QuestionHI,
This question has been asked multiple times on this site and it seems nobody has clearly answered (or solved) the question.
Here it is again. I have a DT175 (2-stroke) that I am restoring. I bought it and it did not run. I cleaned the carb 3 times and believe it is clean now. I got it running, but now the problem is that it bogs down at about 5000RPM, like it is running rich. I can get any power out of it! The bike idles beautifully once started. And when cold it is a bit hard to get running.
I replaced the main jet with 140 (stock). I have inspected the reeds, the coil, the seals, the exhaust, the air filter, the plug. The CDI, and the compression (is 114lbs). I even tried raising the throttle needle up one notch (which I was told would lean it). It did not. :(
What else could be the problem???? How the heck can I get it to lean out on the top end? I have read many threads on the internet from people having this same issue, so it must be common.
Any help is greatly appreciated!
Exhausted me....
AnswerHi David,
The three-quarter to full throttle fuel mixture is
handled by the main jet and partially
by the carb slide needle and needle jet.
If leaning out the main jet had little or no
effect then it indicates that the
exhaust pipe packing is likely
fuel of two-stroke oils and is not
allowing the flow of air through the engine.
This is of course assuming that
you have checked the air filter and
air intake for obstructions.
Raising the needle makes the fuel mixture
richer not leaner. In other words
if you lowered the needle clip it would
get richer as the needle would be raised.
If you lift the needle clip up a notch the
needle drops down which makes it leaner.
The mixture always gets richer when the needle
moves upward as it does when you open
your throttle. Be aware that the clip
goes up as the needle moves down.
I have had many small Yamahas with this
problem and if reducing the main jet slightly
did not help I had to cut the back of
the exhaust pipe open and remove all
the oily packing.
If the exhaust cannot get out it holds
the fuel back inside the engine and it runs rich at high speeds.
The air has to flow in the engine and
back out freely or you will experience problems
at high throttle openings.
If you have excess exhaust smoke it can also
be the pipe or a blown clutch side crank seal.
If your spark plug is black then it is probably
the above. If the plug is light colored
you could have air leaks around the intake
or the cylinder base and head gaskets.
Some Yamahas have an mechanical ignition advance
under the flywheel. This needs to be free to advance
the timing for high speeds.
A stripped flywheel key can cause problems.
If none of the above help then you may have
to remove the cylinder and confirm that
you don't have a broken piston skirt causing problems.
If the engine is okay my guess would be
cut open the pipe and clean it out,
then go with the largest main jet that
will allow you to get full RPM's so
you don't overheat the engine with a lean mixture.
Even if you can blow air or solvent through the pipe
that doesn't mean it is clean enough for high
speed air flow. The oily packing has to be removed.
It is a pain, I know.
Good luck,
Wayne S.
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