I have a 2 stroke lawn boy push mower that was made in 1998. It's a pro gold series with electric start and self propelled. Anyway.. a person I know gave it to me and it had sat for awhile and wouldn't start. I went through the carb and cleaned it well. It will start on the first pull and overall runs perfect. But once it runs for a little bit, (can be 2min or over 10+min) it will start to loose engine speed and then die out. It seems to happen more often if I'm cutting taller or thicker grass areas. If I run the choke for a few seconds it comes right out of it and will run perfectly again for a random amount of time. It will still do this with a new fuel cap and even if I take the cap off it makes no difference. Fuel flow to the carb is great, I've messed with the float height and replaced the needle and seat, as well as replaced the spark plug. No differences at all. Is it possable the crank seals are going bad? Or any other ideas?
I too have had similar sounding issues with my old Lawnboy. Mine is a commercial 6259 model which I bought new in the early 80's. I am not familiar with your exact model but suspect it is similar to mine engine-wise.
The first time it failed it was about 20 years old. It would die out in a fashion that could be simulated by turning the ignition switch off and on extremely rapidly with progressively longer off times till it quit. This gave me the impression that it was a spark problem as there was a definite and immediate off and a definite and immediate on ie like turning the switch. Being such a simple engine, there couldn't be much wrong. I changed the spark plug immediately. It wasn't that so I checked around for a shorting ignition wire or a bad ground of the ignition coil. I checked the gap between the ignition coil and the flywheel but that was OK so the last thing that came to mind was a bad ignition coil. Of course that was it because that was the most expensive part. I bought an aftermarket one for around $30 and it has worked ever since.
The next time I had an intermittent issue, I would say it could be described as very temporarily running out of gas and then the gas would flow back resulting in the engine rpms returning (repeating). Unlike with my first fault with the ignition module, there wasn't a distinct off and on. It was a ramp down and a ramp up. Like you my first thought was a plugged vent. Turned out that my carb needed cleaning. I don't know where the gunk in the carb came from but a cleaning with carb cleaner and compressed air did the trick.
My last and final failure was like my original ignition fault excepting that the off and on wasn't periodic but rather random. I had a visibly fouled plug that would work and fail with a distinct on/off indication. I cleaned away the fowling carbon. All good.
This was my first new lawnmower and I will probably out live me.
My last thought is to check the quality of the carb float. I recent attained my neighbours old Lawnboy for my cottage and it has a cracked cork float which allows the carb to flood.
