Usually just cleaning the carb will fix the surging but sometimes increasing the pilot jet size is needed. The left side of the carb has a sticker on it, if the sticker is gone, that's most likely your problem as you've sucked dirt into your pilot jet. Under this sticker is the pilot jet. Unscrew it and make sure it's clean and don't forget to put tape or something over it when you put it back in. If that doesn't fix it, remove the carb and give it a good cleaning. After that, you can try going up in pilot jet size. Here's the part numbers that go up to the size most people find works well.

.425mm Pilot jet - Toro #98-7025, Briggs & Stratton #801308 or Mikuni N101.043. They should run about $4 to $5.

You need to add a 90 degree shut-off and fuel filter to your Dura Force between the tank and the carb. Good luck.

OK - I just checked my mower which is outside running with a tad bit of carb cleaner in the fuel. On the left side of the carb, looking from the front, there is a brass screw. No tape over it...and black specs of junk (soot) on it. I assume that is the pilot screw. When I remove it, clean it, and then put it back in....do I just tighten it gently until the screw stops...or is there an adjustment involved? Also, I'm assuming duct tape will do the trick. I really appreciate your help and support.

Yes, just snug it down, don't strip it. It has no adjustment.

Uhhh, the pilot jet on my 10424 is on the left side of the carb as you're looking down the throat. Are you sure you's in on the other side?

Sorry, I mispoke. As you look from the rear and stand as if you were operating the mower, you first have the air filter, then the carb, then the engine. As you stare into the air filter from the rear(which I assume equates to looking down the throat), the screw is on your left. No tape was on it. (The fuel hoses run into the other side...and I see no other screws anywhere - only the four mounting bolts that run from the back of the air filter housing, and the screws on the bottom of the carb housing). I removed the brass screw on the left, and it had two small offset holes drilled into it. I cleaned them out with a small wire and carb cleaner. Then I replaced it to snug tight and placed tape over it. Since I ran the mower out of fuel (mixed with a small amount of carb cleaner), I've flipped it upside down to release any stuck floats (another forum suggestion I read earlier). I'll try the mower out shortly and let you know. Also, what is the white plastic piece that sits on top of the carb??? It has a tiny thin wire going to it. I thought maybe it should be moving while I move the throttle...but it doesn't. Is it supposed to move? Thanks so much for your help.

Yes, that's the pilot jet. If the piece you are talking about looks like a sail on top of the carb, it is the governor vane. It should only move when the engine is running and air is blown across it from the flywheel. If the flywheel is turning too fast, more air is flowing and pushes the sail and closes off the carb to slow the engine down. The tiny wire you see is a spring for the governor vane. You might have to adjust the governor but let's take one step at a time.

The carburetor should be completely cleaned as mentioned. Both jets contribute to the mixture at operating speed and any obstruction of these jets can cause surging. It's a good time to replace the fuel line and add an inline filter and fuel shutoff. One correction to orangedotfever's already good advice is that the tape or sticker over the pilot jet is not necessary. the pilot jet does not pull in air to the carb there. In fact the pilot jet flows gasoline from the carb bowl only. At first look it appears that it is open to the the outside like that but in fact that jet opens on the bottom and then has a passage at a right angle farther up. The top hole is not drilled all the way through. So do not worry about the tape, sticker or whatever that was over the jet.

I might be coming down the homestretch. After cleaning the pilot jet and running the mower with some carb cleaner the surging part seems to have greatly improved. I'll order the larger pilot jet on Monday. Will I need to drill anything or just simply install? Will I need to mess with the main jet?
Now that the first problem seems to be on its way to being fixed, I may have created another. I moved what I've now learned is the governor vane. The mower was trying to stall...and the throttle response was very poor...you can choke to start or prevent stalling, and you can carefully find one setting near fast that allows it to run without stalling....slower settings now mean stalling. I moved the governor to prevent the stalling and keep it running but may have actually adjusted the governor by mistake. Can you please advise on how to reset the governor and make my throttle respond correctly?

Thanks for the correction rdaystrom. pgmike, the governor vane should swing freely without binding on anything. The little wire spring swings it back to it's open position. Pushing the throttle back and forth puts more or less tension on that spring. There is a small plastic disk collar at the base of the governor vane that clicks when turned. This changes tension on the spring and changes the engine speed. It should be set to around 3200rpm at full throttle. Clicking it clockwise when viewed from the top will increase RPM. Usually, about 3 clicks will be about 100rpm change.

Go to the link below and chose your model and year and it will take you to manuals that will help. Good luck.

Here is a link that might be useful: Lawnboy manuals

Oops, forgot to answer your question. The new pilot is just a screw in replacement. No modifications to the mower. I would do that as a last resort though. A good carb cleaning and properly adjusted governor should do the trick if it ran good before. The little screen in the carb bowl may be clogged and need cleaning. This is why it's important to add an in-line filter. It helps keep contaminants from getting to the carb in the first place.

Here's the type of shut-off to look for.

Here is a link that might be useful: 90 degree shut-off

The surging seems to have stopped, but I still have to really dial in the governor. I guess I'll eventually find that happy balance where the rpms are not too high at the fastest setting, and the throttle operates correctly at the lower settings.

You guys have been great and I really appreciate all of the help and support. Thank you all.

Here is an article on jetting. Searched on "lawnboy jetting"

Here is a link that might be useful: Link

The Lawnboy model 10550 was designed to run very lean to comply with EPA standards. Usually, after a year or two, the engine starts to surge. I have a 10550 Dura Force that had a real bad engine speed surging problem. I followed the instructions on drilling out the carburator jets -- it now works perfectly, runs like my old lawn boy that I used for 22 years. The pilot jet (covered by the metallic p/n label on the carb bowl, on the muffler side of the mower it can be removed with a screwdriver) was enlarged from .014 to .016" using a #78 wire gauge drill. Drill the jet from the engine side but only through to the cross holes, don't drill it all the way through. Drill the high speed jet to .033" using #66 wire gauge drill. Your Dura Force will come alive!! No more surge, and it now has that happy two cycle sputter.

