SB 350 Timing (vacuum advance issue)
 

HI everyone,

My 350 was running rough in my 1970 chevy c10, sounded and felt like cylinders were dropping in and out while driving. Ran very rough most of the time with a hesitation when first accelerating. I thought it needed a tune up so i put new plugs in it, fuel and air filter. Ran great for about 5 days then the issue returned. So i thought it could be a timing issue. i watched a few youtube videos, and decided to adjust the timing. found out it was running about 14 degrees retarded with vacuum advance plugged up. When i got the thing to 8 degrees advance while idling and vacuum advance line plugged it sounded better. So i shut everything off and hooked up the vacuum line to the vacuum advance. went to start it and it barely started. had to shut it off because it sounded so bad. so i thought maybe my vacuum advance was bad but i hooked up a vacuum pump to the distributor and it held 20 inched of vacuum no problem. So now the only way to run my truck is to not even use the vacuum advance at all. WHERE DO YOU THINK I WENT WRONG. AT A LOSS. THANKS

Have you checked other areas for vacuum leaks? You say it ran rough, and shut it off, was it slow cranking to start?

I haven't checked for vacuum leaks lately. when i hook up the vacuum gauge to the carb i get about 20-25 inches so i think that means its pretty good right? And no, its not that slow to start, maybe 1-2 seconds of cranking. Ive been watching more videos and im thinking i need to set the vacuum advance timing while the engine is running at 2500 rpms. which means that my timing mark will be very advanced when it is idling if im not mistaken. What do you think? and thanks for the quick response. i really appreciate it!

Vacuum advance is on idle till about 1500 rpm, at 3k rpm you are on mechanical advance. I'd suggest not knowing exactly what you have is isolate for a vacuum leak. I'd start with if you have power brakes, disconnect the vacuum booster, and cap it off.