stumped...cant get my 2T started

hi there, i am stumped. i cant get my engine to start.ive taken the
carb (dellorto sha) apart,cleaned it and reassembled.i am puzzled
because after the engine fires and stalls, plug is wet.these are the
observations:

-starts for 1 sec with starting fluid or splashing fuel into the air
intake,
when i check plug is wet with fuel around central insulator
-visually confirmed spark
-float and float needle appear to function well. with carb inverted
and weight of float acting down on valve , i cannot blow air past
valve.
-float does not appear to contain fuel inside it.it is free to move on
its pivot.
-i can blow air and felt blown air output through the main jet to the
venturi & through idle circuit
-i suspected flooding of some sort but this happens even if i try
starting with petcock at off position after i turn it on for a few
seconds to fill float bowl.
-tried starting with almost every combination of choke and throttle
position.most commonly recommended here was choke with half throttle.
-used 2 different sources of gas, tried even raw petrol
-could it be an air leak at cylinder head?if so would the plugs be wet
when it stalled if its sucking air and running lean?
-the exhaust has some leaks.can this affect starting?
-if i have a leak at the carb connection to crankcase, would this lead
to plugs ending up wet?

i have tried everything i could think of. any ideas what is left to
check?
many thanks

B

 

What led up to this difficulty?
Can you see fuel seeping out the carb?
(Is the float leaking , mal-adjusted or saturated?)

 

I'm going to concentrate on the carb since that seems to be the item you
changed last and you indicated that the bike will run when using starter
fluid. You mention that the float and needle are functioning, but is the
float level correct? If float level is too high, the fuel mixture will be
rich. If float level is too low, the mixture will be lean.

More importantly for the problem you are describing, is the needle valve
sealing completely shut when the float bowl is full? It might be worth the
time to pull the carb off and test it on a bench to be certain that the
needle valve is not leaking.

Carl

 

i loaned it.

float appears intact. after many failed start attempts the metal air
cleaner element will feel wet.

thx

 

thanks for all your replies. i think i might have additional probs
with air leaking past carb.

when the motor is turned with my finger instead of plug i can feel
great compression and alternately considerable suction.if i put my
finger on the carb 'spigot' (on the reed valve block where you attach
carb) i feel almost no suction when motor is turned. it can just move
a piece of tissue paper.
how much suction should be at the spigot?

also i pulled out reed valve. the thin metal was not fully closed. at
atmosphereic pressure should the reed valve be *fully* shut or is it
acceptable to have a little curl at the end?

many thanks

 

compression and alternately considerable suction.

Think about what is inside the 2-stroke cylinder. There is a piston going up
and down as you turn the crankshaft...

The exhaust port and the transfer ports are *closed* until the piston reaches
the half-way point in the downstroke...

So the piston creates a partial vacuum in the cylinder and you feel that
vacuum
*sucking* against your finger at the spark plug hole...

Then the piston goes past the open ports and the sucking stops...

No compression starts until the piston again closes the exhaust and transfer
ports at the half-way point in the upstroke. Then the piston begins
compressing
the air in the cylinder and you feel it at the spark plug hole...

Bottom line is, you have some compression...
[/QUOTE]
The key here is "some." From some experiments in my (comparable) youth,
I found that I could only hold about 40psi with my thumb pressed against
the end of a hole. People with stronger hands could certainly hold more
pressure, but just because someone senses "great compression" doesn't
mean that the engine has enough compression to operate effectively.

Of course, I'm not there, and I'm too young to have 'enjoyed' the
two-stroke years, so you might choose to disregard this anecdote.
YP(ressure)M(easurement)MV

 

found that I could only hold about 40psi with my thumb pressed against the
end
of a hole. People with stronger hands could certainly hold more
pressure, but just because someone senses "great compression" doesn't mean
that
the engine has enough compression to operate effectively.
[snip]

The 2-stroke in question probably only has about 90~100 psi cranking
compression. I could tell the owner to warm the engine up and hold the
throttle
wide open to take an accurate compression reading, but he can't get the
friggin' engine started to warm it up...[/QUOTE]

Yeah, and if he can hold 90psi with his finger in the spark-plug hole,
then his compression is likely ok. If he can only hold around 40 or so,
then it seems to me that his compression is rather low. As I said, I'm
no expert on these things, but it does seem a bit surprising to me that
you can hold in the full compression pressure of an engine with your
finger.