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Hi:

I have a Lawn Boy 10683 mower. Its performance has been superb since I purchased it in 2005. The last time I used it was about three weeks ago, when it worked fine. Now it won't start. It has fresh gasoline, a clean air filter, and the spark plug appears to be in good condition, but I cleaned it anyway.

I sprayed a shot of Chemtool cleaner into the air intake and the mower started up right away, so I assume the ignition system is working fine. The engine ran for maybe 10 to 15 seconds, and then it quit. A fuel problem?

So I drained the gas tank, removed the carburetor, and cleaned it with Chemtool. There was a tiny bit of gooey, yellowish gunk at the bottom of the float chamber, underneath the float, but the float was able to move freely. Otherwise everything looked fine. I cleaned the gunk, but I don't think it was affecting anything.

I reassembled everything, and the problem was unchanged; the mower still won't start. Another quick shot of Chemtool to the air intake and it started again, ran for 10 to 20 seconds (kind of pulsing...not at even speed), and then it quit.

I opened the drain plug at the bottom of the float chamber and gasoline streamed out, so the fuel is definitely getting into that part of the carburetor.

I would greatly appreciate any suggestions.

Thanks!

Qualifier, I am new Lawn Boy. It sounds like you have a fuel intake problem. My advice would be to never spray that Carb cleaner to run the motor. Get an atomizer spray bottle and fill it with chainsaw or Lawn Boy mix. When you use Carb spray I believe you are straight gassing your motor.

Rudedog:

Thanks for your reply.

I'm not familiar with the term "straight gassing". I Googled it, but the references I found seem to pertain to the use of pure gasoline in 2-cycle engines that normally require a gasoline+oil mixture.

That's not applicable here. My Lawn Boy engine is a 4-cycle engine, and in any case my use of the Chemtool as a starting aid was a diagnostic procedure, not something I have any intention of doing regularly. I have used it sparingly in the past (not on this engine) without harm. I appreciate your concern, but I've probably learned all I can learn from using it in this case. I don't think there's anything to be gained from using it again.

As I mentioned in my original post, I recognize that I might have a fuel problem. What I'm looking for is any suggestions about what steps I might take to isolate and identify the problem so I can fix it.

Def sounds like a fuel issue. I'd take the carb apart and clean it again.

fizzicist said:

Rudedog:

Thanks for your reply.

I'm not familiar with the term "straight gassing". I Googled it, but the references I found seem to pertain to the use of pure gasoline in 2-cycle engines that normally require a gasoline+oil mixture.

That's not applicable here. My Lawn Boy engine is a 4-cycle engine, and in any case my use of the Chemtool as a starting aid was a diagnostic procedure, not something I have any intention of doing regularly. I have used it sparingly in the past (not on this engine) without harm. I appreciate your concern, but I've probably learned all I can learn from using it in this case. I don't think there's anything to be gained from using it again.

As I mentioned in my original post, I recognize that I might have a fuel problem. What I'm looking for is any suggestions about what steps I might take to isolate and identify the problem so I can fix it.

Click to expand...

I am sorry. I did not look up the model number and assumed it was a 2 cycle. I agree with Parkmower because its not mechanical and its not spark so its gotta be the carb or fuel line.

Yeah...that's kind of what I figured. I did clean the carb, but it wasn't a "get in there and clean every little tiny orifice to within an inch of its life" kind of cleaning. I guess that's what I'm going to have to do.

It doesn't seem likely that it's the fuel line. I mean, if there were a problem with that, there wouldn't be any fuel in the float chamber, would there? I dunno...I'm certainly no expert, but it seems that way to me.

Anyhow, I'll get the carburetor off and see what I can find. I've just read an old post on another forum wherein someone had some tiny bits of rubber from the primer bulb come off and get into the carb. It was quite a task getting it cleaned out.

Thanks!

OK...I removed the carburetor and cleaned it completely...well, except for whatever parts might be behind the Welch plugs, which I don't think I can remove without destroying them. (There are three of them; two on the bottom of the carb, on the roof of the float chamber, and one on the side of the carb, forward of the primer bulb.) But all of the ports and tubes and holes are clear. There's absolutely no question that fuel is getting into the float chamber at the bottom of the carb. If it's not getting out of there, I don't know why.

The nozzle (the column that's lined with a red plastic tube that extends upward from the float chamber into the carb throat) is wide open. So is the carb vent tube, and the fuel inlet port to the float chamber. The fuel inlet needle is working properly; I verified that by hooking a plastic tube up the to fuel line fitting and blowing through the tube while I moved the float up and down. Moving it up (which corresponds to lots of fuel in the float chamber) closes the needle inlet, and moving it down (which corresponds to less fuel in the chamber) opens the needle inlet.

Anyhow, I put everything back together, and the mower still won't start.

The problem is that I don't know any way of testing the carb to make sure that fuel is actually getting out of the float chamber into the carb throat. And how does it get into the throat, anyway? Is it pulled out by vacuum generated by the piston on the intake part of the cycle? As far as I can tell, there's no fuel pump anywhere on the engine.

Now I'm wondering whether my assumption that the ignition system is working might be incorrect. I had thought that, because I could get the engine to start using a shot of Chemtool into the carb, the ignition system must be working. But maybe not. I mean, what if it's a weak spark -- enough to ignite the highly volatile, highly flammable compounds in Chemtool, but not enough to ignite gasoline?

Obviously I'm not a gasoline engine mechanic. Maybe I need to take this to a small engine forum. (?)

