Motorcycle Repair: wr 426, iridium spark plug, fuel air mixture

Question
I was riding my 01' wr 426 in 5th gear and. the bike would surge like it was hitting to governor. I would let off the throttle for a second then get back on it and it would go for a minute, then start to surge again. Finally I lost all power in 5th so I shifted down to 4th and was able to ride with no problems in the lower gears.
Any suggestions would help.
Thanks
Aaron

Answer
Hi Aaron,

These bikes can require alot of trial and error tuning
to get things right.
I suspect your engine was either loading
up and fouling the spark plug or it was
lacking fuel for some reason.
An overly rich fuel/air mixture can
cause a bike to studder and fail to reach maximum rpms.
A lean fuel/air mixture will usually result in cutting out
or stalling.
Sometimes the best thing to do is install a clean spark plug
and ride at the problem speed for a short distance, then
quickly stall or kill the engine and check the plug colors
for clues on what is happening inside the combustion chamber.
http://verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

Once you know if it is running too rich or too lean you
can make the appropriate changes to your carb settings
or check for other problems.

For a starter I would just try running some regular fuel
and an iridium spark plug which resists fouling.
Set the pilot screw on the carb bottom to 1.5 turns out.

The needle inside the carb center can be raised to make the
mixture richer or lowered to make it leaner.
This needle goes into the main jet and controls
all fuel from 1/4-3/4 throttle openings.

If your problem is at 3/4-full throttle then that is
handled by the main jet only. Bigger is richer and
smaller is leaner fuel mix. Use caution when reducing
the main jets size as if it is too lean the piston
could overheat and burn a hole in itself.
Always do the plug reading I described above after
any changes to jet sizes.

Try to use iridium or at least NGK brand spark plugs,
most other brands will not work as well.

Make sure your carb is clean before you change anything.
Also check the tank screen and add a fuel filter.
Even one drop of water in the fuel will cause problems
as the main jet is low in the float bowl.
Use a clean air filter.

Once you have a good spark plug reading then
you can start tuning the carb.

I have seen bikes where the exhaust was partially
plugged and that will also cause plug fouling.

So to summarize use a clean carb, good spark plug,
try regular fuel as it burns faster than premium.
Do a plug reading and make changes as required to
jetting and carb settings.

If it revs out okay on the lower gears you may
gain by some sprocket changes as well.
Fifth gear is only for higher speed riding and
can bog the engine if the RPMs are too low.

Check thumpertalk.com for tons of info on dirt bikes.

Wayne S.
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