LawnBoy Dura-Force carb issue resolved

Discussion in 'Mechanic and Repair' started by Roger, Jul 29, 2007.

  1. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    Some may recall a thread I started about two months ago about a surging problem with my LB engine,

    Much has happened since the last post. I did not continue to post a blow-by-blow account of what was happening. I opted to start a new thread rather than tack onto the old one to bring the matter to conclusion (I hope!).

    I had two carbs, an one one (refer to carb A), and a new one (carb B). Carb B was installed early this Spring, but the engine was not running well, primarily because of surging. Carb A being used last Fall made the engine hard to start, and not run consistently, hence the change over the off-season.

    I tried old jets in Carb B, and new jets in Carb A. Neither resolved the problem. Carb A was much worse, because the engine was "roaming," that is, unable to keep consistent speed. The holes for the throttle vane were pretty badly worn. One observation included the hazard of running such a setup -- unfiltered air being sucked into the carb through the oversized holes.

    I remounted carb B, the new one.

    After trying many things, and concluding the mixture was too lean, I was directed to another forum that discussed rejetting the carb. One response on LS suggested the same thing, but without specifics. The other site gave very specific instructions. Those instructions included using wire bits to oversize the orifice in the jet. I tried in vain to find a set of wire drill bits.

    I did talk to a man who had worked with LB engines for a long time, and he said a simple solution was to get a tip cleaner for torches. A small case holds several file-like bits that could be used to open up the orifice a bit. Obviously, "a bit" becomes subjective, without any specific measure.

    I found a torch cleaning kit at the local NAPA store, and used two of the files to open up the orifice. I believed I was careful not to open up very much -- filing over a blank piece of paper, and found no filings. After reassembling the carb, the engine fired up easily, and ran well. I was concerned about a bit of smoke and odor.

    We took it to the field and after 30 minutes, I knew a couple of things, (1) the surging had ceased, (2) the engine was running too rich. The exhaust had a touch of smoke, and the odor was significant. Also, the sound of the exhaust made one believe the engine was being choked. One more thing: it used about twice the fuel as before the jet alteration, yes, about twice.

    My conclusion, I opened the orifice too far. End of trial #1.

    I took the carb apart, and pulled the jet out of the other carb. Now, working with a virgin jet, I could start over, but exercise more restraint in opening the orifice. I did some filing, and assembled. The surging returned, just like nothing ever was done. My conclusion, the mixture is still too lean. End of trial #2.

    I took the carb apart, ... opened the orifice a bit more ... reassembled ... went to the field, still had the surging. My conclusion, the mixture is still too lean. End of trial #3.

    I took the carb apart, ... opened the orifice (well, you get the picture, right). After going to the field, I was back to the results of trial #1, too rich. Obviously, I took out too much material again. End of trial #4. And, I was back to the twice fuel consumption rate, just as with #1.

    Now, I am wondering if there is a balance between rich/lean mixture, and smooth running engine. At this point, I thought not.

    However, during #4, one more change happened. The throttle vane would bind up in the holes through the throat of the carb, but only after running for awhile. The behavior would not begin until about 2 or 3 in the afternoon, after running OK (albeit rich in mixture) since 8:30 or so in the morning. Sometimes I could release the binding behavior and get a free turning throttle vane, but after a few days of this, I could not make it work at all.

    In the meantime, I had ordered a new jet and nozzle, so I had them on hand. But, I am perplexed about what to do. I did that the carb apart, taking out the vane and governor spring, and found nothing unusual. I reassembled, without any change -- by mid-afternoon, the problem of binding reappeared.

    I got a new vane and throttle spring, although I felt the ones being used were fine. I took apart carb B (the one being used for trial #1, #2, #3, and #4), and installed a virgin jet, the new throttle vane, and new governor spring. There was absolutely no reason to think this would work -- had a new vane and spring when carb B was new in the Spring, and had used a virgin jet in the new carb B when initially installed.

    I made the last change nearly two weeks ago. The engine has been running better than at any time in over three years I have owned it. Go figure. I have no explanation of the changes to make it work right. Remember I am using a virgin jet, so have done no rejetting on the present setup.

    When the engine was new three years ago, I had a terrible problem of surging then. But, it settled a bit. However, it never started well. Warm starts often were a real challenge (load at one job, move 10 minutes to the next, unload, and can't get it started). None of the starting problems have surfaced, and it runs very smooth. Throttle control works perfectly. I think it uses less fuel than at any time since I bought it.

