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Old mower (sister n'law's) was sitting outside for a couple of years. Have a young niece who needed a mower so I thought I would try and get this one running for her. It is extremely light and easy to push.

Brand: MTD walk behind push mower
Mower Model: 11A-084E020
Engine: Briggs and Stratton 500 series
Carburetor Model: 795477 (This is the setup that has the plastic carb directly on top of the metal gas tank. No float/bowl/needle.)

Short version:
What can cause gas to get into the oil? Bad ring allowing gas to get mixed with the oil?

Long version:
Didn't want to start. Here's what I've done, not necessarily in this order:

1. Changed oil. The old oil did not look healthy. Looked as if there was water or gas in it. Thought I smelled a little gas in it and don't see how water could have gotten in there.
2. Compression tested with a schrader compression gauge: 90psi and held for a minute without dropping.
3. Pulled the magneto, cleaned it up with a wire brush including engine mounts, tested with ohm meter. Good. Remounted with proper spacing using a business card.
4. Checked spark plug with ohm meter and checked for spark. Good. Tested with plug out of engine, ignition wire attached to plug, in a dark garage. Could both hear and see spark.
5. Tested kill switch and kill switch wire to magento to make sure it wasn't grounding. Good.
6. I cleaned out the carb real well. Did NOT put in new diaphragm or gaskets. They seemed workable and I didn't have any immediately available.
7. Cleaned the gas tank while I had it off the engine.

Finally did get it to start but the RPMs were very low. It was hard to get started at all. Had a backfire or two also. So I thought, "maybe the muffler is all gunked up and it can't breath". So I put a new muffler on it (needed it anyway). Still low RPMs. Took a screw driver and held the throttle open. Started up and ran steady with very high RPMs. Took off air cleaner, looked at governor and saw that things were all messed up. The idle spring wasn't attached to the proper place. The governor blade/throttle shaft connecting rod was backwards. After getting that all straightened out and making a few adjustments, the mower runs great.

Mowed my front yard with it and it ran great. No surging, steady RPMs. Checked the oil, and level, color, and consistency looked good.Took it to my niece's place and mowed her front yard with 9" grass. Ran great. Governor did it's job. Steady RPMs, mower didn't bog down anymore than would be expected when the grass clipping bog down the deck. Checked the oil, and looked as if maybe it was getting thinned out and ugly looking. Wondering if gas is still getting in there. I told her to keep using it for now. Should mention she has a pretty steep side hill in one area of her front yard and I did mow it horizontally, not vertically.

How concerned should I be about gas getting into the oil?

Appreciate the help.

Would like to add, the mower starts easily when cold and when warm. No smoke out of the exhaust either.

Nessuno said:

Did NOT put in new diaphragm

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I'd do that next. But it seems like a diaphragm stiffening would move less fuel - so puzzling. Maybe tank is heating up in the sun, pressurizing and spitting fuel into intake.

Could water get in the oil? "was sitting outside for a couple of years". Yes, I think it could.

Maybe I'm overreacting. I did check the oil after running it, so it makes sense that the dip-stick tube might have oil all inside it from running it causing an erroneous reading. They say you should check the oil before a cold start, so maybe I should just sit tight and wait a couple of weeks then check it again. My real question was, on that type of carburetor, in what ways is it possible for gas to get into the oil?

It's not surging, in fact it is running great. So I don't want to replace the diaphragm unless that could be the gas-in-oil culprit.

Sat outside for years, starts easily, runs great.

*SULK*

Ethanol fuel, sitting outside with large temp swings daily? Say early spring it's 40F in the morning and 85 in the evening? Yup water is a coming.

Nessuno said:

Should mention she has a pretty steep side hill in one area of her front yard and I did mow it horizontally, not vertically.

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Yup at steep angles gas can/will get into the oil.

Nessuno said:

How concerned should I be about gas getting into the oil?

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VERY.

Pull the oil dipstick and smell for fuel.

slomo said:

Ethanol fuel, sitting outside with large temp swings daily? Say early spring it's 40F in the morning and 85 in the evening? Yup water is a coming.

Yup at steep angles gas can/will get into the oil.


VERY.

Pull the oil dipstick and smell for fuel.

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I know that 4-cycle engines have a tough time on side hills because they use splash lubrication (some might have an oil pump) whereas 2-cycle engines have the oil/gas mixed together. I also know that if you tip the mower to the wrong side that oil can run into the carb/air cleaner, and on this particular type of machine, with the carb on top of the gas tank, I can see where oil might even get into the gas if the mower is tipped to the wrong side. But my question still exists: "How does gas get into the oil?"

