Lawn Boy mower with Tecumseh 4-stroke-cycle vertical shaft engine. Primer
bulb, non-adjustable carb.

I've been repairing / tinkering on mowers and other power equipment engines
since I was 10; not an expert by any means but some level of experience.
This has me stumped.

The engine seemed to have carb issues. It would take many pushes off the
primer bulb to get it to start. Sometimes I would take off the air cleaner
and shoot some gas from a spray bottle right into the barrel and then it
would start easily at least for a few seconds and in some cases continue to
run successfully and sometimes not. I once cut part of the lawn with a
paper towel wadded into the carb to partly choke it to get the mix okay.

So I dismantled the carb for cleaning and afterwards it was a better but
still not totally right. I was looking online for a carb rebuild kit when
I saw I could buy the entire carb--or a knockoff--for like twelve bucks. I
installed it and the engine ran well. They've dumbed the design down to
where there is no more actual speed control...you bend a tab that controls
governor speed tension. I did that; even used a strobe to check speed.

Used it a number of times and it was fine.

Now I cannot even get it to try to start.

My old trick of shooting in some gas did get a few partial starts but now
nothing.

So...no spark or no gas, right? Shooting in some gas manually would
eliminate that possibility.

So no spark?

I took it into a dark area and with the plug off but wire still hooked up
and the plug grounded with a clip lead I could see a spark when I cranked
the engine. I replaced the plug -- new last year -- anyway. No change.

Shooting in some gas as described does put out tiny puffs of smoke when I
crank, puffs that are not there with the plug disconnected. But nothing
like a true partial start. No sound other than what I make by pulling it.

I started to think there is something really unusual going on. Like a
valve stuck open or something. Nope...I pulled the head and they appear to
be working fine.

The only thing I can think of is that the spark is too weak. Or that the
new carb is so bad that the mix is so rich that there is no way to light it
off.

Before, grasping at straws and buying a new, potentially unnecessary,
magneto, I think maybe I will try to give the old carb a good cleaning and
put it back on.