LawnBoy Dura-Force carb issue

Discussion in 'Mechanic and Repair' started by Roger, Mar 3, 2007.

  1. Roger

    Roger LawnSite Fanatic
    Posts: 5,917

    I have a LB Dura-Force powered 21" hand mower, SP. It is about four seasons old, but has been run pretty hard. It has been through some difficulties (no need to spell them out here), but it ran pretty well last season ... until the last month. I did not spend much time, but lived with the problem as it did not impact productivity in any great way.

    The primary problem was lack of speed control with the throttle. After a little searching, I found the throttle butterfly to be worn, the shaft passing through the throat of the carb, with the fin on top catching air flow from the flywheel, and a blind pin on the bottom. The real problem was the pin on the bottom was badly worn, out of round and not fitting tightly into the hole. The movement of the entire shaft caused the bottom of the fin to interfere with the slider mechanism. This mean uneven speed control.

    I replaced the shaft, and the blind pin fit pretty well into the hold. Apparently most wear was on the pin, not the hole. The new shaft now moved freely and did not interfere with the slider. The shaft is the only part I replaced.

    However, the speed control was not working well. Movement of the lever on the handle made no difference in engine speed. Sometimes the engine speed would race, and then settle to the proper speed. It was if the throttle was catching someplace, but that place was not obvious. My thought was that the disc that slides through the slot in the shaft was misshapen from my removal and reinsertion.

    This Winter I got new parts, a new shaft with fin (same one replaced in the last month of the season), a new throttle disc, a new throttle spring.

    I put all the pieces together yesterday, and got the engine running. However, the problematic characteristics remain. I am unable to get proper speed control with the thumb wheel. I have no tachometer, so must judge speed by ear. I am sure it is running a bit too fast, but any movement on the wheel makes no difference. Also, the speed control lever (moving the slider on top of the air box) makes no difference in engine speed. The shaft with the fin seems to be free, has spring tension, and the disc fits well in the slot (don't think it is providing any interference).

    At this point, I am at a loss what to try next. I have removed and reinstalled this carb many times, and did likewise on another LB of former years. My point is that I am working with familiar parts and mechanisms. I have had successes in changes before, mostly making spring replacements.

    Obviously, none of my explanations will make any sense, UNLESS you have had some experience with these engines.

    Anybody have a suggestion for next step?

    I am not sure if an entire carb can be bought as an assembly. If so, maybe it is time to trash this one in favor of a new assembly. My patience is pretty thin with this engine. However, I do want to have it running in top condition when the season begins. It is a workhorse for us, often running 6-7 service hours per day (more than a full tank of fuel).

    Thanks.
     
  2. imograss

    imograss LawnSite Senior Member
    Posts: 796

    I have experienced those same issues. I bought 2 10323 series Lawnboys with the Duraforce. I never used them all that much. I got tired of messing with them. If anyone wants them for parts your welcome to them.Both have new coils, baggers, mulch plates. I replaced them with a Quick 36. Both are behind my shop. One ran somewhat and the other ran fine, and then the crank rope locked up. They have been sitting for 2 years. Anyone is welcome to them. I am an hour west of Nashville Tn. Pickup only.
     
  3. lawnboy dan

    lawnboy dan LawnSite Gold Member
    Posts: 3,712

    yes you can replace the entire carb. these carbs are tricky to work on and its eaiser to replace the whole thing
     
  4. dutch1

    dutch1 LawnSite Silver Member
    from Jayhawk
    Posts: 2,231

    Two things I have experienced that you might check out:

    1. Check for the governor vane hanging up in the opening where it encounters the air from the flywheel.

    2. Check to see if you have overtightened the two screws that attach the carb to the block. I happened to discover this by accident on one of the first duraforces that I rebuilt a carb on. Overtightening can compress the plastic enough prevent the airvane/throttle shaft from being completely free.

    In addition, check your governor spring and make sure the 90 degree bend is still sticking out of the slot of the governor vane/throttle shaft. If you accidentally overturned the governor control ring at the base of the vane, the bend can be pulled inside the ring, thereby losing governor control.

    I'm not a LB fan but the duraforce was/is a decent engine. Nothing complicated about the carb to work on or adjust as long as you consider all aspects of its operation. The cork float is one feature that has always been a problem with LB. Sitting in fuel for long periods of time cause them to expand. thereby allowing the float to contact the carb body before the needle is able to shut off the fuel.

