On Jun 17, 1:51 pm, Dyno > wrote:
> 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
>
> Has the carb been rebuilt yet? Could be gummed up. Another possibility
> is the carb might be loose where it attaches to the engine. This would
> create an air leak that appears at light loads but is less prevalent at
> high load. Also check the gaskets at the mounting flanges.

Thanks for the reply.

The carb is original. I checked for loose connections, and was not
able to see any. I also used carb cleaner near the gaskets and the RPM
did not pick up. The carb rebuilt kid has a couple of gaskets and two
needle screws. I can't see where and how to use the needle screws
since this carb has nothing other than the RPM adj. The idle screw and
high speed adj screw are not there. I really needed the idle mix
screw. Sadly there is a little hole but no screws in to.

The strange thing is that the Sears manual shows a screw there!