Most older engines have several adjustments. High speed mixture--a needle
valve usually adjusted by running engine at high speed and making leaner
until engine rpm drops, back off a bit from this, further tweaking may be
required under load. Idle speed usually a screw to prevent the throttle
from completely closing, sometimes there is also a needle valve controlling
the idle mixture. Usually set idle speed so engine idles smoothly and
adjust idle mixture same way as high speed mixture. Engine should not
stutter on quickly opening the throttle. This may require several iterations
to get right.

"Peabody" wrote in message
news:Zl19g.9654$B42.8982@dukeread05...
I have an ancient 2-cycle Lawn-Boy mower (model 7086) that uses
gas/oil mixture. It doesn't run very smoothly anymore, and the
spark plug fouls pretty easily/frequently.

The other day I shut the main gas line off and let it run until the
carburetor bowl emptied and the engine died. But I noticed that in
the middle of that process, the engine smoothed out real nice, and
also sped up a bit. Well, I'm wonderinging if the bowl is filling
up too much under normal operation, producing too rich a mixture.
That could make sense if whatever shuts off the gas inflow as the
float comes up has gotten old or compressed or whatever in the last
27 years, so that the float has to go higher to shut off the fuel.

After that, I took the carb off, took it apart and cleaned it. And
I discovered a screw adjustment on the side that I hadn't seen
before. Can someone tell me what this is for, and how it should be
adjusted? Could I use it to compensate for the aging of the fuel
inlet gizmo so the mixure is leaned out a bit? The adjustment looks
a lot like the "idle adjustment" screw you used to find on auto
engines, but I don't know if it performs the same function.

Otherwise, do I have to buy a new carb the get the mixture right? I
would hate to have to do that. It would probably cost more than the
mower. Could I just bend the float arm? I don't see any way to
check what the fuel level in the bowl is, or should be.

If you are lucky it is a mixture adjustment, turning it in will lean it
of gas, try it turn it in till it runs best.