Motorcycle Repair: carb problem??, mtx 200, engine seals

Question
hi wayne hope you can help,, i have a 1985 mtx 200 r honda, at 85-95kmh it hesitates(misses)as this is normal cruise speed it is annoying,my friends mtx does the same,, bellow this speed and above this speed its fine,i have cleaned and checked the carb and reed valves, all engine seals are new, it has electronic ignition which seems fine,i have tryed to lift and lower the needle but the problem is still there, in the first three gears it passes this point fast and it is not noticed,but in top gear ,,a real pain,, thanks for any advice

Answer
Hi Adrian,

There are a few things that can contribute to this
problem. The first thing is the design of
the two stroke engine. It fires every stroke
so has a constant push of piston acceleration
to deal with. This is sometimes countered by fitting
a heavier flywheel to keep things moving steadily.
Most enduro type bikes are designed to rev quickly
so the flywheels are lighter and made to rev fast.

This design is prone to surging at times but
a lean fuel mixture is often the main culprit.

The carb is designed so different parts affect
various throttle openings.
Picture: (copy/paste the link)
http://home.everestkc.net/malsin/Motorcycle/carb/mc_carb_theory_101_files/image0...

The problem you describe likely has it's roots
in the half to three-quarter throttle area.
This is the domain of the jet needle and it's jet.
The jet needle is tapered to allow fuel to pass
at different rates depending on the amount of taper.

As you open the throttle and it nears the three-quarter
position the taper controls the fuel flow
through the jet.
The condition of this needle and jet and the amount
of taper controls the fuel at cruising speeds.
If these are worn or the needle taper is too thick
then you will have a lean fuel mixture at that throttle
setting.
This throttle opening is also somewhat affected
by the main jet.

I would suggest trying dropping the needle clip
(raising the needle) all the way and if that doesn't help
try one size larger main jet and maybe dropping the needle
if necessary.

The next thing to try would be inspecting the condition
of the needle jet and needle and finally
seeking a needle with more taper near the bottom.
A thinner lower end taper will increase the fuel at cruising speeds.
In this picture the needle on the left has a narrower taper:

http://www.howmotorcycleswork.com/r6/carb21_needles_zoom.jpg

So, the surge is likely a lean mixture at that throttle setting.
Also,keep the air filter clean, use a good plug (NGK) and check
the spark plug cap.


Wayne S

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