Erratic Lawn Mower Engine Performance - Starts and Runs Great / Stalls or Will Not Start

Hard to believe that the fuel was so dirty in the bowl. I recently installed a new carb on a WX-10 Honda water pump and it ran great. Three days later the guy said it won't start. New carb was full of Rusty gas and water. I replaced it again and took it to his house and saw the old rusty gas can he was using. The fuel must have been 3 yrs. old and the inside of tha can was all rust. He swore he used new gas, and then blamed his kids for using old gas. I told him not to bring it to me again. Thanks for the video.

I give you a ton of credit for your thoroughness and perseverance! Most people would never have found that problem.

I had a Toro mower that did almost the exact same thing. I called it my "Union Mower". It always died about 30 minutes into a mow and wouldn't work again until it had a 30 minute break. I came to the same conclusion as you, it wasn't venting properly.

I thought it was a fuel problem but the gas cap was a new one to me also, great job!

Here is why I really liked this one. Many years ago (many, many), I had a British sports car. It had suffered minor body damage from a parallel parker, near the fuel filler. Picked it up from the body shop, and it died on me on the way home. I was mystified: the car had always run flawlessly, and the body repairs had nothing to do with anything under the hood. (Er, “bonnet.”) Before getting out tools (something one always carried with British cars) and fiddling with the carbs, I decided to smell the fuel in the tank. Spun off the cap and heard the air rushing in. Left the cap off, drove to an auto parts store, bought a vented cap, and drove home no issue. Apparently the body shop had replaced the filler cap and, back then, one had to be sensitive to whether a given vehicle required a vented or unvented cap. All that said, I got all the way through this video and never thought about the cap. Oh, well.

That was almost super unsatisfying until you found the gas cap issue. Always nice to find something you can definitively point to as the issue. Nice work.

I remember the metal caps with a vent hole. I miss those days. I still use my 1965 Briggs. I've replaced the rings once in 1988 or so. Did the valves too, since the head was off. Outside of replacing the rope a few times, never a problem. It was my grandparents mower. I keep the oil changed and run it empty every fall. It'll get passed on. Bad cap....that took some thinking cap. Great work.

Inside the bad cap is probably a valve, similar to the disk valve in a vehicle's radiator cap. That lets it build pressure to a certain level before releasing, but also allows a vacuum to open the valve. Gummy-goo ethanol gas glued the valve or it's holes shut, thus you had excessive pressure and no vacuum admittance. Let's have a cap deconstruction to see what's going on with the bugger 😉

Good video! Who would of ever thought a fuel cap?

It would be interesting to determine how the original cap was supposed to vent and why it failed.

Some times James things like that just slip's pass , no body at fault here, it is just one of those thing's. great catch thou, good video. keep up the good work.

Nice find. That's probably the last thing you would suspect but it just goes to show it's often the simplest thing causing the problem.

What you do is amazing. When I was a kid in school that was gonna be my career yeah, fix small engines, but my high school shop teacher told me don’t do it cuz there’s very little money in it.

I love the the thought process you use troubleshooting these kind of things. I probably would have just drilled a small hole in the old cap instead of buying one, but Im cheap like that.

Once again James’s attention to detail found and issue I wouldn’t have even thought of, and once pointed out made perfect sense! Great video I now have another item to search for when working on small engines

Rance here Jim. What an astute observation ! But that’s what I have learned to expect, another very enjoyable and instructional video.

I would have never guessed the CAP! Nice job Jim.

I think the reason there are 2 very different fuel caps listed for this engine is that the original cap is required to meet EPA clean air requirements mostly for the state of California the replacement cap is a noncompliant version. Walt

I assume a that the original cap vented from those rectangular slots on the bottom outer ring of the cap. This would prevent rainwater from easily entering the tank. Most likely got clogged up due to it having a long passage and the crap that fuel leaves behind. It would be interesting to see if throwing it in your ultrasonic cleaner fixes it up. Again, this is 100% of an assumption.

I love seeing you torque every bolt with a torque wrench meanwhile I’m hitting them with my Milwaukee on low.