Briggs 21hp over revving

[jhn9840]

Hello all,
Have a 407777 0121-e4 030325 that is over revving upon start up. Put choke all the way on and it slows down to normal. Start to release the choke any at all the rpm’s shoot way up with a lot of black smoke coming from the exhaust. I’ve checked the governor linkages and the adjustment of the governor itself. It all appears to be functioning as it should. I took the carb apart and cleaned it out. There was some dirt inside, it helped a very little but was not the solution. Any suggestions are much appreciated.
jhn9840
John

 

[bczoom]

John,

Welcome to the forum.

Is the spring connected on the governor?

What's the idle adjustment screw set at? Turning it clockwise, count how many turns out it is (and write that down so you can go back to it if needed). I'm thinking it should be around 3/4 - 1 full turn out.

 

[Bindian]

Hello all,
Have a 407777 0121-e4 030325 that is over revving upon start up. Put choke all the way on and it slows down to normal. Start to release the choke any at all the rpm’s shoot way up with a lot of black smoke coming from the exhaust. I’ve checked the governor linkages and the adjustment of the governor itself. It all appears to be functioning as it should. I took the carb apart and cleaned it out. There was some dirt inside, it helped a very little but was not the solution. Any suggestions are much appreciated.
jhn9840
John

If you choke it and it starts running normal and taking the choke off races the engine...........sounds like an intake air leak. Did you use all new intake and carb. gaskets? If so, I would take the carb to a good lawnmower shop that can put the carb in an ultra sonic cleaner, then let them rebuilt it. If you are sure it isn't an air leak, then you have debris blocking fuel passages. Most likely at a jet.
I rebuilt a B&S twin cylinder eons ago, but it is foggy now. Does your engine have a vacuum fuel pump? It could be a bad vacuum pump. Or a combination of all of the above. With it running with the choke on, lean the mixture out while slowly taking the choke off. See if you can keep leaning it and taking the choke off to get full power. That system is hard to troubleshoot from here. You may even have a small pin hole in the intake manifold. Check the governor linkage springs to see if they are all installed.
hugs, Brandi

 

[Art454]

You probably have a broken governor gear.
You have to take the sump off to find out.
I did one for a guy last year as I used to be a small engine dealer.
My as well change the oil seal why you got it apart.

BTW don't keep runing it as the broken parts could get between the runing parts inside the motor and due damage be on repair....meaning cheaper for a new motor or lawn mower.
Art