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I was given a non-running mower with a B&S 500 series ~4.5 hp 158cc engine. The mower is three years old. I was told it did not run very well last year or this year. Now it does not run at all. The mower had way too much oil in the crankcase, so I dumped all the old oil and filled to the full mark on the dipstick with fresh oil.

I emptied the old fuel and replaced it with fresh fuel, cleaned the carb, replaced the carb diaphragm, and put in a spark plug from another running mower. It still does not start. I know the primer works because I can see it shoot gas into the carb (I had the air filter screw in the carb without the filter box at the time).

I checked for spark and noticed something: the ignition wire is worn through where the wire comes out from under the engine shroud. When I checked for spark, the bare spot on the wire would arc to the shroud, but when I grounded the tip of the spark plug, the the arcing at the wire stopped. Could this be the cause of the problem? Is there some way I can check without buying a new ignition module?

I thought maybe one of the valves was sticking so I removed the cylinder head (flat head engine). There was some carbon, but the piston bore looked good and the valves and piston appeared to move in time. I rubbed some oil on the cylinder walls before I put the head back on. After I put the head back on I primed the carb, pulled the cord, and the mower ran... for about ten seconds before it quit again. Now the mower will not start.

Any ideas on what to check next?

Try wrapping some electric tape around that bare spot on the plug wire, and see what happens from there. A crack in the heavy insulation of the plug wire grounding out can cause a no run situation.

ILENGINE said:

Try wrapping some electric tape around that bare spot on the plug wire, and see what happens from there. A crack in the heavy insulation of the plug wire grounding out can cause a no run situation.

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After seeing the plug spark during testing, I would have thought the wire insulation issue was not the real problem. However, I removed the flywheel shroud and wrapped a bunch of tape around the ignition wire, and sure enough, the engine fired up. I mowed my front yard with the mower without any trouble. Thanks for the help. Free fixes are the best kind of fixes.

The design on these engines makes no sense. Why on Earth would anyone ever make the carburetor out of plastic and the fuel tank out of metal? Why put the carburetor on top of the engine, necessitating a cranky little diaphragm fuel pump when it is so easy to put the fuel tank on top and let gravity feed fuel into a normal float bowl carburetor.

Whoever put this mower together gave absolutely no care to assembling it properly. There was a little spot where the metal was formed to allow the ignition wire to pass through; the wire crossed past the shroud at a different part, allowing the wire insulation to be destroyed by the vibration of the engine.

Briggs has used the carb on top of the tank design since around the 50's. Only only change is what the carb has been made out of. I have seen the plastic carb various types since the 80's. On some of the newer engines with the tank above the carb and using gravity flow uses a plastic bowl type carb.

pbcrunch said:

After seeing the plug spark during testing, I would have thought the wire insulation issue was not the real problem. However, I removed the flywheel shroud and wrapped a bunch of tape around the ignition wire, and sure enough, the engine fired up. I mowed my front yard with the mower without any trouble. Thanks for the help. Free fixes are the best kind of fixes.

The design on these engines makes no sense. Why on Earth would anyone ever make the carburetor out of plastic and the fuel tank out of metal? Why put the carburetor on top of the engine, necessitating a cranky little diaphragm fuel pump when it is so easy to put the fuel tank on top and let gravity feed fuel into a normal float bowl carburetor.

Whoever put this mower together gave absolutely no care to assembling it properly. There was a little spot where the metal was formed to allow the ignition wire to pass through; the wire crossed past the shroud at a different part, allowing the wire insulation to be destroyed by the vibration of the engine.

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Please tell me why you hate the plastic carb that B&S has been using for 50+ years and they are so dam simple and easy to work on and the only way they become bad is if you tighten one corner more than the other and warp the carb

50 year old plastic carbs????? I don't think so........

Fish said:

50 year old plastic carbs????? I don't think so........

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they have been using that design for 50+ years

pbcrunch said:

I checked for spark and noticed something: the ignition wire is worn through where the wire comes out from under the engine shroud. When I checked for spark, the bare spot on the wire would arc to the shroud, but when I grounded the tip of the spark plug, the the arcing at the wire stopped.

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With the plug installed there was more resistance for the plug to arc because of compression so the current took the path of the least resistance which was to the shroud in this instance.

With the plug removed the shroud showed more resistance at that time and the plug arc was seen.

Tristan2021 said:

they have been using that design for 50+ years

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Can you give an example model number? Type/code too?

I have been looking up plastic carb for some time. I found this on boat engines.
In 1987 they introduced an outboard carburetor that everybody called 菟lastic top and float bowl. The plastic was a material called minlon which is a mineral enriched nylon and soon became commonplace in a lot of different part applications throughout the boating industry.

Yeah, well 20-25 years ago they started the plastic carb thing, but not 50 years ago..........

That is how I read it.