lawn boy dura force surge

eric_2007June 12, 2008

I have a model 10550 lawn boy mower that has a bad surge when starting up and under no load. After the engine warms up its slighty better when under no load at full speed but still very noticable and if you pull the throttle to less then full speed the surge is aweful. Ive had the carb apart to clean the main and side jet with fine wire and cleaner and replaced intake gaskets but no differnce. Ive seen past posts on here stating to drill the main jet to a 67 from a 65 i belive and the side from 46 to 48? What type bits are people refering too? the number gauge sets go up to size 40 and get smaller diameter as number increases just like sheet metal but the posts said to use a higher number size? I never have this problem with any other lawn boy mower although this is my only dura force model. The fuel is also new but im just already sick of this mower and want to stay with only my vintage models.

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Maybe you can find out where to buy the jets here-

And number drill sizes go to 80 (at least)

    Bookmark   June 12, 2008 at 3:14PM
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I get little surge on my V engine 10304 - but overall runs pretty smooth when I keep fresh, stabilized fuel in it (also seems to prefer sythentic 2 cycle oil versus LB oil). When that baby gets under load, the sound and smell of 2-cycle power.......ah too much fun.


    Bookmark   June 13, 2008 at 11:02AM
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'Under no load'
'Part throttle'

Under those conditions, it doesn't matter if it surges. It will most likely surge because it's a lean-running engine. Only one thing matters; does it surge under a heavy cutting load when completely warmed up? If not, there is no problem. If you want a smooth idle, get a 4-cycle.

    Bookmark   June 13, 2008 at 2:35PM
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True - never cut grass at Part Throttle. ALWAYS run at full throttle to get the proper blade tip speed. If it runs with little or no surge at full throttle you don't need to mess with it further. If it only runs smooth under heavy load at WOT - you could look at ONLY doing the main jet to try and smooth things out.


    Bookmark   June 16, 2008 at 3:40PM
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Eric,I understand about the surging, I myself would never allow a Lawn Boy to surge under any circumstances. The drills that you mention are from the 1-60 number set, 1 being the largest diameter and 60 the smallest diameter. You can buy the whole set or buy them individually (cheaper route), check at your local Ace. When drilling a jet(after removing it from the carburetor), its best to do it by hand holding the drill with what is called a pin vise or small chuck. An electric hand drill is to hard to control for something like this and you may end up with a oversize hole. Also, brass tends to grab onto the drill and you will end up with a ruined jet. Determine the size hole in the jet and go up one size remembering that the next larger drill has a smaller number size.

    Bookmark   June 17, 2008 at 7:37AM
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Don't put up with the surging - you will be amazed how much smoother it is when you enlarge the jets. You want a kit that goes to #80 drill bit size. Just go one size larger than fits currently. These bits are really tiny so be sure to keep them sorted by size correctly. You do not need a pin vise to hold these bits - just pliers will work. As mentioned you are drilling just a small amount of brass. Since you already had it apart once - should be a piece of cake.

    Bookmark   June 20, 2008 at 4:26PM
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dkr407(z5 MO)

Mine is doing the same thing. Ran great for several years and started the surging a few years ago. Some one suggested before driling out the jets, check to see if the governer spring has relaxed some during the years.

    Bookmark   June 22, 2008 at 11:25PM
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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) 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!!

    Bookmark   May 27, 2009 at 1:56PM
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Enlarging the jets will doubtless work, but I hold to the fact that if it doesn't surge under a load, there is no mechanical problem that needs fixing. The engine is designed to run lean under no load regardless of throttle position. Throwing fuel at it makes it run like the old LBs, but that is not a valid consideration in the technical sense.

    Bookmark   May 27, 2009 at 3:33PM
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Some owners feel that, with a 2-cycle, its best for the engine *not* to run lean under any circumstances. For them, if curing the surge makes them feel better, then there's nothing wrong with re-jetting.

    Bookmark   May 27, 2009 at 8:05PM
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Mine didn't surge for the first year and started the second year of use, it got to be so annoying that I decided to correct the problem -- they were indeed jetted so lean to comply with emissions standards that it defeated the purpose of buying a 2 cycle mower for the added performance.
I'm really happy with it's performance now. I believe it will last a lot longer - it's getting more lubrication, but it still doesn't smoke except when first started. There is a slight increase in fuel usage but very little.

