Lawn Boy runs high rpm

[Two-Stroke]

I watched your video and I can tell you that my problem is nothing like that. Mine appears to be a lean condition but I can't tell from what. As of today I've changed both crank seals, changed the carburetor from a LB that was running fine, checked the block for leaks visually only (maybe I need to rig up a crank pressure tester) and still the engine runs wide open. I looked at the carb while it was running and the governor has the carb butterfly closed. I'm almost to the point of buying another mower and using this one for parts.

I think there must be a fairly significant leak for it to run fast with the butterfly closed. Keep in mind that, unlike a chainsaw, a rotary mower has a fair amount of drag (even without any grass underneath) due to the power necessary to turn the blade -- it's like a propeller.

The fact that it does this with another carb (that you know is good) is a real puzzler.

You are sure that the butterfly is closed when it's revving high -- correct? That's the most baffling part. :confused2:

 

[WhyZed]

Ya, baffling :confused2:

The confusing thing for me is, "it was running fine, rubbed a small tree then it started doing it again".
That is key but what it unlocks is ??? :confused2:

 

[Mary Wilson]

Ya, baffling :confused2:

The confusing thing for me is, "it was running fine, rubbed a small tree then it started doing it again".
That is key but what it unlocks is ??? :confused2:

I should have used the word "tapped". I didn't hit it very hard, just bumped into it. I thought maybe it stuck the float but I changed carburetors and it does the same thing. This week I'm going to try to fix up something to check the engine crankcase pressure, like they do on chainsaws, just haven't got it done yet.

 

[Mary Wilson]

I think there must be a fairly significant leak for it to run fast with the butterfly closed. Keep in mind that, unlike a chainsaw, a rotary mower has a fair amount of drag (even without any grass underneath) due to the power necessary to turn the blade -- it's like a propeller.

The fact that it does this with another carb (that you know is good) is a real puzzler.

You are sure that the butterfly is closed when it's revving high -- correct? That's the most baffling part. :confused2:

The butterfly has a hole in it set for a fast idle, I suppose. I figured that plus the air leak, if that's what it is, might be enough to make it speed. I don't think the problem is in the tank because when you open the valve with the fuel line disconnected fuel pours out. The gas cap has a good vent also.
When the mower is in fairly tall grass I can kinda control the rpms but it's when the grass is light or short that it runs away with itself.
I might add that when I changed the carbs I didn't do any adjustment to the good running one, just unbolted one and replaced it with another so I wouldn't have the confusion of whether I set it right or not.

 

[impalass]

The butterfly has a hole in it set for a fast idle, I suppose. I figured that plus the air leak, if that's what it is, might be enough to make it speed. I don't think the problem is in the tank because when you open the valve with the fuel line disconnected fuel pours out. The gas cap has a good vent also.
When the mower is in fairly tall grass I can kinda control the rpms but it's when the grass is light or short that it runs away with itself.
I might add that when I changed the carbs I didn't do any adjustment to the good running one, just unbolted one and replaced it with another so I wouldn't have the confusion of whether I set it right or not.

Did you check to see if the bolts are tight on the bottom end cap.

 

[Mary Wilson]

Did you check to see if the bolts are tight on the bottom end cap.

No, I haven't done that. I guess I should check all the engine bolts...I just assumed they were okay since they haven't been messed with. I'll try that next.

 

[bwright1818]

This is a great thread. I also have an F engine which seems to have the exact same problem, although I have done little to solve it yet, other than a carb cleaning. (I currently own 13 Lawnboys, 10 chain saws and am a boat mechanic by trade; so I have done a few carbs.)

I wonder: When you say the carb is "closed," it's sort of relative as you know, because it doen't take much throttle opening to rev high with no load. Can you take some sort of a hooked tool to actually pull the vane dead closed while it's running? If it STILL revs, boy would you have an air leak then!

Can't wait to hear how this one tuns out!

 

[Mary Wilson]

This is a great thread. I also have an F engine which seems to have the exact same problem, although I have done little to solve it yet, other than a carb cleaning. (I currently own 13 Lawnboys, 10 chain saws and am a boat mechanic by trade; so I have done a few carbs.)

I wonder: When you say the carb is "closed," it's sort of relative as you know, because it doen't take much throttle opening to rev high with no load. Can you take some sort of a hooked tool to actually pull the vane dead closed while it's running? If it STILL revs, boy would you have an air leak then!

Can't wait to hear how this one tuns out!

Me too, except I'll probably be glad when it's actually fixed! I'll let you guys know what I find (if I find it). I'm pretty determined on this one so I'll probably come up with something. Joe

 

[Mary Wilson]

This is a great thread. I also have an F engine which seems to have the exact same problem, although I have done little to solve it yet, other than a carb cleaning. (I currently own 13 Lawnboys, 10 chain saws and am a boat mechanic by trade; so I have done a few carbs.)

I wonder: When you say the carb is "closed," it's sort of relative as you know, because it doen't take much throttle opening to rev high with no load. Can you take some sort of a hooked tool to actually pull the vane dead closed while it's running? If it STILL revs, boy would you have an air leak then!

Can't wait to hear how this one tuns out!

I'm proud to announce the mower is fixed! The problem was an air leak at the base of the carburetor caused by the pulse hole leaking there. My last stab at fixing it involved plugging up the hole with RTV and making a new, thicker gasket for the carb with the hole deleted. I started it up a few minutes ago and it ran normally, actually a bit slow, so I may have to adjust the governor a notch or two. It's set on the fastest of the two settings and is just a little slow. It two cycles in the taller grass like it's supposed to so maybe now all I need to do is adjust. Thanks for the input. Joe

 

[bwright1818]

Hmmm, I ALMOST took mine out of service for a new deck, today. I drained the gas out today, so I could bring it in the cellar. Maybe I should have a look at my gasket, too. I have had this machine for 16 years and have put many, many, MANY hours on it. And I got it, used! I thought it was done. It would be unbelievable if all it needs is a gasket! But my machine has pretty much EXACTLY the same symptoms....

Good job on yours! Persistance pays!