It has been a long time sense I have done any of this. When setting the timing on a SBC do you disconnect the vacuum advance hose?

Yes And Plug It Off

Thanks, I thought thats how it was done but just could't remember for sure.

Please keep in mind that after you have set the timing to re-connect the vacuum hose. If you are connecting to a full manifold vacuum source, the idle RPM will increase when you re-connect the vacuum advance hose. At this point lower the idle speed with the idle stop screw. The idle should be set with the trans in Drive if an automatic. Be sure to have the emergency brake applied and have someone else holding their foot on the brakes when adjusting the idle speed with the trans in gear.

An alternate option for this adjustment would be to set the idle in Park at about 800 RPM; then with your foot on the brake pedal, drop the trans into Drive and check the RPM. It will drop some. You should look for around 650 RPM in Drive at an idle for most applications.

After I got the timing set, I adjusted the idle speed. Its a three on the tree. I set the idle to 800rpm. then I adjusted the air fuel mixture, using a vacuum gauge. Then set the idle again to 800rpm.
I was hoping this would cure a problem for me but it didn't.
The truck backfires through the carb when accelerating up hill when the rpms aren't very high. It will do it on a flat too but not as bad. If you keep the rpms up some.

how long has this engine been in the truck

What is the timing at idle (without the vacuum advance)? What is the Total Mechanical Timing (without the vacuum advance) and at what RPM? Are you using Full manifold vacuum or ported vacuum? Does the backfire occur when slowly accelerating or when going to WOT?

I'm not sure. I just got the truck. Its a 77 chevy 2 wheel drive three speed on the column. The truck used to be an automatic. The trans is newer. it has metric bolts. It needed a clutch when I got it, thats done. I'm currently trying to find out what engine it is. It has a stamp code of 1A1185020. I couldn't see the casting code on the back of the block.

I set the timing to 10 degrees at idle. I don't know what the total mechanical timing is. Not sure how to check that.
It is hooked up to full manifold vacuum.
The popping happens at heavy throttle.

check your vavle lash .if the valvas arent opening and closing proper the engine willbackfire .you could have a worn cam or stretched timing chain.any idea of milage

Speedometer don't work so no idea on the mileage. one guy that know a little about the truck told me it is supposed to be a relatively fresh 350. Another guy looked at it and said it was a 305. I don't know. It runs pretty smooth now that I set the timing and air fuel mixture. It seems to sound and run decent too.

I just got the casting code. Its 14010207. From what I can tell it an 80-85 350 car engine.