    I wish I had an explanation. I have waited nearly two weeks to post, fully expecting each day to have it move into a tailspin again. But, that has not happened. I seemed to have the right combination of jet, throttle vane, and governor spring -- the combination, to make it run well. I know there are so many of these machines in use, many in the neighborhoods I work, and they are running fine. Perhaps I have finally reached the same point with this machine.

    Sorry for the long post. Perhaps my experiences will help somebody else.

    Thanks for all the responses and PMs to my other thread.

    BTW, I consider myself an expert at removing and reinstalling a carb! What a useless, unmarketable skill! Perhaps my next challenge would be to do it blindfolded.
  2. lawnboy dan

    lawnboy dan LawnSite Gold Member
    Messages: 3,716

    these carbs are a real pos arnt they. another answer to the jet prob is to install a suzuki carb low speed jet (which is slightly larger tan stock ) and leave the high speed jet stock sometimes this will cure the surging the binding of the gov vane is from the carb body warping from heat . you can mimimize this prob by not over tighting the carb to engine bolts. there is a site that we lawnboy users hang out on and all this info is avalible of these sites is garden web.
  3. khouse

    khouse LawnSite Bronze Member
    Messages: 1,465

    I'm glad it's working for you. You can't beat a LawnBoy when they are working good!
  4. Roger

    Roger LawnSite Fanatic
    Messages: 5,943


    Last Thursday, the engine went dead. After running well for about four weeks, it is seized, or nearly so. I have not yet pulled it apart, but I believe the cylinder walls must be scuffed.

    It was working so great. It probably ran only about 130-150 service hours since the last post above when the carb issue was resolved. After it died, I pulled the plug and it looked good - brown-grey, but not white. The failure was quick and sudden. It started fine on the last job and ran for about an hour, then slowed, and in a couple of minutes was dead.

    Strange as it might seem with all the troubles, I am going to rebuild. I have a short-block in the transit system, expecting it to be delivered in the next day or so. Nobody locally had one in stock.

    I am no longer convinced these mowers are intended to run 35 hours a week, week after week, day after day, hour after hour. Yes, it is run hard, often in stretches of 3 or 4 hours without stopping. The DuraForce engine is unlike the former generation that would hold up running under those circumstances.

    If I knew of an 80# mower that would produce the kind of results, I would make a switch. My Toro ProLine with a 5.5hp Sizuki is in the garage, but at 110#, or thereabouts, it is just too heavy on all the slopes -- to say nothing of the cumbersome bagger, and inability to work in discharge mode.

    I am using my old 8 year old, previous generation LB as a temporary fill-in. I hope I can get my DuraForce model back in service in a couple of days, after getting the new short-block installed.

    Are we having fun yet ....?
  5. lawnboy dan

    lawnboy dan LawnSite Gold Member
    Messages: 3,716

    roger-what oil were you using?
  6. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    I have been using 80:1 Opti-2. The original engine lasted only about four months with 32:1 LawnBoy oil.

    I know you will question the wisdom of Opti-2 oil because I know you are in the pool of objectors. However, the old LB (8 years old), has run thousands of hours on Opti-2, and it remains strong. After an early start on the old machine with 32:1 LawnBoy oil, and many, many exhaust port cleanouts, the change to Amsoil or Opti-2 eliminated that problem. And, the engine has outlasted the DuraForce several times over. Therefore, I cannot believe using Opti-2 oil is the source of the problem.

    The old LB ran all day today, purred like a kitten. A logical question would be: Why not just use the old machine? It is pretty tired in other departments. The holes in the cast aluminum deck are well worn out -- height adjuster levers are held in place with a spring and string running from side to side. The front running gear is very, very worn, and loose. The machine is OK for "second-machine" use, and fill-in, but I would be reluctant to expect full-time use for the rest of the season. I rebuilt the DuraForce machine in the off-season; new front running gear, including new wheels, bolts, levers, new bushings and pinion in the transmission. I intended it to run for 800 or more hours this season.

    I also use 80:1 Opti-2 in all my Stihl handhelds, without a problem.

    I just don't believe the DuraForce engines were ever designed for heavy duty use. Or, maybe better stated, never able to withstand heavy duty use without failure. I would characterize our usage as "heavy duty."
  7. khouse

    khouse LawnSite Bronze Member
    Messages: 1,465

    I've uses Amsoil since around 1979 with no problems. I was running 100 to 1 but went to 80 to 1 mix since I give the 2 strokes a work out. I run Amsoil in my Stihl saw, Redmax, Shindaiwa trimmer and my old Lawnboy with the OMC engine.
  8. lawnboy dan

    lawnboy dan LawnSite Gold Member
    Messages: 3,716

    i would use amzoil at 80/1 over opti 2 at 80/1 in a lawn boy.

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