    Dutch
     
  5. lawnboy dan

    lawnboy dan LawnSite Gold Member
    Posts: 3,712

    every lawn boy owner should install a fuel shut off valve because of the pos cork float. the older lawnboys with metal carbs have a metal float and never have carb problems. the plastic carb is a pos
     
  6. dutch1

    dutch1 LawnSite Silver Member
    from Jayhawk
    Posts: 2,231

    LB Dan,

    What LB model had a metal float? LB used a Walbro carb(similar to what you find on Briggs) on some models in the 70's-80's that used a plastic float. As far as reliability, I believe I have encountered more brass floats that were defective than plastic. Just last week I worked on a Tecumseh on a MTD snow blower that had set for a couple of years. Rebuilt the carb and the unit sat for 10 days. After a little snow the customer used it and after shutting if off, he called saying that fuel was running out of the bowl vent. Pulled it apart again and found a small pin hole where the float sat on the bottom of the carb bowl where moisture and varnish had aaccumulated. I've seen seam seal leaks in plastic carbs but never a pinhole. Brass floats are susceptible to both. I agree, the cork floats are crap.

    Dutch
     
  7. Roger

    Roger LawnSite Fanatic
    Posts: 5,917

    Thanks guys for the insights. I will take these suggestions to the next work session. My working area is outside -- one day last week was acceptable, but nothing since and won't be for the next couple of days.

    The idea of the bolt-up being too tight, thus distorting the carb is a good tip. I have checked the 90 degree tip extending through the hole in the knurled knob. I have had my share of "revelations" on this over the years -- getting all back together, only to discover the tip has become dislodged from the hole. That spring is so delicate, as contrasted with all the other work to get the carb fitted up right, getting the sliding disk through the slotted shaft, etc.

    I will return to this project when the days get to be 40 again. At 35 my fingers get so cold, no feeling, and just cannot get all those little parts in the right place without feeling in the fingers.

    Thanks.
     
  8. lawnboy dan

    lawnboy dan LawnSite Gold Member
    Posts: 3,712

    dutch-i stand corrected. walbro had plastic and not a metal float. too bad lawnboy returned to the pos plastic carb. the walbro was great and trouble free. and yes trying to fit that carb on a dura-force into place on a cold day isnt my idea of fun
     
  9. Roger

    Roger LawnSite Fanatic
    Posts: 5,917

    Update:

    I spent more time working with the carb, throttle spring, and anything else I thought might be a problem. But, my efforts met with no success. I was very frustrated, nearly ready to take it to the junk pile.

    In a desperation move, I ordered a carb assembly.

    It arrived, but the weather didn't permit outside work for a few days. I was a bit surprised by the contents of the package. When the parts manual says "assembly," I presumed I would find a complete carb, ready for mounting. I had a fully assembled unit in mind, because I thought a new assembly would rule out any problems I may have inadvertently created with the old one.

    Anyway, the package came with a bare, black plastic carb, a plastic bag of parts, and another smaller plastic bag inside the outside one, with even more parts, plus a small hard plastic box with a screw top containing the delicate parts (governor spring, choke spring, and a couple of other very small items). This was not a case of "some assembly required," rather "total assembly required." Nothing was put together.

    I pulled off the old one, laid it beside me on the workbench, and had a good hard copy of the carb assembly from the parts book (downloaded from Toro web site). It all went together pretty easily. The most difficult part was the float valve and float spring.

    After getting it all assembled, I mounted it on the DuraForce engine. After a bit of tinkering with the governor speed control knob, it works fine. The engine starts well, runs pretty well, and idles back to a proper speed. I say runs "pretty well," because I think it needs some run time at load to iron out the cobwebs a bit. I wish it ran just a bit smoother at full speed, but I am optimistic that after a couple of hours of service, it will be better.

    I have ran it for awhile, let it sit, restarted. I have fired it a few times when cold, and it has started well. I don't know what else I can do for testing, so I will have to wait until I put it out on some properties for a full workout. I expect that to begin in about one week.

    So, the question remains about the reasons for the old one not working the way it should. I am without answers.

    Thanks for the help offered here. I wanted to report back. Maybe my experiences will help somebody else.
     
  10. lawnboy dan

    lawnboy dan LawnSite Gold Member
    Posts: 3,712

    well i would have been pretty pissed if i had paid for a whole carb assmbly and had to put it together! you can buy the whole thing as i have done so .like i said the plastic carb is a pos and very trouble prone . do your self a favor and install a fuel shut off valve and inline filter to minimise trouble caused by leaky needle/seat and dirt in the carb. some times a carb just cant be fixed and you just have to replace it. good luck
     

Share This Page