Dual Solex side draft carburetor issues

My boat is a 32' 1981 Bayliner Explorer. It has twin Volvo 140 4 cyl engines equipped with twin Solex side draft carburetors.
The recent symptoms are that when the port engine is warmed up, I can't get a smooth idle at 900rpm! It's not too rough of an idle, but when you chunk it in gear, the engine sounds rougher and drops down to 700-750 rpms. It only smooths out with more throttle(because of the shared intake manifold to all 4 cylinders). The starboard engine doesn't behave in this manner. When in gear it only drops around 50-100 rpms and purrs like a kitten.

A possible culprit could be the rear carb that feeds #3 and #4 cylinder.
One test I performed was that during idle, I pulled #4 plug wire and it didn't change idle or make engine run any worse!!
I also did the same with #3 plug wire and it changed slightly for worse.
When I pulled #2 it ran a lot rougher.
I suspect the front carburetor is compensating for the rear carburetor's performance. Oh the joy of sharing the same intake manifold!

Other trouble shooting I've done was:
1.) swap dist caps and rotor- same results! (starboard engine runs good with swap)
2.) replace port engine plug wires and plugs with brand new ones- same results!
3.) adjust idle speed and idle mixture screws- same results!
4.) pulled rear carb, split open, inspect (looked very clean), reassemble and re-install- same results!
5.) checked timing- looks good!

Years ago, I replaced the points and condensers with Pertronix on both engines, so I can't trace it any possible timing issues.
The old spark plugs I pulled, all looked the same as if they were burning normal. The electrodes had signs of wear, but their color was not brown or black, just whitish.
More trouble shooting I can do is:
1.) do another plug check on all 4 cylinders.
2.) double check timing
3.) compression check on all 4 cylinders
4.) swap rear carbs with starboard engine to see if I get the same rough idle problem. If this fixes the problem then I need to think about having the carb rebuilt or replaced!

I might have it narrowed down to a carb problem, what do you think?

I did the carb swap with the starboard engine and now the port engine runs great! Now to replace the bad carb. Does anybody know where I can purchase one online?

Swapping the carbs may have cured the symptoms, but there is not much in a Solex sidedraft carb that can go wrong. You may be better off overhauling your faulty carb. Also, I think that your best bet to obtain Solex 44phn carbs is eBay, but you never know what yo will get.

The other day when I first split the carb, I inspected, cleaned and re-assembled it. I did noticed that when I screwed accelerator pump nozzle passageway screw back into the body, the hole had no threads! It's like it's being held in to by the bowl when I reassembled it! I suspect this is not normal and may be the reason I can't get a smooth idle out of it. Is this screw supposed to be a cap for the passageway or is it a metering screw? The Seloc manual states (on page 4-28), "Thread the screw covering the accelerator pump nozzle passage into the carburetor and tighten it just snugly".
It didn't have threads to screw back into! I do remember that it unscrewed when I removed it. The screw is also made out of aluminum, which I suspect is odd? Maybe when the previous owner had the carb rebuilt, the company he used might have failed to do it correctly and just stripped it out?? Finding another Solex Side draft carb body is rare, unless somebody has an idea where I can locate one?