The number one thing is ignition timing. Don't think you know how. If you haven't done it, you don't know how. And even if you have, you still don't. You need to find out how, and it really needs to be done. But it is a difficult job. I know of no other seemingly simple maintenance task that requires as much skill as this one, and I've been at this for almost 47 years now. And unfortunately, the need for this adjustment is not reduced by an aftermarket ignition system. Worse, the proper procedure is not explained in the official manual. And it's many times more important than carburetor work.
Close behind ignition timing in importance is the use of the very best ignition parts. This starts with factory Honda TEC brand points. Be sure to get the first-generation Honda parts, preferably not the current crop of inferior Honda points and definitely avoid the Chinese stuff such as Daiichi.
The carburetors aren't difficult to service, but they are quirky enough that familiarity with other carbs won't help much, and mere intuition is likely to simply get you into trouble. These are 1960s-era Honda car carburetors that were converted by the factory for use in the debut Wing in 1975 and are unlike any other Honda carburetor, though they superficially resemble 70s CB450 units.
Nearly every GL1000 I see in my shop has poorly-maintained spark plug caps. They are usually very loose on their wires, and inevitably have aged into the 10K ohm range.
If you have Dyna ignition coils on your bike, you should have been told two things. One, the stock GL1000 ignition coils are as powerful as Dyna coils. So you won't gain anything from that quarter. And two, Dyna coils come default with automotive type resistive plug wire. You can order from Dynatek non-resistive wire and that is what you should do.
Cylinder compression is vital in this machine. If it's below 150 psi using a quality (not a $49) tester, forget about engine tuning until that is corrected. And note: Worn cylinders and valves are the issue and finding someone to machine this engine's cylinders is a challenge, as is also finding quality rings and valves.
The idle jet is critical. It must be removed from each carburetor and "sized", that is, its size confirmed with a wire of known dimension. Sixty percent of this engine's operational finesse depends on this jet. And it is frequently neglected, and unless very recently looked at, almost surely at least partly plugged. Don't overlook this.
The rear wheel flange demands a goodly application of high-content moly grease. Honda still sells this grease. It comes in a squeeze tube and is also used as engine assembly lube.
Unplug your crankcase breather from the airbox. This will slow the sludge and carbon formation on the carburetor slides.
Do not use premium gasoline in your GL1000. This engine is already prone to accumulate carbon. Using anything that will accelerate carbon buildup is a bad idea, and this includes that snake oil of snake oils, Marvel Mystery Oil.
Do not use carb rebuild kits. No professional carb rebuilder uses them, and no one who cares about their carburetors should either. Kits containing replacement jet needles and needle jets result in the most seriously negative alteration you can make to these carburetors.
One of the bad things about carb rebuild kits for the GL1000, other than the near criminal replacement of factory metering parts with Chinese substitutes, is the unfortunate fact that the o-rings for the primary and secondary main jets in these kits are incorrectly sized and tear when installing the jets. No o-ring supplier and no kit contains the correct o-rings. They are available from only one source, and that source does not sell to the public. Fortunately, finding a business that has an account with this source and will sell the o-rings retail is not difficult. One such retail outlet is Amazon, believe it or not.
The throttle shaft felt seals on the GL1000 carburetor are not vacuum seals, they are dust seals. As such they cannot be expected to solve an engine performance issue. If you're experiencing such an issue, diagnose and go after the real problem.
Twisted throttle shafts are virtually endemic to GL1000 carburetors. It happens when the machine is resurrected from the dead and someone inadvisably exerts extreme force on the throttle grip to overcome the varnish buildup inside the carbs. The delicate shafts easily distort.
Unless rebuilding your carburetors each season is your idea of a good time, use Sta-Bil fuel preservative. And since the time it takes for fuel to start to gel is less than the period between rides for most of us, use it full-time.
The GL1000 carburetor is very much at risk for corrosion. This is because it is made of zinc, not aluminum. Actually, it's an aluminum alloy but one that is unusually high in zinc, making the casting extremely susceptible to corrosion. The result is that these carbs can corrode badly. And it has little to do with ethanol.
An inexplicable design flaw on these carbs is that they have no overflow telltale system. In fact, no Wing carbs do, four-cylinder or six-cylinder. (Many of Honda's late model carbs-- V4s, CBRs, even cruisers, don't either.) This means that when the carburetors overflow-- and they inevitably will-- there is no outward sign, no indication. This is not helpful for maintenance and poses a potential deterioration problem for the engine as the engine oil is continually at risk of fuel pollution at any given moment.
Eventually every GL1000 will start to overcharge its battery. The design of the charging system (permanent magnet, three-phase) coupled with the historically inevitable failure of the voltage regulator, makes this a sure thing. Constantly feeding 16V @ 9.5A into the battery theoretically can make it explode. It can damage the lights also. The only reason that may not happen on your bike is the fact that the GL1000 is a voltage sponge-- its electrical system, and particularly the huge 18 A/H battery, sucks up all the electrical energy it can. But don't count on it. I see this overcharge often, and it is not a safe situation.
Stay away from K&N and Uni air filters. Neither filter well-- especially the K&N, and the oil in the foam type tends to creep downward into the carburetors.
Easily two-thirds of the advice on popular GL1000 websites is not good information. Most of them don't even know the purpose of the hoses between the sides of each carb and the air chamber (plenum) and have been debating the necessity of static ignition timing for decades, despite Honda's documented preference and the experience of pro mechanics. There has unfortunately been a long history of misinformation about this model online.
All years of the GL1000 exhibit an off-idle hesitation. Even the better-tuned 78-79 models. There are different approaches to eliminating this performance glitch. But bear in mind one thing: The GL1000 engine will never throttle like a GSX-R Suzuki. It has a massive inertia-storing automotive style flywheel that prevents instant throttle response. But careful tuning can make a significant difference.
Although considered a matter of preference and vigorously and incoherently debated on the 'net, the GL1000 like most 50-year old machines is ridden so little that it is an excellent subject for switching over from glycol brake fluid to silicone to prevent brake cylinder corrosion.
Like many 70s Hondas, the GL1000 starting system's freewheeling (sprag) clutch is negatively affected by heavier oil and by synthetic oil. Also, long periods of sitting promotes moisture accumulation which results in rust in this part, further deteriorating its function. Watch out for this.
Rusty fuel tanks are a hurdle on this machine. There are many approaches to repair but removing the tank and removing the rust followed by zinc plating is probably best practice.
It's a great bike, but a GL1000 is not an investment. Every quality part you will ever put into a GL1000 will be diligently sought out over long periods online. And virtually anything you do to this bike will exceed its resale value. This is generally true of most 40-50 year old Hondas. But it is all the more so of this one because the model just hasn't appreciated in dollar value. Maybe this is why so many of these bikes are chopped up these days, turned into "cafe" and "bobber" and similar variations.