Very low power on 1989 gl1500, idles smooth but stumbles when you try to accelerate in N. top speed about 75mph.

no miles stated, 400k miles> or 100 = 100k?
if any 95 has 100k miles that be like a new car. no ?
5k /year, wow...
trouble is i cant drive car. and see this effect.

the issue is acceleration, right? :called Hesitations !
and only happens on a hot engine , yes?
and never happens wide open throttle.? correct?
HAPPENS:
only when tipping in the throttle at cruise. (then self corrects )
THE TPS sensors does that.
The TPS is a demand sensor. it sees driver DEMAND
acceleration, and this is detected and the PCM goes to ENRICH mode. this is done or the engine goes LEAN until the MAP BARO READINGS, catch up and correct fueling.
that is my wild useless guess. (not driven and me not scanning the PCM while it fails.)

The TPS is a rate of change sensor.
This is because all air meters made are slow.
The TPS hides this fact. until bad.
there are other causes,but im not there with my scan tool..
and (box of gauges)

A blockage in the exhaust would limit top speed but not power off idle.
Sounds like lack of fuel, either because of a sensor or low fuel pressure. Or a vac leak.

Clean the mass air flow meter.

This is a common problem related to plugged or very dirty low speed idle jets in the carb, When they get that bad it is best to have the carbs cleaned by a reliable mechanic.

The high speed mixture screw is initially set at 1 & 1/2 turns out. Then adjust it to give good acceleration and top speed, while not closing the screw more than you need to. The low speed mixture screw is set at 3/4 of a turn out. Adjust that, to give a smooth idle. The 3rd screw, the idle speed screw, is set to give a low idle speed without stalling.

Your engine is stumbling "off idle" - as it switches from idle to throttle control. The idle air motor regulates the air flow and engine speed at idle. If the idle is smooth but the stumble occurs just as the throttle is pressed this isn't the problem, The MAP sensor is mounted in the middle of the firewall behind the engine just below the hood. It has to sense the manifold vacuum drop when the throttle is opened. This my first suspect. Second would be the fuel pressure regulator. If it runs O.K. after you get it going, this is a weak suspect unless it loses power when pulling. A bad fuel pump, again low fuel pressure. Just like the squirt from the accelerator pump in the carburetor days, the injectors have to deliver a good stiff shot of fuel when the throttle opens to keep the engine from stumbling. The MAP sensor will tell the ECM a squirt is needed but the fuel system has to have the pressure to deliver.

Faulty idle air control valve. Fuel filter clogged. Damaged or wet ignition system wires. Faulty idle control system. Low fuel pressure. Ignition timing or timing chain.

After warming up the engine at part throttle for a couple of minutes, clear it out with a 3 second burst. Then idle the engine.. If the chain rotates, turn the idle speed screw to the left (CCW) until the chain stops.
Turn the LO NEEDLE slowly to the right (CW) to find the fastest engine speed. If this causes the chain to rotate, repeat idle screw adjustment to stop the chain from rotating.
High speed (with throttle wide open). Set needle 1-1/4 turns open, checking for smooth acceleration by opening the throttle wide for 3 seconds. If the saw stumbles, turn needle to the right (CW) 1/8 turn @ a time until smooth acceleration is obtained. Try a saw cut to see if adequate power is available. Continue making 1/8 turn adjustments (CW) until saw shows a power drop when asked to cut. (This indicates too lean a mixture setting.) Then, from the setting in step 3, turn the HIGH NEEDLE 1/4 turn to the left (CCW). Check the cutting ability of the saw again. This should be the best setting for your particular altitude, temp. and humidity conditions.
To pre-set before initally starting the saw, turn LOW and HIGH and IDLE screws in gently till each one just stops. Then turn each out (CCW) 1 - 1 - 1 1/4 turn.
Hope this is helpful. Try www.zamacarb.com/tips.html Select the "Tech. Guide"
Good luck and God Bless.
oft