Stubborn lawnmower keeps stalling

May 14, 2009 3:06 PM

I need help with a stalling 4-stroke lawnmower engine. It will pull start, run for 30 seconds and then die.

I've ruled out everything in this AskMe here. The problem is that the engine will only run if I sit and press the primer bulb when the engine sputters. I've had it running for about 15 minutes that way, but as soon as I stop pressing the primer bulb, the engine stalls.

So far I have:
-changed the sparkplug
-cleaned the carb
-installed new air filter
-made sure that the float is not stuck
-drained the tank and refilled with brand new gas.
It started doing this before I drained it and put it away in the fall. Any ideas welcome.

(Note to those who will tell me to get a reel mower: this lawnmower was a legacy when I bought the house and is shared with 3 other urban households. I'm keeping it running as long as I can.)

posted by kuujjuarapik to home & garden (8 comments total)

Hypothesis: There's a bad seal on or around the primer bulb, letting air into the line when fuel is flowing, which interrupts the flow and shuts the engine down. When stopped, the air rises to the top, allowing you to restart the engine (which also restarts this problem). Pressing down on the bulb closes the seal, and keeps the fuel flowing.
posted by Cool Papa Bell at 3:12 PM on May 14

Have you fooled with the choke?
posted by musofire at 3:25 PM on May 14

Yeah. It starts best with the choke open, the idle will bounce up and down somewhat, then stall. With the choke open, closed, halfway, whatever, it's always squishing the primer bulb that keeps the engine running. Vrooom!...sputter...squish-squish-squish-Vrooom! and repeat. Like that.
posted by kuujjuarapik at 3:35 PM on May 14

My bet is that something is blocking the needle valve. That's what happened to mine. You can replace it for about 20 bucks.
posted by disclaimer at 5:17 PM on May 14

Primer bulbs typically work by squirting fuel a slosh of fuel straight into the carburettor's airway, bypassing all the carefully designed float valve stuff that would normally regulate the fuel-air mixture for continuous running. Sounds to me like your motor is only running on the fumes from the squirt of fuel from the primer, and is not actually getting sufficient fuel via the carburettor. The most likely cause for this is a blocked carburettor jet - it doesn't take much buildup inside the fine channel through the middle of the jet to reduce its effective diameter to the point where your motor starves for fuel.
posted by flabdablet at 6:39 PM on May 14

Fuel starvation. There's a bit of grass blocking your main jet. It's very simple to strip your carb- the key is to be methodical, gentle and thorough.
posted by BadMiker at 8:41 PM on May 14

It could also be a blockage in the fuel line, especially if the mower sits idle during the winter months...gas becomes varnish, or varnish-like, and when you try to start the mower after a prolonged period of sitting this gluey substance gums up the fuel line and prevents fuel from entering the fuel bowl on the carburetor. Easiest solution is to replace the fuel line and/or soak the carburetor in Gum-Out, if this is in fact what's causing the problem.
posted by motown missile at 12:25 AM on May 15

Thanks everybody. I disassembled the carburetor and found there was a bit of something between the fuel meter and the jet. Googling for carburetor disassembly instructions led me to this comprehensive and useful website, for anyone else with this problem.

(CPB- your hypothesis was what I thought too. I'm glad I didn't test it, though. That way lies madness!)
posted by kuujjuarapik at 2:32 PM on May 15