12.5 hp B&S only starts with airfilter off. runs rough

[Willowick Al]

My 12.5 hp Briggs And Stratton on my 38", MTD riding mower is not running right.
Two weeks ago, the first time I pulled it out of the shed since October (7 months), I got it to start and run - without charging battery or anything. I was very pleased. I cut my front yard in about 30 minutes. All good.

Last week, I could not get it to start normally. Noticed it spun fast and for a long time with the electric start - like not much compression? I pulled the plug - looked okay. Took off air filter and tried to start it and it started!! Sputtered and missed as it clearly was running rough. When I put the filter back on it would not start or would stop if it was running. I decided to run it around the yard without the air filter off for a little while and after a few minutes I could tell that it was running normal/smooth. So, I put the air filter back and it ran normal and I cut my backyard - about 45 minutes with a couple of stop and starts to empty bags.

Okay. Today it would not start. Took the air cleaner off and it started - but runs rough. Let it run without air filter, but it did not run smooth, even after a few minutes. I can see gas splatter out of carb top when starting/running without filter. I tried check compression but got zero (slight needle jump). Don't know if gauge works.
Took carb float bowl off and checked that float moved and I sprayed some gumout around needle.
Everything same - only starts with air filter off and runs rough. What is wrong?

 

[pugaltitude]

If fuel coming back through carb then it could be a compression related component like head gasket or valve problem.
What is the model, type and code of engine?

 

[Willowick Al]

The engine model is 286707, type 0420 01, code 91053011

I had to put gas in the mower after that first 30 minutes of running well. When I got the gas, the pump shut off at two gallons and the pump next to it was tagged 'out of order'. Suspicious. The gas doesn't have the normal blue color; it's pinkish.
Do you think this could also effect the start only with the air filter off?

I'm trying to figure out how to check the valves - tomorrow.

 

[Willowick Al]

Okay. Today it would not start. Took the air cleaner off and it started - but runs rough. Let it run without air filter, but it did not run smooth, even after a few minutes. I can see gas splatter out of carb top when starting/running without filter. I tried check compression but got zero (slight needle jump). Don't know if gauge works.
Took carb float bowl off and checked that float moved and I sprayed some gumout around needle.
Everything same - only starts with air filter off and runs rough. What is wrong?[/QUOTE]

I figured it out. Actually, parts supplier advised me.
I discovered that my oil had a lot of gas in it. While trying to start it with air cleaner off, I noticed that the spatter from the aircleaner would evaporate with an oily residue. Checking the oil, I could tell that it was not only too high, but it had a gasoline smell.

Online, I found out that the carburator could be bad. At the parts store, he said that if I try to replace the needle/float valve that I would have a 50% chance of ruining the carb. He suggested that I put a shutoff valve in the gasline line and gasoline whoud not run into the oil sump while it sat in my shed. It worked. Used my lawn mower the rest of the summer without a problem. Just have to remember to open valve when I start and shut it when I'm done.

 

[bertsmobile1]

I do hope you have changed the oil twice or you will not have an engine at all next year.
If you have not then change it now and once again after the next mow.
Fuel taps get fitted to every mower I service & I always recommend owners turn off the fuel & run the carby dry when they have finished mowing.
This has a double barrel benefit being that the engin will run a bit hotter for the last minute or so as it runs lean which will burn all the crud off the spark plug and prevent shut off fouling.
Secondly it will stop fuel draining back into the engine .
The tap should be between the pump / tank and the fuel filter so you can shut off the fuel when you change your filter every season.

 

[SeniorCitizen]

QUOTE: At the parts store, he said that if I try to replace the needle/float valve that I would have a 50% chance of ruining the carb.
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Ridiculous. Fix it and learn how your engine fuel system works.

 

[Rivets]

The guy at the parts store sold you a boo boo strip to fix the problem, not solve it. He didn't know how easy it is to solve, because he is a parts guy. Replace the needle and seat, plus adjust the float level. To many techs use this method, which will cost you more oil changes every time you forget to shut the valve. Are you going to tell me that every time you have to stop the unit, you're going to shut the valve off? Fix the problem, don't cover it up.

 

[SeniorCitizen]

Check the float to determine if it will actually float. Floats fail a lot more often than needle valve seats wear out. Replacing the seat is where a carb can be screwed up.

 

[Rivets]

All depends on which carb he has. If he has a metal needle, he'll replace the viton seat and needle. If he has the viton tipped needle, just replace the needle. At some point in time he will need to do this anyway.

 

[bertsmobile1]

According to Jacks it could be fitted with either a Nikki or Walbro
Briggs and Stratton 286707-0420-01 Parts Diagrams for Carburetor, Carburetor Overhaul Kit
However both have replaceable needles & seats but you would have to be a little carefull as there appears to be a few variations.

No arguement about fixing the carb or at worse replacing it.
A lot cheaper than a crank.