1966 coronet,318,727,both stock.......Have newer plugs,cap, rotor,wires,air and gas filter.The car has electronic ign.,Petronix..Well just got back from a 300 mile trip and was getting only 15 mpg highway and less in town driving.On the highway it seems like when cruising the car seemed to be jerking or chugging just slightly but ran pretty smooth overall...Also latley mostly when warm the car stalls when put in R or D.but runs fine in neutral.I just rebulit the carb and the CARB guy gave me the correct gaskets and all the holes were blown clean but I still have to take both air mixture screws almost 3-4 turns out before I get a good idle.The carb guy seemed to think I have a vaccum leak but when checking with a gauge it was right aroung 18 and the needle did not jump,just stayed steady.What do you guys and gals think..Need more info just ask....

CHECK your exhaust heat crossover in the intake. I will BET it is plugged. MPG will suffer BIG TIME if that is so.

Where exactly is the exhaust heat crossover and how do I get to it.......

Will this also make the car run rough and stall in gear...

It is the center passage of the intake running side-to-side uNder the carb. To check it out ... run the car for a couple of miles and the spray that area with a mist of water. It should "boil" right off.

Run ruff and stall ? ... THAT plus get very poor MPG.

If no vacuum leak......it sounds lean. The surging at speed and poor idle in drive are the clues.

Check your total ignition timing advance, if too much, that can cause the surging at speed.

Vaccum reads a steady 15,rechecked timing and is fine at 10degrees.Total timing,what is the best way to do that.
Could my petronix be worn????

did you check your crossover ?

Side to side slop in your distributor shaft. Up and down slop?

Slop in your throttle shaft on the passenger side of the carb?

I am guessing you have a BBD 2bbl.

Is this is an A motor 318 with mechanical lifters?

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did you check your crossover ?

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I just let the engine warm up and sprayed some water and it is boiling away......I have had the carb off recentley and have looked down there and it was pretty clean,not much carbon,

It is a carter BBD 2barell,318 poly stock.....

One thing to check if you haven't already. If you used the thick gasket between the carb and intake, check to see if the carb is still tight. Those thick gaskets will compress after a warmup when new sometimes giving you a vacuum leak even though you tightened the carb down snug when reinstalling it.Dave

For best MPG I found that 15-18 initial was best, then you shorten up the mechanical advance to get it to about 35 maximum mechanical. And you want it all in by mabe 2000 2500 RPM. Then you need minimal vacume advance, look for a can marked around 5 or 8 degrees. This was on an LA but a poly is probably gonna be similar. A quick way to figure out if the advance is causeing it , unhook the vacume advance and take it for a drive. My car would do exactly what you are describeing untill I really shortened up the vaccume advance. The factory distributer is desighned to run about 10 degrees initial and had to make up for it with lots of vaccume advance so when you advance the initial where it needs to be then the vaccume is too mch. I think I found the short advance can in a mopar performance distributer.