I have the same problem with my Lawn Boy exactly. I've cleaned the carb more than once, checked everything out including changing the ignition module, still no start. I've worked on lots of engines this summer and still I'm perplexed. This engine has a Walbro carburetor and if I can find one I'm thinking about putting one of the plastic carburetors on it that has a primer bulb which mine doesn't have. I did find out a few days ago that mine will start if I tilt the mower over on it's left side. Mine may have a float problem or something like that even though I've checked that and it seemed okay.

As it turns out, the problem was the carburetor. The engine was not getting fuel. I verified that by removing the spark plug and squirting some gasoline into the cylinder head. The engine started up, and then stopped when the fuel ran out. The same thing can be achieved by removing the air cleaner and squirting fuel into the carb throat. If the engine starts and then stops, it's a fuel problem, not an ignition problem.

In my case, the solution was to rebuild the carburetor. You can find the details here:

Rebuild Tecumseh Carburetor Model TEC-640350

I'm not sure whether that will be of any help with your Walbro carb, but at least the "squirting fuel into the head/carb" technique should tell you whether it's a fuel problem or an ignition problem.

As you can read for yourself in the post linked above, I never did find out exactly where the problem was, but it's most likely that there was something plugged somewhere due to problems with three gaskets inside the carb:

1. The upper gasket on the main fuel nozzle was completely gone.
2. The lower gasket on the main fuel nozzle was damaged.
3. The primer bulb valve gasket was in pieces, some of which were still behind the bulb, and some of which had worked their way into the float chamber.

There are numerous tiny "jets" and ports inside the carb, and any of the gasket pieces could have been clogging things up. The problem with blasting carb cleaner through everything is that you can't see when it blows some little obstruction out of there. I think that's what happened in my carb. There was just some little piece (or pieces) of gasket plugging the fuel system inside the carb, and there was no way to get at it without completely disassembling the carb so I could spray carb cleaner directly into the ports and chambers behind these parts (refer to this diagram):

• The primer bulb and valve assembly (Part No. 35)
• The three Welch plugs (Part Nos. 47, 48)
• The fuel jet screw (Part No. 20) and its plastic cover (Part No. 20A)

ALL of those areas are inaccessible without removing the parts listed. Blasting away at the carb with Chem Tool carb cleaner did nothing to solve the problem until I completely disassembled it and sprayed cleaner into those otherwise inaccessible areas.

Oh...one other thing. I'm not sure whether this applies to your Walbro carb, but as it turns out, most of the fuel that the engine uses at idle runs through the fuel jet screw. (See this thread: Question about fuel jet screw in Tecumseh Carb No. 640350) If the tip hole in that screw is plugged, the engine won't run.

The screw is covered by a little black plastic cap to keep people from fiddling with it. It's not an adjustment screw; you screw it all the way in and leave it alone. But you can remove it and inspect it without disassembling or even removing the carb from the engine. Just dig out the little black plastic cap and remove the screw. The tip hole must connect to the transverse hole for fuel to pass from the float chamber into the three jets near the throttle shutter. If not, the engine won't idle...which means it won't start.

I hope that's of some help.

Good write up Fizzicist.

Thanks Rudedog. I have to say that much of the credit goes to Bob ("Nevada Walrus" on The Lawn Mower Repair Man's POWER FORUM). I did the research and the work, but it was his helpful tips that got me over my stumbling blocks. There really wasn't anything I couldn't do for myself once I figured out how everything worked...with Bob's help.

One bonus that came out of it: I realized that my carburetor was actually in pretty good shape. There was very little gunk or goop in there, except a tiny bit in the bottom of the float chamber. That's pretty good for a mower that's seven years old. Actually, the weakest parts of the carburetor turned out to be the two main nozzle gaskets and the primer bulb valve gasket.

After that, I'd say the next weakest part is the primer bulb itself. Mine was showing some signs of wear---there was some cracking in the rubber at the fold above the base; however, it was still in decent shape...no holes, and it still would have primed the engine properly if the valve gasket hadn't self-destructed. My recommendation to anyone who replaces the primer bulb is to replace the entire assembly. The O-ring gasket on the plastic valve is definitely the weak link in that sub-system.

In retrospect, I might have been able to tell that the O-ring gasket had failed if I had understood how the primer works. I didn't, but I do now. There's no fuel behind the primer bulb. It draws in air from the breather tube through the primer valve, and when you first push the bulb, the base of the bulb collapses first and pushes the valve closed. If the O-ring gasket is intact, the valve gets a good seal and prevents the air under the bulb from escaping back through the breather tube. But the valve won't seal if the O-ring gasket fails.

Then you keep pushing to actually squish the bulb flat, and that forces the air out of the bulb. That pressurized air passes through a port at the base of the bulb and runs down through an opening at one of the Welch plugs in the top of the float chamber. That air pressure in the headspace at the top of the float chamber forces fuel through the various fuel ports and into the carburetor throat...and that's the prime that starts the engine.

I could tell that the primer wasn't working. It just didn't feel the same; there wasn't the same resistance that I usually feel...and of course the ultimate proof was the fact that the engine wouldn't start. But I now realize that the fact that there was no resistance when I pressed the bulb meant that the valve gasket had failed, and all the air was leaking back out through the breather tube.

If the primer bulb doesn't feel right, it probably means that the valve gasket has failed. It's such a skinny little thing that it probably has broken, which means it could be floating around inside the carb plugging things up. Next time I'll know better. :wink: