235 runs rough under load

[Bob's 49]

I was given a 1959 – 235 engine and used Deve’s 1959 235 Project Manual to rebuild it. It ran fine for 1200 miles and now I am having a problem with it. The engine was given to me so I don’t know what it came out of. The block was checked and then bored 40/10. The head was cracked so I acquired a rebuilt head with hardened valve seats. I bought an engine rebuild kit with new pistons, cam, bearings, timing gears and hydraulic lifters. I bought a remanufactured Rochester one barrel carburetor. I replaced the distributor and coil with a new HEI distributor and new plug wires. I replaced the exhaust header that had a frozen riser with Williams dual headers. I replaced the draft tube with a PCV valve. The engine ran fine for 1200 miles. Then one day it started bucking and stuttering under load.

It will run fine at idle and any RPM up to 1500. I have checked the timing, vacuum advance, mechanical advance and idle mixture. I ran a compression check and got 155, 151, 160, 156, 156 and 155. After each test, I would test drive it with the same results. The plugs were fouling with dry black carbon after each test. I have leaned the idle mixture down to just above where it runs rough, 1 ½ turns out.

I have run out of ideas. Any suggestions?

[Deve]

If the engine runs at idle just fine, I would suspect too little fuel getting into the carb. Sometimes there is enough flow to idle properly to a certain rpm but not enough for full throttle. Do a valve adjustment check to make sure something didnt get loose, and the last thing I can think of is an HEI module that isnt working as planned. I hope someone else can chime in with other ideas. Good luck Bob!

[Bob's 49]

I checked the valve adjustments. They were fine. All the carbon deposits suggest a rich mixture. Since leaning it out didn't have any effect, I thought maybe it wasn't getting enough air. And yes, I checked the air filter and also tried it with the filter removed. Perhaps the exhaust was restricted limiting the flow. I disconnected the exhaust pipes from the headers, but no change in performance. I haven't checked the fuel flow yet, but if there wasn't enough fuel at higher rpm, shouldn't it be lean instead of rich? What I don't understand is why there is so much carbon deposition on the plugs and in the exhaust system.