Mine ran like crap for a couple of years till I replaced my modified head with a stock one! I used dad's flow bench and ported the exhaust side and added bigger valves. On the intake side, 250 ci, I added epoxy to raise the floor and get a better short turn radius into the port. I gained a decent amount of air flow doing this with out increasing the port size, I actually decreased the port size and figured the air speed would increase which helps low to mid lift power. On paper it looked really good, about 15% air flow increase.
Driving was a different story, it ran noticeably better at full throttle and off idle. It ran terrible at pulling hills, and like yours ran good up to a point then start to cut out, it even broke a motor mount it was running so bad. All I had to do was stay in the throttle (keep pushing down) or let up. I tried everything I could think, changed cams, distributors ( points to HEI and back several times), intakes (one carb to three and back) harmonic balancers, torque converter, even a rear axle I thought was bent.
Turned out the head was causing it. The exhaust would flow close to 90% of what the intake was, normally you would want 75 to 80%. This high exhaust flow was sucking the fuel mix right out the intake during valve overlap causing a extreme lean condition. At full throttle the air speed was enough to over come it. Those long slight hills were the worst, sometimes I was down from 65 to 40 by the time I made it to the top.
That head is now on the rack collecting dust.
My point of all this is to keep working at it. Lean conditions do strange things. Yours sounds a lot like mine did.
Go back into the carb and make sure the fuel enrichment circuit of working right. You might swap in a different spring if it uses one or a different metering rod. It should get worse if you go the wrong way.
Joe
