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Please advise on 12 hp i/c briggs

Posted by slowpoke_gardener 6/7 (My Page) on
Sat, May 12, 12 at 9:33

My daughter's lawn mower quit. She thinks that her Alzheimer's husband poured diesel in it. When I went to check it, it was low on oil, will still fire, seems low on compression, sucks pretty hard when cranked with finger over plug hole, but does not blow very hard. I could get it to hit a few times and try to run, but blows a lot of mist and a little smoke out the carb. At this time we don't have a lot of money to spend on repair. I am hoping to tear into it and check for damage, hoping for a burnt valve or maybe a compression release problem, but because of the low oil I fear something worse.

The engine is an older flat head design.

Thank you in advance, I know this is not a lot of info, but it is all I have at this time.


P.S. it is on a zero turn with joy stick controls if that makes any difference.

Follow-Up Postings:

RE: Please advise on 12 hp i/c briggs

1. Did you find diesel fuel in the mower? If so, as fuel or oil? Would it simply not run vs damages as fuel?

2. A compression check is usually my 1st test if a lack of compression or power seems to be an issue. From the quick finger test, could you hold the compression in with your finger (if so, that is usually a sign that it's low)?

3. If it is that low, I would check the flywheel key 1st, valve clearance 2nd and pull the head to check for blown gasket, burnt valves, cylinder wall and ring damage last.

RE: Please advise on 12 hp i/c briggs

Best to properly identify the engine-
Click the link to see how

Here is a link that might be useful: Model Type Code

RE: Please advise on 12 hp i/c briggs

Loger, thanks. The fuel I remover looked like a gas /diesel mix. I don't know why there would be diesel in a small can, but my son-in-law's mind is such that he cant be left alone and often does not know what he is doing.

I don't have a compression gauge anymore but will try to follow your instructions. I expect it has been 25 years or more since I have torn an engine town, and never one with a compression release. ( which I expect the 12 hp may have).

Would diesel mixed in with the gas make the engine run hotter?


RE: Please advise on 12 hp i/c briggs !

Bill, thanks for the codes. I hope to be going back over to my daughters house next week, I will try to learn more.


RE: Please advise on 12 hp i/c briggs

"Would diesel mixed in with the gas make the engine run hotter?"

Your question is above my personal experience but there is info online related to your question. Type your question in the search window and search. You will probably get some good personal info on this message board also. A youtub at one link look interesting but I like personal individual info and my hands on info. I feel the minor inspections will get you some solid answers quick. I know! You have to get motivated for this downer. If you find some solid info and I can help, I'll jump back in.

Again! Were you able to hold the compression in the cylinder with your finger? Experience Techs, would that be normal with compression release?

My questions to the Experienced Techs:
1. How much starting compression difference would you see in a system with a compression release vs w/o?
2. Is this system equipped with compression release since it is probably battery started?

RE: Please advise on 12 hp i/c briggs

  • Posted by baymee LehighValleyPA (My Page) on
    Sat, May 12, 12 at 21:56

I just tested a 15HP Briggs and it has 60# of compression, but I can't be sure if the compression release is working, although, it has one.

A small lawnmower engine will have compression of 90-120# without a compression release.

Gasoline will burn (the fumes) if you throw a match into a puddle of it. Diesel will not. Diesel needs to be atomized to burn, or compressed. In other words, gasoline vapor will burn where diesel vapor alone will not.

I'm not sure what even a small amount of diesel will to do gasoline, as far as running in an engine.

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