69stang390

Concerning the high RPM shifts in the trans, could i just shift it manually w/o hurting anything?

Oxnard Montalvo

Maybe so.

Using the formula at Pony Carburetors for calculating CFM, a 770 carb is big unless you're pulling close to 7,000rpm.

390/2 x 6823/1728 = 770cfm

A "stock" 390 has a max rpm of 5,556 based on piston speed.

390/2 x 5556/1728 = 627cfm

Too big of a carb causes the speed of the air going in to slow down which will cause improper fuel atomization. An idle or off idle stumble will result.

A good tune up will mask the problem and the car's going to run, but if you're not looking at 7,000 rpms I would think a smaller (650cfm) carburetor would give you much better response.

That's my view from this side of the street, but I'm old enough to know there's a big difference between numbers and the real world.
I hope the tune up leaves you smiling.

69stang390

The formula on the holley site also reccommends a 650/670. Thats actually the size carb that i had ordered, but they were out of stock, and sent me a 770 instead.

4reboy

I don't know if this has been said before but check your vacuum lines. A small vacuum leak can turn into a big problem. The hard shifting could be vacuum related as well, or it needs a rebuilt (mine was the latter ). My 351 had a leak at the carburator which in turned made it hard to tune, so make sure all the vacuum lines and the gasket at the carb all check out.

One trick a tune up shop taught me was to use two carb gaskets to ensure a good seel, they are kinda thin and sometimes an edge might be off so they use two to get a good seel there.

Ever try driving around in first gear during traffic then shifting when you tell it to? Thats what I would do with my auto if i had this problem.