Can't find a decent answer on Google, so I figured someone here might know.
Briggs & Straton 3.5 HP lawnmower. 20+ years old. Runs decent, if you can get the damn thing started. Doesn't seem to burn an excessive amount of oil. No leaks, fresh plug, oil and gas.
Cold, you'll pull and pull until your arm is sore. If you can get it to catch even once, it'll start on the next pull or two. Shut it off immediately (i.e. no time to warm up), and it'll start on a single pull. Run it for an hour, shut it off, and let it sit for another two (completely cooled off), and it will start again fairly easily. Let it sit for two days, and it's back to pulling your arm out of the socket to get it going again.
I'm thinking it's something fuel related, like you have to prime it with the first two dozen pulls. So, the carb isn't holding the fuel in, or it's draining back out somehow? I can rebuild a Rochester Quadrajet in my sleep, but small engine carbs are like a foreign language to me.
I can't answer it better than "carb related". With such small fuel passages, it takes little varnish to hose them up and let's face it, most people run bad/old gas every spring. My tiller is the same way. Starts fine after it pops or while hot. I have just resigned myself to hitting it right from the get-go with a bit of ether for the 2-3 times a year it gets used and it pops right off. Can't see spending money on fixing it as it works fine after starting.
So, the carb isn't holding the fuel in, or it's draining back out somehow?
Is the carburetor on this engine gravity-fed from the tank, or is there an actual fuel pump?
I've always assumed (and perhaps quite wrongly) that the priming bulb simply shoots a bit of fuel down the throat, similar to the accelerator pump on an automotive carburetor. It seems plausible that if this mechanism has dried/cracked/etc with age, then you'd be unable effectively to prime the engine, and thus, it would be rather like trying to start a Megasquirted engine if you forgot to set up the priming and cranking fuel.
If you can get access to the throat of the carburetor (ie: looking down through the venturi) try spraying a shot of starting fluid or gasoline into the manifold just before you give the rope a tug. If that starts it, then I'd wager the priming bulb (or whatever diaphragm lives behind it) may be bad.
At any rate, carb rebuild kits seem to be quite cheap for these engines.
(edit: looks like a couple folks beat me to it while I was researching Briggs carb rebuild kits)
Bad/weak fuel pump. It is a pulse arrangement with a diaphragm. The diaphragms get hard and do not pulse well and lose efficiency/leak. The newer alcohol fuels mandated by the bastards at the government attack the membranes as well. It is an easy replacement.
The newer engines also have a primer bulb to alleviate the extra pull or two to start problem. I would find a blown up newer model and swap for the friendlier carb setup or dribble some gas into the air cleaner to get her primed up easier anyway.
Edit: There is also typically a rubber elbow on the pulse tube for the diaphragm. If it is leaking crankcase pulsations slightly it can reduce the effectiveness of the pump.
No priming bulb. It's an 80's vintage Snapper, with the B&S "pulsa-jet" carb. Irregardless, which is a whole 'nother ballgame, I all of the sudden seem to have found an answer.
"an 80s-vintage B&S. That would be about the lowest point in the armpit of quality of B&S carburetors, and it agrees with your symptoms."
Further on, another poster states it's the choke (which makes sense), that is spring operated, and vacuum opened by a diaphram. That diaphram also controls a gravity feed resevoir, which feeds the carb.
Several posts indicate where to get the diaphram, with one stating, "the best selling B&S part of all time".
So, the first thing to check is the choke, to see if it's closing all the way. If it is, it might be that diaphram.
Edit: sixshooter, I know there's a rubber elbow on there as well. I checked it last year, and it seemed fine. This problem has been going on for a while, so I doubt that's the main problem. The past couple of years just lived with it, but when the wife offers to cut the grass, but can't start the mower, I hafta fix it!
This is gonna sound so stupid. I guess it'll be good for a laugh at my ignorance/laziness.
Took off the air filter - surprisingly dirty, as is was new last year - and checked the carb. Very dirty! Pushing the control cable all the way forward resulted in NO movement of the butterfly. Hmmmm.
Took off the cleaner housing to reveal lots of crud, and a small plastic slider thing. This had teeth like a rack on the back edge, engaging the throttle lever. It was kinda gummed up, so I doused the whole mess in carb spray. Turns out it's supposed to push on the butterfly via a pin, but the flapper was stuck. Also, full movement of the cable wasn't enough to move it. Apparently, after it got gummed up, I jammed the cable forward, and instead of moving to the end of the stroke, slipped back in the bracket that holds the cable shroud. Shitty, cheap-*** design.
Cleaned it all up, lubed it well, and adjusted the cable. Fired up on the first pull.
Can of carb cleaner: $4
New air filter: $9
Having the wife mow the lawn: priceless.