[Erik]

My neighbor gave me his MTD lawn tractor when he moved away a couple years ago. It sat for several years because the dog had ripped the wiring out of it. The fuel in the carb bowl looked like mucus. The engine is a model 422707 type 1510.

At that time, I cleaned out the carb and have used it since. It runs OK but not what I would call great and it is somewhat temperamental.

Yesterday I replaced the starter gear and drive clutch after it jammed for the second time and decided to also freshen up the carb for the season with an overhaul kit and a soaking in cleaner. It runs better but still not as well as it should.

Here are my questions because I'm really not much of a carb guy. I've noticed that it always has and still does have fuel spattering out of the angled brass tube that comes out of the bowl area above the throat of the carb. Is this a bowl vent or is it actually supposed to be a part of the fuel delivery system? The fuel dribbles out of it and onto the throttle plate below.

When I advance the throttle up, a mass of fuel splatters out of this tube and the engine bogs for a moment and then regains its speed.

My plugs look like the engine is running rich with black soot on them and it smells rich when running. The soot will begin to appear after only a few minutes of run time after being cleaned. It of course gets heavier the longer it runs. The engine does not appear to be burning oil.

The idle mixture screw has no affect on how the engine runs whether it's all the way in or backed out.

I didn't change the float level. The guy at the mower shop told me that it should hang parallel to the top carb plate when held upside down and it did. I don't have any actual measurements of where it should be set.

The kit did come with a new needle valve for the float but not a new seat.

The plugs probably have about ten hours on them. I haven’t checked the strength of the spark because I’ve been suspicious of this carb all along.

I hope this is enough information to help. Thanks much.

 

[30yearTech]

I would check the valve lash to make sure the clearance is sufficient particularly the intake valves.
The brass tube is the main nozzle and fuel will dribble out of it when the engine is running, check for a possible air leak in the intake manifold or where the manifold mounts to the cylinder heads. It's possible that there is still some obstruction in the low speed circuit of the carburetor if the adjustment screws has no effect on engine operation.

 

[Erik]

I didn't think the valve lash was adjustable on an L-head engine. Could it be worn and have too much?

I pulled the welch plug and everything was spotless and open in there. I blew out all the passages while it was apart and everything was clear. I haven't checked for an intake leak. Would that not lean it out?

Thanks for the ideas to check.

 

[30yearTech]

Yes, the valve lash is adjustable on an L Head engine. Yes a leak at the intake would make it lean, a possible reason for the hesitation.

 

[Erik]

Thanks 30yearTech for taking the time to help. This is like solving a puzzle. I didn't mean to appear to question your knowledge and vast experience of these engines, to the contrary I very much appreciate your sharing- I'm just trying to learn and understand the process on how they work. I'm much more experienced on auto engines but am finding these mower engines very interesting and fun to work with. This forum is great and I'm reading issues that don't pertain to my engines because it's all so fascinating.

How do I go about checking the valve lash on this engine? I've not had the engine apart to really see how it all goes together but I thought it had the camshaft in the crankcase/cylinder assembly with solid tappets pushing straight out to the valves. If I remove the heads I'm looking at the bottom of the valves. Is that correct? I'm just trying to orient myself with this engine. Do I have to break into the case to adjust the valve lash?

The unapproved but effective method to check for an intake leak on a car is to allow a very small amount of propane to leak from a hose and move the hose end around the intake areas listening for an RPM increase. Is there any reason this wouldn't work on a mower? Is there a better way?

What do you think could be causing the black sooty spark plugs?

Again, thanks for your time. I'm learning a lot.

 

[30yearTech]

I did not gather from your questions that you doubted anything about my answers, I was just in a hurry and did not describe the adjustment procedure.

There are valve covers that can be removed just above the area where the tappets operate the valves, measure the lash there at tdc. Use the check chart http://www.perr.com/graphics/spec_chart_usa.pdf (copied from an another poster) to find the specs for your engine.

I have always just sprayed carburetor cleaner around any suspected leak, but I guess you could use propane just as well.

The black soot on the plugs can be coming from a rich running condition and can also be caused from the intake valves not seating properly or from insuffient valve lash.

To adjust the lash on this type of engine you have to grind a little off the valve stem, do it a little at a time and check the clearance so that you don't cut too much off that you can't put back on

 

[Erik]

Thanks, I'll take a look at it next weekend.

 

[Erik]

Just used the tractor to spray Roundup. Ran OK but like I said when I up the throttle, it bogs and I paid attention today, it also throws a puff of black smoke.