Fellow LB addict here

[Joe Kuhn]

Pushed primer button down and held it to stop possible air flow and it made no difference. Later analysis of the primer tube showed no leaks.

Rather than drill out the main jet of the old carb, I installed a new carb. Now it doesn't rev up like it should although it runs fine. I can make it rev up by moving the blade at the top red arrow with my finger. With the machine off, this blade moves easily and the spring at the bottom red arrow makes it go back to a low idle position with no problem. The choke mechanism is set correctly as shown at the right. It does seem to work.



I ran out of daylight, so next I'll take the engine covers off and be sure the fan of the flywheel can blow air to the white throttle blade on the carb. Thoughts?

 

[Joe Kuhn]

Here's a broader view. Even though the white blade appears to be hitting the machine at the top, it isn't. The photo was taken at a bad angle.

 

[FuzzyDriver]

When you say it doesn't rev up like it should, are you saying you can't make the engine speed up with the throttle control? I had an F-engine one that had this issue and it took me forever to figure out what was happening. With mine, the vane sometimes could rub on the shroud opening and stick until the engine speed changed a LOT (to overcome the stick - and then surge). Not saying that this is your problem, but your comment reminded me of mine. It is kinda interesting that the gaskets and heat shield move the carburetor slightly, just enough (in my case) to get it close enough to touch.

 

[Joe Kuhn]

When you say it doesn't rev up like it should, are you saying you can't make the engine speed up with the throttle control? I had an F-engine one that had this issue and it took me forever to figure out what was happening. With mine, the vane sometimes could rub on the shroud opening and stick until the engine speed changed a LOT (to overcome the stick - and then surge). Not saying that this is your problem, but your comment reminded me of mine. It is kinda interesting that the gaskets and heat shield move the carburetor slightly, just enough (in my case) to get it close enough to touch.

Yes, you've got it. Will check the problem you've described asap. Thanks.

 

[JDeere101]

I Love those old 2 stroke Lawn Boys,, I wish I had one now.

 

[FuzzyDriver]

I Love those old 2 stroke Lawn Boys,, I wish I had one now.

Old Lawn-Boys aren't that hard to find. I have two for sale right now and there are always some on eBay. I also see some on Craig's List every once in a while.

 

[Joe Kuhn]

Vane was clear:



But the spring controlling the vane was much weaker on the new carb compared to the old.



The sprocket at the red arrow above can be moved one way or the other to alter tightness of the spring and the top end. Tighten the spring by moving the sprocket clockwise to increase the top end and move it counterclockwise to decrease it. This got us to an rpm the owner was familiar with.

When I was tightening up the spring I noticed hot gas hitting my hand from the hole in the exhaust can. The heat plate between the carb and the engine got so hot, I couldn't touch it. This may be why the old carb was surging so bad. It got overheated and was permanently changed.



That hole is from the heat shield that was removed from the exhaust can. The bracket on the shield at the screw hole broke and is long gone. We found a screw that fit tightly into that hole:



...and called it fixed. We did some test passes in the front lawn, tweaking the throttle spring until the owner was happy with the speed (because it matched the old speed). Here's where we ended up.

Done.

 

[Joe Kuhn]

Thanks for the help on this. Not so easy of a fix, but we got it.

 

[Joe Kuhn]

It's fixed...or so we thought. Owner mowed his back yard the next day and it started surging again. He was able to finish by running at a lower rpm. I had him using 50:1 fuel and he asked about switching back to 32:1, so we did. Still died in about 30 seconds. Then we checked the rpm, which was at 4000 with a recommendation of 3000 - 3200 (that I could find). I turned down the rpm by 10 counterclockwise clicks and mowed my whole yard with no problem. Seems fixed now.

Old discussion here

Lawnboy Duraforce Surging Fix!

www.mylawnmowerforum.com

lead me to try drilling out the throttle plate hole as shown here.



Used a drill that measured .105, very close to the suggested .10. This made no difference at all. It still died within 30 seconds. That old discussion from 2015 suggested using one of these as well:


Be here Friday, so we'll see what difference that makes.

I believe the darn thing moves fast enough across the yard. I've never mowed my yard that fast and I've had self-propelled mowers for a long time. It is what it is.

I'm tempted to put the original carb back in and see if lower revs fixes the problem with the original parts.

My son loaded an rpm checker onto my phone which works via sound. You put in the make and model number and it gives you a standard setting (3000). Problem is that's the base at which it starts. We saw a top end of 7000 for a difference of 4000. Got old reliable out which gave us 19 turns in 30 seconds or 3800 top end.



The problem with this device is you have to take all the covers off the top of the engine and start it with a separate cord. This is a bit inconvenient to say the least. I like the idea of one that senses pulses at the spark plug cord.

 

[FuzzyDriver]

It did seem to me to be running very fast in your video. I use an old Craftsman tuneup meter to do my RPM checks. It works good, but it's not that easy to set up since it is supposed to be getting power from the car's battery, which the Lawn-Boy doesn't have, of course. I run it to a 12 VDC power supply for that and do the math for reading the RPM. IOW, multiply by 2 since our Lawn-Boys fire on every revolution instead of every other one.

I would have been really surprised if changing from 50:1 to 32:1 had helped a surge issue. I still use 32:1 with Klotz full synthetic, even though it is supposed to be good for 50:1. Call me old fashioned..

Good catch on finding that exhaust blowing on your heat shield. Hope that's the final fix. Are you thinking it warped the old carburetor?

I have heard that the DuraForces don't last as long as F-engine LawnBoys due to them having sleeve bearings instead of the F-engine's ball bearings. ?‍ I know F-Engine Lawn-Boys seem to last forever.