42" riding mower 917.289272, Briggs engine model 4456 77 1188 81. Engine suddenly starting reving at top speed, uncontrollable, unable to decrease rev speed. Can flood engine out in choke position. Must immediately turn off ignition to prevent damage. Dismantled plastic engine shroud to access Carburetor and linkage. Spray cleaned all and into carb itself. Also cleaned throttle cable. No change to over revving. Hesitant to further dismantle carb area itself. Any tips? Thanks very much for consideration.
#2
sgkent
I don't know your engine but that engine has a governor. I'd study the manual to see how it works and check it. There seems to be a gear and a crank plus a lever. There is also a spring either 795224 or 798921 depending on the date code on the engine. Just a wild speculative guess that the spring broke or is dis-attached.
Other way round
Governor makes the engine run slower
Spring works against the governor to make the engine go faster
Run away engines are never going to fuel related
So either the clap at the base of the governor arm is loose & the governor is no longer able to control engine speed
or
the governor itself has fallen apart inside the engine
Sometimes I've seen governor and throttle linkages and springs get crossed over and
cause a runaway situation on start up. It's easy to do if you get the carb off and don't
make sure there is free movement. Could also be a throttle shaft binding due to crud.
#5
Fish
I have also seen a carb lose a screw in the throttle or choke plate, and an off center plate stick open in the throttle. So you might look at that.
You guys ever seen what happens when an engine eats a throttle screw? If it goes down
on top of the piston all sorts of bad things can happen. Piston gets beaten and scarred up
sometimes the cylinder wall gets scored too. Rings get pinched in the groove and ruins the
piston. I've seen that happen on chainsaws where the needle bearings let go and the piston
grabs one and drags it up and down in the cylinder. I even had one on a Husq saw somehow
get the needle bearing up and on top of the piston from below! Yikes!
#7
sgkent
that is why many throttle screws are brass and why they are staked. Sometimes if one is lucky it gets passed thru but usually it beats the spark plug to death first.
I don't know your engine but that engine has a governor. I'd study the manual to see how it works and check it. There seems to be a gear and a crank plus a lever. There is also a spring either 795224 or 798921 depending on the date code on the engine. Just a wild speculative guess that the spring broke or is dis-attached.
Broken spring will not cause it to over rev. it will go back to idle. Gov. works like this the spring is trying to blow it up and the gov. is trying to save it. Sounds like it blew the gov. Briggs engine are prone to do this. The gov, shaft that the gear runs on get loose in the block and cause this problem. Anybody ever fix one when the shaft comes out? Never had any luck with it. If you have it would be great to learn your secret since I have one like that now.
#9
StarTech
445677-1188-B1
First check that the static governor is set correct. Sometime being off just a little can cause a racing engine suddenly. Then when done start the engine and attempt to manually return the engine to idle by closing the throttle plate at the carburetor. Now with the engine at idle release the throttle plate (Be ready to manual close it again to prevent engine damage and shut it down). If it return to full speed immediately the governor has failed.
Also note that if the idle speed setup of the carburetor is not setup correctly it can cause engine speed problems such as engines that will not return to low idle speed but to have higher then normally operating speed idle speed. This means the engine must be tune for 1200 rpm idle speed when the throttle plate is held closed and a governed idle speed of 1750 set when the plate is released with the controls at idle position.
Anybody ever fix one when the shaft comes out? Never had any luck with it. If you have it would be great to learn your secret since I have one like that now.
