Summary of repairs: Correcting stumble/jerk problem below 2,000 RPM
Now that I've acquired more experience on the GL1500 carbs, I believe I can now decipher what was causing the problem.
The carbs had two main problems:
1. Sticking float needle valve
2. dissolved jet cap o-rings which control the high and low speed circuit.
It is my opinion that my fuel related issue of only getting about 28 to 32 mpg had to do with a combined factor of failed float needle on the left carb, but more the dissolved jet cap seals which control the low and high speed circuits (jets). I believe the lack of o-rings on the jet caps was preventing the proper metering of fuel since fuel was entering behind the jet caps and feeding uncontrolled/non-metered fuel levels for mid range power. With the RPM's below 2,000 RPM, the high speed jet was receiving too much fuel in conjunction with the low speed jet due to the dissolved o-rings. This became obvious to me when looking at the jet-cap itself, as it has a specific hole size to limit the amount of fuel entering into the cap, which supplies metered fuel to both the low and high speed circuit. Thus at about 1,500 to 2,000 RPM's, too much fuel was entering the intake and causing a misfire, which also fouled out some of the plugs. By rebuilding the carbs and installing replacement o-rings for the jet caps, the engine now responds quite well below 2,000 RPM, albeit with a slight hesitation upon throttle roll-on, until the fuel has a chance to catch up; the accelerator pump momentarily supplied sprayed fuel to handle the tiny delay, which when combined, makes for a smooth transition of power. Thus below 2,000 RPM, the slow speed jet feeds fuel to the engine until RPM exceeds 2,000 RPM at which point the high speed jet supplies fuel.
A secondary issue was the lack of fuel at times due to a failed auto-fuel petcock likely caused by the use of ethanol based fuel. Albeit the bike has been running on ethanol since before I purchased the bike. The bike is roughly 20 years old, and has taken a long time for the ethanol to begin dissolving much of the internal rubberized components of the fuel system. Thus what caused my engine stumble and eventual shutdown last year on Thanks Giving day (and a few days ago) was a flimsy open/close diaphragm on the petcock valve to where at times it would open just enough for cruising, or else close altogether and cause a stall.
The spark plugs were fouling due to excessive fuel, which would also cause a spark-knock issue when insufficient fuel was metered causing a lean condition.
Thus in summary, the failed jet caps due to destroyed o-rings, a sticky float needle, and a failing auto-petcock also leading to spark knock and plug fouling, contributed to the pre-2,000 RPM jerking, poor fuel economy, and a nasty gassy smell. Since all of these repairs, the bike now runs quite smoothly and I appear to be getting approximately 38 to 40 mpg; only time will tell. Since repairs, engine speeds are now smoother and much more peppier. However, my GL1200 which I am still restoring, seems to be much quicker off the line, with a hell of a lot of torque compared to the GL1500. I suspect this is due to the GL1200 Interstate being a much lighter bike.
Hope this information helps. If anyone has a different opinion, feel free to contribute. I hope this will help others who experience the pre-2,000 RPM jerking/stalling issue.
Joe