Somebody with more knowledge than I will probably give you the solution. If it were me, I'd pull the muffler off and clean the screen behind it and scrape away the deposits that build up in there over a period of time. This is sometimes the problem with 2-cycle engines. Good luck.

That's one I hadn't thought of, so I pulled the muffler today and the spark arrestor was clean. The unit is only about 2 years old, so everything is fairly clean. Anybody know if there is a way to test the control module?

What color is the spark? A strong spark will be blue in color, a weak spark is yellow to orange. What color was the first spark plug you pulled? If the electrode was light brown, the gas mixture and delivery are probably OK. Dark brown to black running too rich. Is your plug gapped properly? Too wide or too narrow-improper spark.

Spark is blue (with the new plug. Was a yellowish color on the old one). Plug is gapped to manufacturer's spec (.020).
Mixture is good, the old plug looks like every other 2-stroke plug on my other equipment. I'm starting to think the timing must be off.

Check the primer bulb for a crack in it. check the fuel hoses for breaks in them. Check to see if the fuel filter in the tank is in the fuel, and not hung up out of it! I hardly think the timing is off, unless you got a stick caught in the works, and which may have sheared the flywheel key!
In case one of the fuel lines is broken, go to the lawn mower dealer, if they have parts and get a new length of hose. To get it into the teeny hole the old one came out of, using a razor blade or razor knife, cut one end of the hose back on a long taper, about 2-3 inches, which will let you feed the hose into the tank! You must have about 1-1/2 ft. of hose. Then fish the tapered end out the filler hole, until the other end is where it goes onto the carb. trim off some of the taper, and install a new filter on that end, and stuff it back into the tank! End up with the same amount of hose length when you are done!

I checked all the fuel lines and primer bulb when I took the carburetor off and disassembled it for cleaning. Everything seems to be working OK since the thing will flood after about 8 pulls. I can pull the plug and see atomized fuel spray out of the spark plug hole.
I've also checked the kill switch and all the grounding straps. Piston is free and moving like it should. I've checked everything I can think of to check short of completely disassembling the motor.
I'm getting spark, fuel, and compression. I can think of no reason on God's green earth why this thing won't start, but I can't get so much as a cough out of it, even though it ran fine right up until I shut it down the last time.
Ther only thing I can't check is the electronic control module. Could this be the problem?
If the reed valve was bad, would the engine lack compression?
Could it be a case of demonic possession???

Woodsrunner,

"...ran fine all morning, but when I shut it off for lunch and let it set for about an hour, it would not start back up."

In what position did you leave it during lunch? I notice you didn't mention the air cleaner. Could it be fouled? Can you start it with the air cleaner off?

MM

Shut it down as per the instructions in the manual, closed down the throttle and hit the kill switch.
The air cleaner is/was clean, I've tried starting it with it off and on as well as every possible choke/throttle setting I can think of. I keep thinking I must be missing something, but I'll be danged if I know what!

Might want to try putting a carb kit in there. If the diaphram is slightly damaged it could cause the problem you are experiencing. Can you take the carb number off the carb and post it? Also make sure the intake manifold is tight and that the gasket to it is good. Would not hurt to replace that gasket when you rebuild the carb. Pull the mixture, high and low speed screws out when you rebuild the carb and make sure you spray them good with contact cleaner or carb cleaner. Blow out everything with compressed air.

Woodsrunner,

I bought a 19-inch magnesium deck Lawn Boy lawnmower in the mid 60's and I spent more time working on that thing than running it. Within a couple of years I had bought the factory service manual for it and, with that, I knew enough to be dangerous.

When it was running, it was very powerful for it's weight. I bought a spare blade so I could always have a freshly sharpened blade and spare air cleaners as well. When it was running it could buzz through the thickest bermuda grass and its light weight made it very easy to handle. But it taught me to hate two-cycle engines.

As the years wore on I bought all kinds of spare parts for it, including a new rope pull assembly (that thing got pulled an awful lot), a new carburetor and, surprise, surprise, a new reed valve assembly. If I ever buy a new lawn mower it will definitely have a 4-cycle engine.

