Engine speed problems


Old 05-24-03, 09:45 PM
Andrew's Avatar
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Question Engine speed problems

Hi All,
I have an older mower with a 3 1/2 HP Briggs engine. Here's the deal: After it is started, the engine runs well for maybe 20 minutes, then starts to slow down and lose power. At this point, I need to adjust the needle valve in the carb, and this increases the engine speed again. In fact, I carry a screwdriver to "fine tune" the carb while it is running. Also, the throttle control doesn't really seem to control the engine speed. As I decrease the throttle lever, the engine doesn't seem to slow down. It finally just quits when I get near the lowest marked speed.
I did disassemble and clean this carburetor a few years ago, so I don't think it is too dirty.
Thanks for any help!
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Old 05-25-03, 07:58 AM
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Hello: Andy

Depending upon when this problem first appeared may help to provide the source of the problem. If the problem existed after you cleaned out the carb a few years ago and continues presently, it's possible the fuel level in the carb bowl is too low.

Also possible there is a fuel flow restriction between the carb and the fuel tank or the vent hole in the fuel tanks cap is restricted.

Linkage adjustment of the throttle cable may not be set correctly. The throttle cable may not be adjusted correctly or has slipped at the point where it is secured to the engine frame.

The gov spring may be weak or was reset into the incorrect location on the link rod or carb throttle plate, etc. Also possible there is a vacuum leak at the carb body or the mounting to the engine. Gasket may be leaking or damaged causing air to enter after the carb. {Downstream}

Most likely I haven't covered all of the potential possibilities. The other resident small engine service and repair professionals may offer additional suggestions, advice & help.

Check back on your question several times over the next few days for additional replies.

Regards & Good Luck. Web Site Host & Small Engine Forum Moderator. "Accurate Power Equipment." Small Engine Diagnostics Services & Repair.
Old 05-26-03, 11:09 PM
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Hello Andrew!

The 2 most common causes of your symptoms would be: 1...the carburetor diaphragm needs replacement (sounds like you have a pulsajet carb with automatic choke). OR, 2...the valve clearances are too small, causing you to have to richen the mixture to compensate for loss of vacuum when the engine warms up.
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