not a 240 as well. Thanks for the reply