Motorcycle Repair: Wont stay running!, fuel air mixture, vacuum leaks

Question
Hi, I own a 1992 Kawasaki kx80 2 stroke. I am having problems with keeping it running. I can only run the engine with the choke on and when I take the choke off, the bike dies after 20 seconds. I have cleaned the carburetor and tried it again i still have the same symptoms. I even replaced the pilot jet with a brand new #55 jet and with the new jet it wont even start. I have inspected the fuel petcock and the fuel lines. I have also check the reed assembly and nothing appears to be wrong. I can ride around on the bike but the choke has to be on. And after a longer period of waiting for the bike to warm up with the choke ON it will die. Any ideas?

Answer
Hi Sean,

There are a few things that can cause this problem.

Since the bike only runs with the choke on it sounds
like your float level and fuel supply might be okay.
The float setting has to allow a full bowl of fuel
to run well.

If it has enough fuel then the next logical thing to check
is if the engine is getting the fuel inside the combustion
chamber.

Sounds like it only pulls the fuel in when the choke is on
so that means the engine is pulling in either too much
air or does not have the vacuum to pull the fuel/air mixture in.

A lack of vacuum is related to either worn piston/cylinder
and low compression (under 100psi)
OR air/vacuum leaks around the cylinder base gasket
or the crankshaft end seals.

The way to check these is with a compression test to verify
the engine top end and then
a pressure test to make sure the seals are not leaking.
This is done by sealing off the intake and exhaust ports
and adding about 6 psi air pressure to the spark plug hole
and then watching if the engine can hold this pressure
for a short time.

Alternately you could check around the carb flanges and cylinder base for air leaks
with some carb spray and then check the crankcase seals for leaks
or oily residue around the seals.
You would need two of these crank seals: 92049-1188 SEAL-OIL,SD20407GS

Sometimes carbs can be very hard to get clean too.
Above the main jet is a long needle jet that
people often fail to clean. It has many small horizontal holes
that need to be cleaned physically with some small guitar
wire or whatever fits the holes.

See if your spark can jump a gap of 1/4" and
check the coil frame is well grounded not corroded.
Check plug cap and plug too.

WS
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