Old dt175 "bogs" down

So I bought this old dt175 on ebay for about $100...


Repaired a bunch of stuff over the winter and spring and it starts on the
first kick (after I fixed the kickstarter).

But it seems to bog down at higher rpms. Did my best to clear the exhaust
of carbon build up, compression seems good, dissassembled and cleaned the
carb, but it really lacks power, doesn't want to "wind up" and go like a
2-stroke should. Besides a fouled plug what else could be wrong with the
ignition?

I have two of the bikes, the other one pulls much stronger (after the
top-end rebuild) and doesn't bog at higher rpms. But its carb has new
internal parts.

Any ideas?

Thanks,

Bryan

 

Does the left hand side oil seal leak air under the magneto cover? The
engine will run really rough and crappy at part throttle if that oil
seal blows out...

Does the engine "four stroke" when it fires? That is, does it fire
every other compression stroke instead of every compression stroke?
That makes the engine sound like it's gargling on excess unburned fuel.
An over-sized main jet would do that...

Can you find an online parts fiche like www.partsfish.com to see if the
main jet and idle jet are the right sizes? The main jet size affects
operation from 3/4 to full throttle. The idle jet affects the engine
from idle to 1/4 throttle mostly, but there is always some gasoline
coming through the idle jet until full throttle...

Is the jet needle clip in the right slot? The jet needle affects
operation from half throttle to 3/4th throttle.

Has somebody tweaked the idle mixture screw and gotten it all out of
adjustment? That usually affects 1/4th to 1/2 throttle more than the
main jet...

Is the ignition set at the right timeing before top dead center?

 

Hmmm, I have a mysterioul leak, left side it appears... But the problem is
at wide open throttle.

Yeah that is a good description. I have 2 of these beasts, bike 1 and bike
2. Bike 2 has bike 1's carb, with new parts. Bike 2 is using bike 1's
cleaned carb... No explanation, that is the way things turn out.

Bike 2 has new upper end. Piston rings and cylinder machine work.

Would excess fuel cause fouled plug?

Bryan

 

excess fuel/oil will foul the plug, whats your air filter like?

 

Well, engines are supposed to get all their air from what passes
through the carburetor. But, if the oil seal on the dry side of the
crankshaft is leaking air in behind the magneto cover and the magneto
cover gasket is holding air, the
space under the magneto cover would act as extra volume in the
crankcase, reducing it's ability to pump fresh fuel/air mixture up thru
the transfer ports.
Are the magneto coils getting really oily? Do they smell like gasoline?

I've had the crankcase of a 2-stroke fill up with gasoline twice. The
float valve stuck and I'd left the petcock in the ON position. The
engine would fire and turn over few times and the gasoline/oil mix in
the crankcase would immediately wet foul the plug...

Yes, it would. But Yamaha has always injected oil into the intake port,
so oil would mix with the gas downstream of the carburetor and whatever
fouled the plug was always a mixture of gasoline and oil making it hard
for riders to understand whether it was rich jetting or excess oil
causing the problem...

Have you ever removed the oil filler cap on the clutch cover and felt
for pulses of air while the engine is running? If you're getting air
come out of the oil filler cap, the wet side oil seal is leaking and
transmission oil can get into the crankcase and foul the plug...