2-stroke LawnBoy issues

[zrxkawboy]

I have a LB 10304 made in '97. For a while, the engine had been surging at low speed, but ran fine at full throttle. Last year, I took the carb apart to clean it, but it was absolutely clean inside. Still, I shot carb cleaner through the passages, and set the float level to the high side of specs. After this, it ran the same.

This spring, it was hard to start, and once I got it going, the engine speed has started to vary at full throttle. Now, I understand 2-stroke theory, but have very little hands-on experience with them. Does this sound like maybe crankcase seals? (I removed the muffler and checked the exhaust ports and they are completely clean.) Any advice is greatly appreciated, as a new mower is not in the budget right now. Thanks!!

 

[skyactiv]

Maybe you didn't get the float adjusted quite right. Thats my guess. If its hard to start, the primer bulb might be split. I hope you didn't leave fuel in it over the winter. There are other things like spark, compression.. .

 

[Panzerman]

I have three lawnboys. One I keep out back under a outside roof. Seems like I constantly have issues with it loping. The other two in the garage never. I think it's bad gas. Did you try changing the fuel? Try new gas. I ran Sea Foam through it, recommended dosage and it got better to its hardly noticeable. Try new gas and sea foam. I think it's bad fuel.

 

[Panzerman]

BTW. Crank seals bad make them run wild. No surging, just full blast. I have yet to have a crank seal go on a lawn boy. I had some from the late 60s.

 

[zrxkawboy]

Thanks for the replies. Like I said, there was no change after the carb work: still surged at low speed, ran fine at high speed. I replaced the needle & seat and bowl gasket. Set the float to spec referencing the FSM. Also replaced the primer bulb at that time. Honestly, I've never seen an OPE carb this clean inside...I shot carb cleaner through all passages anyway, but everything was spotless.

I have owned this mower since '04 when I bought it from a co-worker who was moving. I've always used non-ethanol gas and Conklin Head-Start syn oil, which contains a fuel stabilizer. Always just filled the tank in the fall and had no issues the next spring. Just to be sure, though, I ran some fuel system cleaner while I mowed for about 30-45 min, with no change.

Just to be clear, it does not surge at high speed, but the engine speed varies at random. The low speed surge has not changed.

 

[Cujet]

Maybe you are not aware of the "FIX" for surging lawnboy Duraforce engines.

They are jetted lean. This is from another site, but it's spot on:

1) Usually no fuel shutoff valve. You can add one for $4.99 (you need this, if you don't have one already)

2) The float valve needle and seat can need replace so that fuel does not leak out when the mower is not running. A fuel shutoff valve can solve this too.

3) There can be a surging problem with some of these carburators (they tried to be EPA compliant). This is usually solved with a larger "pilot" (idle) jet. You can drill it out 1 drill size bigger (you will need a set of 61 thru 80 drill bits) or you can replace the jet for about $4.00 The standard pilot jet is 37.5 and you want to drill it to 42.5 If you decide to buy a replacement jet, you want to ask for Briggs and Stratton part # 801308. A whole nother story as to why it is a Briggs part.

If the speed is changing, it can still be jetting and the governor is just a bit slower to respond. To use an analogy, it's chasing it's tail. The throttle closes a bit, RPM drops off way too much, the throttle then opens and the engines fails to respond rapidly, the throttle opens more and the engine speeds up too much.

Spend the $4 and purchase a Briggs jet if you don't have access to a micro drill set.

 

[CT8]

Originally Posted By: Cujet
Maybe you are not aware of the "FIX" for surging lawnboy Duraforce engines.

They are jetted lean. This is from another site, but it's spot on:

1) Usually no fuel shutoff valve. You can add one for $4.99 (you need this, if you don't have one already)

2) The float valve needle and seat can need replace so that fuel does not leak out when the mower is not running. A fuel shutoff valve can solve this too.

3) There can be a surging problem with some of these carburators (they tried to be EPA compliant). This is usually solved with a larger "pilot" (idle) jet. You can drill it out 1 drill size bigger (you will need a set of 61 thru 80 drill bits) or you can replace the jet for about $4.00 The standard pilot jet is 37.5 and you want to drill it to 42.5 If you decide to buy a replacement jet, you want to ask for Briggs and Stratton part # 801308. A whole nother story as to why it is a Briggs part.

If the speed is changing, it can still be jetting and the governor is just a bit slower to respond. To use an analogy, it's chasing it's tail. The throttle closes a bit, RPM drops off way too much, the throttle then opens and the engines fails to respond rapidly, the throttle opens more and the engine speeds up too much.

Spend the $4 and purchase a Briggs jet if you don't have access to a micro drill set.

 

[bchannell]

Surging can be caused by fault throttle governor sping, broken or damaged governor, or flywheel vanes, incorrectly adjusted carb, or float. But, I have to say more times than I care to admit it's caused by a partially sheared flywheel key.

 

[bchannell]

Surging can be caused by fault throttle governor sping, broken or damaged governor, or flywheel vanes, incorrectly adjusted carb, or float. Air leaking around the carb. But, I have to say more times than I care to admit it's caused by a partially sheared flywheel key.

 

[zrxkawboy]

Thanks for the info...no, I did not know about the surging fix. Now, I'm wondering if it applies to my engine. I thought the Duraforce was the 6.5HP. Mine is a 4.75HP Silver Series. Does the same info apply to my engine?

 

[AnthemBassMan]

Some of the Duraforce engines have the wrong butterfly in them. They were sent out with the snow blower carbs that have a smaller hole in the butterfly plate. I forget the diameter off hand, but you can find it online. Also some people have gone to using two governor springs.

L8R,
Matt

 

[zrxkawboy]

Thanks. Does anyone know if the Duraforce fixes apply to my 4.75 Silver Series?

 

[Cujet]

Originally Posted By: zrxkawboy
Thanks. Does anyone know if the Duraforce fixes apply to my 4.75 Silver Series?


Yes, I believe it does. However, the jet size may be different. All of the later model Lawnboy's were just slightly too lean for proper operation. As they age, the problem seems to worsen.

Even so, it's a good idea to ensure there are no other leaks or issues. As mentioned above, some use two governor springs. But I think this is not the proper solution.

The requirement for a fuel shut off valve remains.

 

[PhilsSmallEngine]

The LB 10304 is not a Duraforce engine. However, you should consider doubling up on the throttle vane spring. The Duraforce is notorious for throttle vane spring failing too. It's the same setup on that one. The jetting on the 97 supersedes the major changes that pushed the Duraforce into service in 98. I haven't heard too many problems with the 4.75hp other than just being a disaster of a design from a long term perspective which is one reason Toro abandoned that design in less than 10 years.

Check the primer line. I had my old Duraforce act the way you're describing and all that was happening was the primer line had a nasty crack in it and it was sucking in too much air making the mixture wrong.

The 4.75hp Lawn Boy is a hard to find info on. The hardcore LB enthusiasts ignore that version completely and I've never had one in my shop to tinker with personally.