Loosing speed top end dropped by 15-20mph - 2003 Suzuki GZ 125 Marauder

adjust the idle mixture screws first and then adjust the idle speed screw to compensate
adjusting the idle mixture screws will alter the idle speed but when properly adjusted then adjust the idle speed screw,

which of 3 engine do you have, gas or diesel and what size?
Are battery terminals clean????????????
Ill assume yes.
below will be 4.0L answers ,gas.
id say lost 12vdc power to EFI system and cluster.
the cluster dropping is lost power.
the cluster is computer in its own right.
and has 2 power pins, one is power to run, and power memory.

can be simple as a loose fuse.
to find it i inspect first. (im retired ET and can find them all)

if you had used a scan tool ($10) it would tell you why.
for example see all sensors go dead.
or the scan tool drops comms mind scan, lost DC POWER>
the scan tool will also tell you if the ASD relay drops and WHY!!!
if 12vdc did not die, first.

i start at the battery, after al batteries are weak technology, (acid/lead) some only last 3 years, (weak tech)
battery lugs corroded (at 4 ends)
2 cables 4 ends.
the hard part, (only) is the intermittancy.
cant fix working perfect cars, can not.

might be a loose fuse?
to isolate the true cause next time have all things ON , in car
head lights, HVAC blower on max. radio. all things.
now what dies????? a big big clue that.......!!!!
the power system is like a tree.
what branch is dead, puts us on track.

04 WJ jeep , i have the full factory FSM
can take you to the spot it fails.

lets say cluster power pin C1p7 drops dead (02 fsm names)
that means power from JB fuse 22 dropped. JB is cab box.
or its bad, (hair cracks in fuse, loose, or rusty) try a new fuse.?
or Fus22
it's input died, and that his the key switch bad, or before it.
called ST Run 21 (fsm)
see photos below for St run 21 feed, sources.

the PDC fuse top seen here, is PDC fuse F8 (engine bay)
if that top path is good, the the key switch is bad.
end story.
finding it, i use a logging voltmeter,
in the olden days, id have an analog meter sitting on passengers
seat, and at stall glance over, and go, gee, the F8 feed is good
so move my test wire to ST21 side, see it die or not.
walking the test wire from top of below to the cluster.


battery is top. last item below is cluster,
this is the fail path.

Loosen the knob at the rear of the top arm - turn it counterclockwise several turns. The blade should now be loose. Push the "blade end" of the top arm down slightly with one hand while you unhook the top of the blade. Lower the blade a little to unhook it from the bottom arm.

Replace the blade. Hook it onto the lower arm first - you can look underneath to see what you're doing. The blade's teeth should point DOWNWARD - it's easy to accidentally install it the wrong way, so check that now. Again push down slightly on the end of the upper arm, then hook the top of the blade into the upper arm's blade holder. It should how stay in place, but loosely.

Now return to that tensioning knob at the top rear. Tighten it slowly until the blade is BARELY tight, not loose. Tighten the knob 1/2-turn to one full turn tighter to tension the blade.

Compare cranking speeds cold to warm most diesels require about 375 RPM's cranking to start. NOW possibly a bad starter (dragging) voltage drop (high resistance) in cables. loose or dirty battery cable ends. To test voltage drop requires 2 people one cranking engine other watching volt meter. Place one meter lead on battery cable end and other lead on the other end of cable. Do this to all 4 battery cables. What voltage that can not go through cable will go through meter. So the closest to zero is the best. good readings are .02 volts for connections and .05 volts per foot of cable.

The screws are roughly 8 turns in from fully out when starting to set the speed. On the lowest speed 13 revolutions of the planetary in 15 seconds is a good speed.

what sort of noise clunks&bangs could be anything from shock mounting bushes exhaust hanging mounts or suspsion bushes.it would be a good idear 2 have a mechanic put it up on a hoist 2 check it out

you can change the sprockets.If you go to a 16 tooth front sprocket. two teeth on a front sprocket is like droping 4 teeth on the rear. You could drop 2 teeth on the rear sprocket but a 43 tooth is probably harder to find. the usual choices for front are 14,15,16 tooth.
For the rear they are 44,45,46 tooth

The way to remeber gearing changes is on REAR sprocket is smaller=faster and larger=slower top end.
the FRONT is OPPOSITE to the rear smaller=slower and bigger=faster.
You may have to lengthen or shorten the chain depending whjch way you go.

Your catylatic converter may be clogged if your loosing power or top speed is less than it was. You can check this when its running with a battery operated infared heat gauge. the temperature of the muffler should be around the same temp. as the catylatic converter.

These bikes never had bottom end power, they only make it at the top. You would have to drop compression to help some at lower end but there would be sacrifices to high speed where this engine was designed to run at. Does it still have the factory pipe, if not it will be even worse! Sorry but in this case there is no good answer. Or at least one that you will want to hear.. Again sorry...

It may be that the speedo cable has come loose at 1 end and will need to be tightened back up. Sorry but not sure where ends are attached on your car.