21HP B&S Engine

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

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.

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!

> 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!

Am I reading this right, there is a hole there but no screw? Either
the screw vibrated out or there is a 'tamper' cap over the mixture
screws that you will have to pry off to get at the adjustment and the
hole you are seeing doesn't actually go anywhere. Do you have any
pictures you can put up somewhere for us to look at? Have you tried
spraying carb cleaner into the hole you are describing with the engine
running? Any change? If so...chances are something is supposed to be
there and it's probably out laying in your yard right now ;-)


Chris

On Jun 17, 3:08 pm, Hal > wrote:
> > 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!
>
> Am I reading this right, there is a hole there but no screw? Either
> the screw vibrated out or there is a 'tamper' cap over the mixture
> screws that you will have to pry off to get at the adjustment and the
> hole you are seeing doesn't actually go anywhere. Do you have any
> pictures you can put up somewhere for us to look at? Have you tried
> spraying carb cleaner into the hole you are describing with the engine
> running? Any change? If so...chances are something is supposed to be
> there and it's probably out laying in your yard right now ;-)
>
> Chris

Chris,

Thanks for the reply.

The hole seems to have no treads in it. Factory smooth. It appears to
be some sort of a seal or something. I'm not sure if I can remove
that. For sure it reminds me of what car makers used to do to make
things tamper proof. If it is a tamper proof cap, how do I remove it?
In the old days we used to drill a 1/8" hole in to it. The hole size
seems just right for the screw, but it seems sealed. I'm concerned if
I drill a hole in there and there is an screw, then what

I did spray carb cleaner in the carb and things did not change.
However, when I was maintaining a slow stream of carb cleaner the
engine stopped its hunting and ran smooth. This somehow tells me I'm
not getting sufficient gas in there.

> The hole seems to have no treads in it. Factory smooth. It appears to
> be some sort of a seal or something. I'm not sure if I can remove
> that. For sure it reminds me of what car makers used to do to make
> things tamper proof. If it is a tamper proof cap, how do I remove it?
> In the old days we used to drill a 1/8" hole in to it. The hole size
> seems just right for the screw, but it seems sealed. I'm concerned if
> I drill a hole in there and there is an screw, then what

No need to drill anything. Most likely what you are looking at is a
port that could have been drilled and used for another purpose on a
different model engine than yours.

>
> I did spray carb cleaner in the carb and things did not change.
> However, when I was maintaining a slow stream of carb cleaner the
> engine stopped its hunting and ran smooth. This somehow tells me I'm
> not getting sufficient gas in there.

Sure sounds like a plugged up idle circuit. There should be either a
removable idle jet or an idle mixture screw somewhere on the
carburetor. If not you'll have to take the carburetor off the engine,
remove the float bowl(f any) and use carb cleaner/compressed air to
try and clear all the passages.

What model engine do you have? Model/Type/ser from the fanshroud would
help.

Chris

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