Plugs my plugs are blackened and the engine misfires somewhat and backfires thru the carb. am i running to rich and do i need to get a vacuum gage to set the carb to the best vac. reading i can get and what would that be?

It sounds like the engine is running very rich, causing excessive exhaust and exhaust heat. The exhaust heat ignites the unburned fuel in the exhaust and causes the backfires. I'm guessing the engine runs quite rough with a noticeable decrease in power as well. You need to determine the cause of the misfire and fix it. How long has it been since the spark plugs and spark plug wires were replaced?

That sounds like a fuel/air mixture problem.

Just to confirm .. it is a carburettor, not fuel injection?

As a general rule of thumb (with carburettors..) if it backfires through the exhaust/muffler .. the fuel mixture is too rich.

If it backfires through the carb/air filter, the mixture is too weak.

It could also be a 'sticking' float in the carb.

When an engine is cold the fuel is rich .. lots of fuel .. that's how a choke works.. However, when the engine warms up a rich mixture causes problems. The car may splutter and stall, run sluggish or cut out.

You either have a bad catalytic converter causing too much back pressure or you are running rich. I think cylinder 3 and 6 are on the same side of the engine (except opposite banks) right? Locate the fuel pressure regulator. Is it on the same side? I've seen those go bad and allow fuel to be sucked into the vacuum line causing a rich condition/ misfire on two or three cylinders on that side of the engine.

bad need of a tuneup? like new spark plugs, ?
172 , is 02 rich/ lean warning. this one is low oxy for rich.
this code can just be a victim. for sure , with misfire.
means 02 sensor stuck at rich,
what is water temp in engine coolant, at failure? less than 150F?
could be lots of things.
leaking injectors, bad fuel.
injector sticking> causing misfire.
id use a scan tool and watch all sensor when it fails.
too high fuel pressure.
misfires cold, but only luke warm cold starts not fully cold.

just one DTC, no P0300 or 30x misfire codes, not even P0300 pendings?

P0105 Manifold Absolute Pressure/Barometric Pressure Circuit Malfunction,

Symptoms of a P0105 check engine light code may include:

• Poor running engine
• Engine runs rich
• Engine won't idle
• Engine backfires through tailpipe
• Engine misfire under load or at idle
• MIL (Malfunction Indicator Lamp) illumination
• In some extreme cases there may be no symptoms other than MIL illumination, could be caused by:
  • MAP sensor vacuum hose disconnected or plugged
  • Bad MAP sensor
  • Bad TPS good-day !

As you know, you have one of two things going on. it is running too rich or too lean. Run it hard for 15 minutes pull the plugs and check for black, wet looking soot = rich. Brownish, whitish dry looking soot = lean. Be sure your vacuum advance on the carb is hooked up to the proper port. Check the full advance of the timing with a light to be sure you have proper advance during rev up. You want 22 degs. full advance. If you have a pull choke try to close the choke to see if that will level out the hi end. If that works you are running to rich. adjust as necessary to lean it out. Too big a carb can also be your problem. For the best carb to engine size double the cubic inch of you engine. If your engine is a 290 ci you max size carb should be approx 550 cfm, Now if you engine has a hi rpm cam profile you have to factor in needed cfm at hi rpm. Thats why such and engine for racing will not have good bottom end or low rpm power. Sounds like you have just the opposite going on. Which can still be caused by a over sized carb. Too much fuel not enough air or room in the engine to burn it.

if is doing it thru the carb then the fuel mixture is too lean, if is in the exhaust (also called after-fire) the fuel mixture too rich.

I would check the float level first if you are going to adjust the carb., carters like to flood and if you check your spark plugs you can tell if your running rich. The other adjustment you will make with a simple vacuum gauge.You should probally check distributor points,and timing as well. More info would help, like is it back firing up the carb or out the exhaust or when accelerrating or decelerating.

check for vacuum leaks especially around the intake manifold gasket. Although the engine appears to be running rich, it could also be misfiring due to being too lean in one cylinder. When the cylinder doesn't fire the charge, it goes into the exhaust completely unburned and looks like a rich running condition due to the unburned fuel. If it is a vacuum leak causing the problem, the engine will run fine at road speeds but misfire at idle and not pass smog.

misfires can be lean/rich conditions at 1 cylinder only ..or spark or mechanical issue...sooo....compression/leakdown test to determine health of mechanical rotating parts ...valves pistons timing......timing belt......vacuum leak at intake manifold affecting mostly one cylinder...or no/intermittent spark to one area...coil/igintor wire plug ..read plugs... the misfiring cylinder will be different looking