    Bookmark   May 29, 2009 at 8:06PM
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When Lawn Boy engineers started tuning the engine to satisfiy the epa, they got very close to falling off the cliff (surge point). The engine was not designed to surge, but when the carburetor is set that lean it doesn't take much to start surging. This is a type of design error the Lawn Boy engineers had to live with, knowing it could damage there products reputation. It's nice to know many loyal owners were able to make the corrections.

    Bookmark   May 30, 2009 at 10:35AM
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I couldn't see how a jet would work for two years and then become unsatisfactory. A condition would have had to change. As you are looking down at the mower as you would when mowing, go to the left side of the carb and remove the plastic sticker. Under the sticker there is the idle jet. You need to unscrew it and clean the passages with a small wire.
Your surging mower should return to it's former self.

    Bookmark   June 9, 2010 at 8:57AM
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Thanks for a great resource and spot on info! You guys are GREAT. I had a surging 10324 DF with the 98-7000 carb. I got it roadside rescue and it is very little used. Judging from the tread on all the tires including the rear self propel wheels it is almost new. I suspect the reason it was tossed was the way it ran. Real bad hunting condition. I did just as you suggested with the #66 for the main jet and the #78 for the pilot jet. It now runs great.

One thing I don't see mentioned here is the ageing / wearing of the throttle shaft where it goes into the throttle body, main body of the carb. This I think is the reason that a new mower runs OK and after a few hours / mowings it starts to act up and continues to get worse. The plastic Carb combined with the throttle shaft are not a constant you can count on. In the old days we had an adjustable High Speed , Main Jet to compensate for this wear. We had for the most part an Aluminum Carb Body and a brass throttle shart with a little sponge ring to keep the dirt and dust from wearing the shaft out. This air leak is not supplied a fuel ration because it doesn't pass through the Carb venturi.

On another note if anyone is looking for a #61-#80 drill bit set I got mine off the shelf at my local Woodcraft store for $6.99 + tax. This company also has a great web site of tools and does a bang up mail order business.

Thanks for all the help. The Admiral

    Bookmark   August 27, 2010 at 1:44PM
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Ah Yes the Trials and Tribulations of the Epa Regulated 2-Stroke Engine . Throw Ethanol Based Fuel and
Sweet Dreams Baby ! lol .

    Bookmark   August 27, 2010 at 5:35PM
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lawnmowerdan(ne fla)

as usual saxmans advice is bogus-he gave up duraforce engines cause he isnt a good enough tech to cure the surge.

    Bookmark   August 29, 2010 at 9:01AM
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I drilled out the jets. the pilot with a #78 and the main with the #67. The mower blows major smoke now which is making me think I might have oversized the jets. I have spare jets for both the pilot and main jets. Should I swap out the main back to a regular size or the pilot back to a regular size? Opinions??? Thank you.

    Bookmark   October 23, 2010 at 6:12PM
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Problem solved. I took out the main jet and replaced it with the stock jet. This left me with the pilot jet drilled out with the #78 drill bit. Very minor smoke now, just like my 22261 after drilling both jets. Thank you for your advice. Two very happy Duraforce owners now.

    Bookmark   October 24, 2010 at 12:29PM
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Thanks to the info found here, my 10247 surging problem has been cured. Drilled the jets out with a #67 and #78 and it now runs great! Thanks!

    Bookmark   April 12, 2013 at 9:30PM
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Hey Dan, WTF is the matter with you? Evidently you can't read, or you've become senile and can't remember that I disposed of my 10550 because I came to despise Personal Pace. Mine ran fine and the guy who bought it was very satisfied with it in every respect. You should consider keeping your mouth shut when you don't know what you're talking about. The info I posted here was to do a few quick checks/adjustments to see if the mower really needs re-jetting. Note the use of the word 'needs'. It sounded to me like his mower was running as-intended, so it probably 'needs' nothing. But sure, you throw a little more fuel at it and it'll run smoother, although there is no improvement in function. That has been my argument with re-jetting all along - there really is no malfunction - ignorant owners think a 2-cycle should idle like a 4-cycle. So what if it doesn't? You can't cut grass at idle anyway. Who gives a damn how a lawn mower idles? Morons.

    Bookmark   April 16, 2013 at 10:55AM
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