Mis4: Wow reminds me of my 1980 FLH (Harley) that I had to initiate the same measures on it's fixed jetted Keihin Carb.
It was running so lean through the Smokey Mountains just North of Ashville that even though it was supposedly an altitude compensating form of carb it still required the re-jetting. I can still remember my buddies astonishment when he had to gap his spark plugs at .048 to run on the unleaded full of the time lol . Our area is approx 250' above sea level so when we hit the Smokies it was an adventure ! Thanx for the memories Dude !

I am reading these posts on "surging" problems on the Lawnboy's.

You are incorrect to assume that this is a carb problem and tampering with the orfices as a fix.

Lawnboys operate on an "average vacuum" principle within the engine housing. There is a vacuum created within that entire housing that must be developed before the engine will draw sufficient air across the throat of the carb and "suck gas" up thru the orfices.

The problem of low or NONE vaccum developed and lower speeds and development of vaccum at higher speeds is caused by problems within the piston and cylinder itself. ie. wear and tear,damage, or a stuck piston ring and so on. Thus vacuum occurs at higher speeds and none or very little as the governor calls for a lower speed. (surging) as the governor reverts to the high throtle.

To fix these engines, remove then engine from the mower, pull off the head, remove the piston. Whatever the problem in that area, FIX IT. You will find the surging is GONE upon remedying the situtation in the piston and cylinder itself and NOT in any carb orfice problem.

Unfortunately for your theory, they all did it when brand new with all crankcase seals in operating condition. Further, it has been our experience that LB 2-cycles do not seem to exhibit surging when the compression is low and/or the crankcase is not holding compression/vacuum - they just will not respond to the throttle or develop enough power to be useful. This same effect is seen with clogged exhaust ports/mufflers.
It has been conclusively shown that the surging in the 10550 was the result of the attempt by the manufacturer to lean out the mixture to meet emmissions mandates. In fact, the mower was able to serve it's purpose in this condition. The surging was mainly an irritant to those who were somewhat sensitive to machines in 'duress', if you will, and those same users usually have mechanical apptitude and sometimes experience, so 'fixing' it immediately became the goal. The fix is to make or install metering jets of a larger diameter to enrich the mixture throughout the operating range. This enrichment brings the mixture to just about the same ratio as the LB 2-cycle mowers preceding the 10550 and is not a gross over-correction, so while it is true that (in engines generally) enrichment can cover up many other problems, in this case the normally-expected metering jet sizes for the volumetric capacity of the engine are restored rather than being increased.

Saxman,

You have explained the last of the "true" Lawn Boy's surging issues better than anyone else on this forum. Just for the heck of it, I wonder how much the emissions were actually affected by the "jet-orifice" fix. Does anyone know if this puts the mower way over the EPA mandate or just borderline.

Wally Richardson
(blisterbubble@yahoo.com), Your explanation makes no sense whatsoever. Even older Lawnboys with 90% worn out pistons and rings do not surge. Saxman your explanation is right on. As experience shows the Duraforce surging was a lean carburetor issue. It has been widely studied and repeatedly shown that correcting the mixture problems eliminated the surging problems.

Very true. The leaned out Dura Force engines were border line surgers out of the box. Very little dirt is needed to cause the carbs to start surging. Sorry Wally, properly carbed 2 cycle Lawnboys very rarely surge. Even with high wear as rdaystrom indicated.

Cordes; Most likely a properly-running 10550 would be putting out more emissions than recommended by the EPA. However, I don't think anybody on here cares what the EPA recommends for lawn mowers.:) It's the burble.
Unfortunately, I got out of the 2-cycle mower deal so I don't have any to experiment on.

Saxman, Thanks for the input. I'm with the crowd here meaning that I don't care either. Just was wondering how close they were to having it pass the EPA testing rather than lean out the carb so much.
PS: You need to get back into the 2-Cycle deals. I've followed your posts here for awhile and you're definitely one of the true experts here.

I just replaced the lower crankcase seal in my gold series 10550 2 cycle and it cured all the surge issues, it also starts and runs a whole lot easier, it got to a point that I had to run with the choke partially on for the first 5 minutes or so, another hint was the small puddle under the mower that would be where I had the mower parked from the last time I had ran it.
It wouldn't surprise me if they have it running lean to pass epa, but it should run as well as it did when I bought it new (so long as the compression and fuel system are in good condition) and it has ran almost 9 years flawlessly until this started late last year.

To any who might see this post: I am so glad I found this sight. My Lawnboy was not much fun at all to cut with. Surging, hard to start once it had run for a while, and excessive smoke. I drilled out the jets and it runs so much better. It surges a little for a couple of minutes, then smooths right out. I can now walk away, come back at any time, pull the rope one time and I'm off to the races. Thank you all for your helpful posts.

Cordes; thanks, but I'm not really. I do like 2-cycles in general and always have. The 2-cycle mower is such a strange case because it can't use the greatest attribute which is high RPM to get it's power. Therefore it has to run richer at 3600 RPM max to be useable. If they had made the 10550 with the Variable Speed instead of Personal Pace, I might still have it. What I got to replace it turned out to be a very rare bird, the LB22271 commercial with Honda GSV190 OHC engine. So, I'm still 'sportin' the green', and not just on St. Pat's day, but sometimes I do miss the 2-cycles.