I am having issue with my fresh rebuilt. One cylinder is not firing. I was having similar issues previously. I ruled out many options. I am fairly certain it is a carb issue.

I have weber 44 idf's, engine is 2258.

Cylinder 4 wasn't firing. The spark plug on that cylinder is fuel fouled. Engine backfires through exhaust at around 2000rpm

Here is what I have done so far-
compression test- all cyl 180+/- 5
changed spark plugs- no change
changed spark plug wires- no change
changed distributor- no change
swapped main jet stacks- no change
swapped 1,2 carb with 3,4 carb and now cyl 4 fires fine but cyl 1 now doesn't fire (see why I think its a carb issue)
made sure the carbs were synced- no change
checked to make sure all jets were properly sized, no clogged and no burrs.
I looked down the carb throat that wasn't firing when the engine was on and I do not see any fuel dripping.
Double checked to make sure float height on both carbs is the same.
Looked at the idle needle- looks ok to me

Any other ideas? I'm sure it some stupid thing that I am just overlooking.

Clogged idle jet. Pull air cleaner and toss a rag over carb top. Pull out idle mixture needle and blow compressed air into mixture needle hole holding rag on top so fuel doesn't mist everywhere. If it's clogged up good you might have to remove idle jet and clean and blow it and the passages out.

clean the spark plug. Clean the jets. Reset the mix to 2.5 turns off fully seated. Try again.

This is a very common new engine start up issue.

You might also want to check your pump jets. Verify that you get an even stream from both. If one is clogged up or the check valve is frozen, all the fuel will squirt down one cylinder.

Also verify that your idle jet vent isn't clogged or covered with a gasket. Blowing it out will clear a clog but not a gasket. Someone had a gasket issue a while back. If this is the case your idle mixture won't be mixed with air.

LOL- that was me who had the issue with the gasket. These are different carbs and different gaskets. Everything seems to line up perfectly.

I thought it was you but wasn't sure. Barring machining or casting issues your problem is most likely in the idle circuit or accelerator pump circuit. So you need to think what would cause an excess of fuel in those circuits on that particular throat.

Idle
1) plugged vent
2) mixture screw seat/needle is hosed

Accelerator
1) Plugged jet on opposite side
2) check valve issues
2) mismatched pump jets

Slim chance on this one - Are your vents the same size? If you had a small venturi on your problem barrel you would draw more fuel. Are these carbs used or new?

I checked my idle circuit- the vent was patent. The idle jet was clogged with some gunk however it was clogged in the area where the fuel flows into the idle jet. Could that cause my issue?

I took out the mixture needles and they look good. I do not know how to check the mixture screw seat.

Both accelerator jets seem to shoot the same amount of fuel. I switched the jets to a different carb so if I get a new cylinder that isn't firing, that is probably my answer (however I guess that my issue is in the idle circuit).

I did check my vents and all are 36 mm.

These carbs are lightly used (by someone else). They are spanish 44 idf's.

A clogged jet in that area would allow little to no fuel to flow at idle and you'd have a rough 3 cylinder idle until you transitioned to the main jets. It should run much better now. Run it and see if you foul up the plug again. Cross your fingers. From your description it sounded like you were too rich and fuel fouling your plug.

With a lightly used set your seats are likely fine. Tough to check with them mounted. It's easy when they are off and you can eyeball both sides with a light.

Yep. Clean your idle circuits and try again. You will get there!

Zach

Update- go figure that cleaning boogers out of the idle jets would make the engine run normally!!! Everything works fine now!!!