wrote:
> I have a Sears lawn tractor which is 6 yrs old. It came with a 21HP
> B&S engine. A few days ago it started to run real rough at idle. If I
> add about 50% chock, it run ok, but still at times can misfire.
>
> At a 100% speed, it seems ok and can cut grass and move around ok. I
> got this condition a few years ago and was able to clear it by
> cleaning the air filter. So far I have clean the air filter, replaced
> and gapped both plugs, replace oil, replaced the fuel filter. With all
> of that no change.
>
> The engine surges up and down at idle real bad. I removed all the gas
> and tried new one, no change. In the Sears manual it talks about a
> idle mix adj. I have none of that. No screws to adj other the the RPM.
> I used carb cleaner and cleaned the carb. No change.
>
> What could this be? I called Sears and they want $96 to come over and
> provide diagnostics. Then parts plus about $100 or so per hr for
> labor
>
> Thanks,
> Fred
>

Idle circuit in the carb is plugged. Only real way to repair it is to
pull the carb off and open it up. That engine has a tamper resistant
carb that only has a fixed jet for high RPM and a hidden idle mix
needle. You need to drill and remove the plug over the needle when you
take the carb apart.

--
Steve W.
Near Cooperstown, New York