Erratic Governor Behavior On A B&S Quantum Engine

So I've got a bit of an odd one here. Back in 2019 I picked up an old Bolens mower equipped with a 4 HP B&S Quantum (model 122782) and front wheel drive, all for free, free because the owner hit a rock with it and sheared the key on the flywheel. I got it home, tried to pull it over and sure enough, it was out of time so I pulled the tank and shrouding off, threw a new key on and it fired right up (after some carburetor work first).

Sparing you the details, one issue it's always had since day one is very erratic governor behavior at random times. Sometimes I'll be mowing along, and it will start running away almost like a diesel would, steadily increasing in RPM, not stopping unless I let go of the handlebar to cut ignition. In this state, it will easily reach over 4000 RPM, and I loose all throttle control (this mower has a throttle and choke on it). Interestingly, when the governor isn't busy running away it governs like it should through heavy grass, though I have noticed if I go through anything taller than normal it will tend to provoke it's odd behavior, forcing me to shut it down to prevent damage.

It could've just been me, but I felt like it was happening more often this past summer, so with snow on the ground now I decided to tear it apart and figure out what's broken inside... to find nothing wrong. Upon pulling the sump I was met with a clean engine and gears that are in great condition with no missing teeth or anything. The weights and cup on the governor are in great shape as well and open and close like they should.

One thing I haven't mentioned up until this point is governor adjustment on the outside of the engine, which I have adjusted correctly following a B&S service manual I have for this engine. The linkage atop the carburetor is also in good shape.

At this point I'm truly stumped. I tried looking around online for any similar experiences others might have had, but came up empty which leaves me here. Since it's apart, I went ahead and got a picture with the sump off, so maybe you can see what's wrong when I can't.


(oh, and the auxiliary gear on the camshaft is just used to power a small external shaft on the engine for the drive on the mower)

Please let me know what you think about this!

Thanks.

I had a wore out weedeater with an l head briggs that had the plastic air vane governor. It broke off and was doing the same thing before it died.

That's pretty odd, never had that happen. When I read the title, I figured it was just running up and down, like a carb issue. But that's really weird. Can't really think what would cause that and at random times. Unless something just gets stuck like a worn throttle plate kinda thing? But you said it kinda creeps up, not just gets stuck open. Only thing I can think is something must hold it open.

Are the little springs still on the weights? Everything looks good to me so the only other thing I can think of is maybe there's gunk on the governor shaft where it goes through the engine case? Or someone has put the wrong spring on/tweaked the bracket?

@mr.modified Yeah, oddly enough the governor arm outside the engine as well as the butterfly on the carburetor move with just a touch, so nothing is sticking as far as I can tell.

@Brianator Springs? Well, I can't say this engine has any on the governor internally, and if they were there at some point then they must have been removed intentionally since I didn't find any after I got the sump tore down, but you might be onto something there. As for the arm coming outside of the crankcase, it is clear of any dirt, and moves freely.

I don't believe anything has been wrongly tweaked on the carburetor side, but I'll go ahead and upload a photo of that as well just in case.

Springs look good to me, but the thing that really stands out to me, (note: I am not familiar with the engine)

See arrow below.  What is that hole for?




Also a lot of these have plastic intake manifolds.  Make sure there are no cracks and all the seals and bolts are tight.

@MightyRaze Good spotting! I actually hadn't noticed that hole before, but upon inspection however, it doesn't appear to be significant. It isn't threaded, doesn't lead anywhere, and doesn't appear to have any air passages or anything like that inside it either. Odd, but it probably is for adjusting something, or just for manufacturing purposes I would guess.

The intake manifold on this engine is (thankfully) all aluminum, probably thanks to it having a date code of 12/14/1990 and it is fully intact.

I don't actually know much about the Quantams, my old flathead I swear I saw little tiny springs on the pins for the weights,  I need to open it back up to change the sump gasket (I'm still setting crankshaft endplay) so I'll have another look, I could have just imagined it AND just because mine has them doesn't mean yours does. I'll double check and report back either tonight or tomorrow

I was wrong about the spring thing, must've been a hair or spiderweb or maybe my imagination! Lol. A difference I do see between yours and mine is your weights seems to tuck into each other where as mine only touch or "butt" up to each other. It may just be a difference in design as they're many years apart but it is something for you to look into at least.

Update 5/9/2022: After putting the engine back together back in February I fired it up and ran it around a little bit, though I didn't have any grass to mow so I put it away for the time being. While I did have it running though, I did feel like I noticed the governor generally behaving a bit better but it was just a hunch at the time.

Fast forward to May and I've mowed through heavy grass with it twice and am happy to report it is running smoother and much better than it ever has. The runaway issue is completely gone, and the governor works exactly as it should through any depth of grass. Oddly enough, throttle response is much better too as previously if I went to idle it down or rev it up, it wouldn't do that right away, instead taking a few seconds then it would adjust to my desired speed. Now, the second I move the throttle in either direction the engine changes in RPM right away.

I'm still fairly clueless as to what the exact issue was, but my guess is that something was either put together wrong at the factory or someone was into the engine at some point and didn't put it back together correctly. Thus, tearing it apart and putting back together properly fixed the issue.

Now that the governor is working properly, I'll be retrofitting a Tecumseh carburetor to the engine so I have full idle and high speed mixture adjustments for much better tuneability and I will also be installing electric start for the fun of it, since I have the parts and the block will accept it.

Thanks again for the help everyone.

Right on glad it's sorted out! Maybe something was interfering with it operation or was gummed up? Either way I'm glad it's worked out for you!