I grudgingly accepted a 2-cycle engine on my Stihl MS 361 chain saw, but if that thing ever goes on the fritz I am definitely taking it back to my Stihl dealer to have it worked on. Sharpening the chain and adjusting its tension and cleaning the saw and bar is all of the work I do on that saw.

You seem to have covered all the basic stuff on your Ryobi. If it were a 4-cycle it would definitely be purring by now. Have you had it apart enough to inspect the reed valve? A reed failed on my Lawn Boy apparently due to simple metal fatigue and that stopped it cold. But I could see the cracked reed.

If you plan to have an ongoing relationship with your Ryobi, maybe you should order a shop manual for it. But only if you want it to be your new hobby. I have no idea how you can test the electronic control module other than buy a spare and put it on. And that approach can run into money.

Is there any chance your Ryobi 340 BV is still in warranty? If you bought it from Home Depot, maybe you can take it back and simply return it for a refund or a replacement.

MM

I can get the parts for the carb (Walbro) from Outdoor Distributors-the kit is $18.00 and the diaphragm is about $10.00, or I can buy a whole new carb for $32.00. I don't think that is the problem, though. I learned long ago (out of necessity) how to take those things apart without destroying parts.
MaineMan, I hear what you're saying about 2-stroke engines. As a former auto mechanic I hate them, but have come to view them as a necessary evil since I've always needed weedeaters, chainsaws, and other gas-powered tools as my yard is just too big for the electric versions. Not to mention the kid's dirt bikes and go-karts. I've just never run into one that I couldn't get to run........eventually.
A former schoolmate runs a small engine repair shop, I stopped over yesterday to see if he had any suggestions, but he is in school and won't be back till Dec. 1. I'll probably check back with him then.
I want to thank everybody for all the help and suggestions, and if I ever do get this thing running, I'll post back about the solution.

Your might be on the right track, maybe out of time, check the flywheel key. Maybe try another new plug first. Steve

Woodsrunner, this is a real puzzler. If you've got fuel, oxygen, and fire the doggone thing oughta run. If there's fuel on the plug, then gasoline is getting in there and it should at least fire ... maybe not RUN, but fire. Like you, I suspect that there's not ENOUGH fire ... I'd gues that the problem is the ECM rather than the carburetor at this point. By all means, let us know how this plays out. I, for one, copy various solutions that appear in this forum and store them in a data base file (cut and paste into askSam makes it easy). This is one that I'd like to keep!

Well, I was down in the garage today (finally a nice day-above freezing) working on some other stuff, and decided to give it one more try. I pulled the starter a few times, nothing. Then, for no real reason, I pulled the plug and poured a few drops of 30w into the cylinder and replaced the plug. Started on the first pull!! So all this time, lack of compression was the culprit. Any way, I ran the thing for a good 45 minutes, ran great, plenty of power, no indication of any problem whatsoever. So, now for the hard part-what is causing the compression loss?
Way back when this problem started, I assumed compression was OK because I felt a noticeable difference when pulling the starter with the plug in vs. the plug out, so I never put a compression gauge on it. So much for assumptions.
I have had this thing disassembled down to the jug, but haven't taken the head off because I didn't see any indication of leakage. Guess I'll take another look!!

The rings are varnished(gummed,gunked,etc.);take off the jug,take off the ring(s),and clean the groove(s) in the piston,as well as the ring(s) itself;the beauty of a 2 stroke,is the ease of wich this cane be done;if this was a 4 stroke,you'd have to play with valve's, lifter's,and probably camshafts too,taking all the fun out of doing this kind of job......................Marc

A lot of small two strokes have compression release valves that help in starting the motor. If yours has one a compresion test will probably do little good. If compression was tha issue the motor would not have run for 45 minutes. I still think you have carb problems mainly with the prime section of the carb. Do the test you did with the oil using gas mix or fogging oil and see if it starts and runs that way.

"A lot of small two strokes have compression release valves that help in starting the motor."

Are there some that release compression automatically? I have only seen a Stihl where you depress a button.

I had the same problem with my Ryobi blower vac.On mine, the two bolts on the engine had vibrated loose causing a lack of compression.Tightened both of them and added a bit of locktite and it worked fine after that.