I got a six pack on a 350 sbc and it's only hitting on 4 cylinders. It's getting fire. Timing is right .what could cause this. It runs off middle carb the front and back kick in like a 4 barrel sort of

It's called a progressive linkeage, Look for a vacuum leak, I saw a dual quad setup once where someone forgot to install a plug on the base of the forward carb, youcouldn't see it but found it with either. I hope this helps you

If the intake manifold has a common plenum all 8 cylinder should fire but there might be lean cylinders if a carb is not working properly.

This link might help you to set up the carbs correctly. https://www.speedwaymotors.com/the- toolbox/how-to-set-up-multiple-carburetors-on-a-vintage-engine/28770

Do you have the correct carbs in the correct locations? The primary carb needs to be in the center. Secondary carbs go in the other two spots.

Doesn't the middle carb have a choke plate and the others don't? I saw that some where but don't remember where.

That would be correct for a progressive linkage setup. The secondaries are not set up for idle or choke.

Isn't that what this is I may have misunderstood

Another link that might help. https://www.speedwaymotors.com/9-Super-7-3x2-Carb-Intake- Manifold-Kit-1957-86-S-B-Chevy,37448.html

The point is to figure out why your engine is running on 4 cylinders. Make sure you have spark on all 8. Pull the plugs to see what they look like and check the cap, rotor, points, wires etc.

I put new plugs in it new distributor button and cap new wires. It ran really good for about a year. Was wondering if it could be the intake gaskets.

Yes, spray around the intake especially the intake runners to the missing cylinders use either or carb cleaner