Think I have a GL1100 carb problem

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I'm sure someone will be along soon to help you out, but while you're waiting
Welcome from the state of Pennsylvania!

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sounds like you have 2 problems - one dead or slightly working cylinder and rich condition causing performance problems

To answer your question, yes there is an adjustment screw for the idle/pilot circuit, it is located behind the runner tubes leading to the cyl head on the carb. It is somewhat difficult to turn, if the caps are off I use a right angle screwdriver. Sometimes you have to remove protective caps to access these screws- you have to remove the carbs to do this.

I agree you may want to try the Seafoam route due to ease, but anticipate having to clean the carbs to achieve good performance again.

Here is a test: get bike warmed up, then unscrew the vacuum test screws located on the runner tubes. As you pull each screw out, you should hear the engine drop rpms in an equal amount as each cylinder drops out. If you have pilot/idle jet trouble the drop in rpm will be a lot less. Since you are complaining about lack of power, sounds like you have one dead cylinder...this should help ferret it out. Good luck!

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going back to what you said originally - the mufflers have a "black coating" - what do the plugs look like? are they all the same? do one or two appear to be running rich? This is the first question to answer.

It is not unusual to see the emulsion tubes above the jets become clogged if the bike has been allowed to sit and the petcock is left in the "on" position. I had one come to me the other day and one carb was clogged with what looked to be algae in the bowl and in the emulsion tube area. Cleaning that would have solved the problem, but anyone who has ever removed the O-rings connecting the fuel lines/vent lines or seen the overrun diaphragms after exposure to fuel for nearly 30 yrs knows replacing all the elastomers in the carbs is a good thing.

There is also a question about the choke mechanism - could it be sticking on? This could cause an over-rich running condition. This might be seen if ALL of the plugs appear black and sooty.

In comparing the GL1100 to a CB750, it is easy to say "well, my 750 was faster"... the 750 was a different bike, sportier and lighter. A good running GL1100 is smoother, but does pull with authority when revved. It should have plenty of torque for acceleration at 65-70mph, and more as it revs higher. There are a host of small things that can affect how the carbs run, but let's start at the simple ones... does it crank easy? are the plugs looking good? does it smoke during cranking or overrun? does it pull smooth up to redline?

Lets' start with these questions, go from there

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The "runners" are the tubes connecting the carbs to the cylinder head - black rubber tube attached at the "inner" side, flanged 2-bolt connector at the other. It is typically the most visible part of the intake system, you may look at it and just see "carb" but they are removable.

There is a vacuum port screw down near the cylinder which points at the front for cyl #1-2 and toward the rear on cyl #3-4. Be careful when you remove these to not lose the aluminum seal washer... you will not find one at Ace Hardware. I typically use a long #2 phillips screwdriver to make removal easy. Loosen, then tighten them back slightly. Then crank the engine - by removing each in turn you can find out if one cylinder is not firing or not synchronized with the others. Do this at idle and at approx 2500 rpm, if the drop on all 4 is similar for both situations, you have a problem with all 4 carbs.

It is common for the carbs on the left to clog up and not on the right... mainly due to the bike being kept on the sidestand leaning to the left.