1968 Chevy P10 Step Van...Any help? 250ci
 

Okay, this is a 250 straight six. Seems like mis firing issues. Have replaced plugs, wires, cap, rotor, distributor which came with new condenser and points, manually adjusted timing, carb and adjusted valves while vehicle was running. While testing spark by pulling one wire at a time, it doesn't seem to affect much on
cylinders #1,2 & 3. Also seems like not as much vacuum from intake on carb to vacuum advance as should be. A lot of vacuum on PCV valve when pulled tho. Still pops under load but stops at high RPM. Can also kinda "feather" the gas pedal and get it not to pop, but if heavy foot it... pop ...pop..

Does anyone have any helpful hints.

This is my work van, recently purchased.

Love the Van, Love to drive, until Friday, when it started losing power when put under load and lots of popping coming from carb ....

Any help would be appreciated..

Thanks..

If you were to do a thread search in the General Tech, or the Silverado section, you could prolly find the answer. I know recently someone was having this same issue on a carbed 350. I have no clue about those old engines. Sorry I can't help.

May be Your timing chain jump a link,,

I was having very similar issues. It was a systemic problem that included having to drop the tank and clean the fuel line, put on HEI and break down the carb. Long story but I dropped in a rebuilt 250 and it was still doing what you described. I had to replace the sending unit on the tank because it was allowing air in the fuel line....when feathering the accelerator not much would happen, some noticeable issues but not enough to make me think of the sending unit. Under heavy load it would be very noticeable as you describe.
Set up a secondary fuel system as a way to check if it is the sending unit or pin holes in the fuel line.
You can pm on FB if this doesn't work and I have a couple of other ideas.