"Most" splash lubed push mowers are not meant to cut anything over 15-20 degrees. This is a splasher. Long story short, splashers are for mostly flat ground.

Ski slopes require a full pressure lube or similar engine.

slomo said:

"Most" splash lubed push mowers are not meant to cut anything over 15-20 degrees. This is a splasher. Long story short, splashers are for mostly flat ground.

Ski slopes require a full pressure lube or similar engine.

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Yes, I know that. But it still doesn't answer the question.

Nessuno said:

Yes, I know that. But it still doesn't answer the question.

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Need your engine number to determine what you are working with. Briggs 500 tells me nothing specific. Briggs could of used several carbs and such.

Have you determined you have fuel in the oil?

Have you pulled the oil dipstick and smelled for fuel like I suggested?

Nessuno said:

Yes, I know that. But it still doesn't answer the question.

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I'm leaning towards what dougand3 touched on earlier and perhaps fuel is being pushed into the crankcase between mowings due to air expansion in the fuel tank. If the gas cap is vented perhaps the vents are blocked. Check the fuel cap and/or leave it loose a bit to see if the problem persists. Just a thought.

Flot valve does not completely shut off the fuel supply
Gravity is causing the float bowl to overfil
EPA regulations require the bowl vent to be internal
So fuel runs out the bowl vent then dribbles into the cyllnder, passes the rings and fills the sump

Probably the No 3 most common problem on the entire forum

bertsmobile1 said:

Flot valve does not completely shut off the fuel supply
Gravity is causing the float bowl to overfil
EPA regulations require the bowl vent to be internal
So fuel runs out the bowl vent then dribbles into the cyllnder, passes the rings and fills the sump

Probably the No 3 most common problem on the entire forum

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There is no float bowl on this model. Carb sits directly above the gas tank. I posted the details in the original post.

slomo said:

Need your engine number to determine what you are working with. Briggs 500 tells me nothing specific. Briggs could of used several carbs and such.

Have you determined you have fuel in the oil?

Have you pulled the oil dipstick and smelled for fuel like I suggested?

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The mower is at my niece's place for now, so I can't get the engine tag # (I thought I had a pic of it somewhere) and can't verify the oil for now. My original post has all the needed details. But if not, maybe this will help: Carburetor 795477

She's a diaphragm beauty. Air pulses from the oil sump are pushed towards the carb to pump fuel into said carb.

How about pulses in engine intake vacuum draw the pump diaphragm inward. A spring returns the diaphragm outward. A cup covers the spring to prevent damage to the thin rubber diaphragm (#13). The in and out motion of the diaphragm working in concert with the flapper check valves causes fuel to flow through the pump.

Different carb below, same principal.

I bet fuel lines are cheap for that one.

slomo said:

She's a diaphragm beauty. Air pulses from the oil sump are pushed towards the carb to pump fuel into said carb.

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I thought those worked by suction from the intake.

Richard Milhous said:

I thought those worked by suction from the intake.

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That's what I thought too, but perhaps 'slomo' is right. It's a very good question even for the gravity fed/float/bowl type carburators. Is gas 'sucked' into the combustion chamber or is it 'pressurized' into the chamber? Never gave it any thought bvefore. Here's a better pic of the carb in question. The long tube is the gas intake. I assumed the other was used in conjunction with the primer bulb. Carb 795477

Richard Milhous said:

I thought those worked by suction from the intake.

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You are correct. I was off my meds that day LOL. I corrected my mis info above. Good catch Richard.

Check this out. GREAT site with tons of info.

slomo said:

You are correct. I was off my meds that day LOL. I corrected my mis info above. Good catch Richard.

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It's okay. I was too. I thought the spring was a second tube for the primer. Oops.

slomo said:

Check this out. GREAT site with tons of info.

https://outdoorpowerinfo.com/repairs/briggs_vacu-jet_vert_auto.asp

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That is an excellent breakdown of that type of carb. I bookmarked it in my browser. That link alone, makes this whole thread worth the time to read. I learned quite a bit about how the diaphragm, intake tube, needle valve, all work in conjunction; also learned a lot about the needle valve itself. Excellent photos and breakdowns.

I've had motorcycles that got the oil flooded with gas when the fuel shutoffs were left on. People think that the carb floats will prevent that but sooner or later that is not the case. Vintage floats were not compatible with current pump gas and the eventually leak and don't float, letting fuel run. In the bikes some floats were remanufactured of a material not sensitive to the ethonol and were aptly named "stay up floats". Being that the oil is diluted when it happens it would be bad to run it like that.

If you are sure that gasoline is getting into the Crankcase, perhaps someone put it in accidentally.
Look for another problem although it seems you have carefully checked them all. 30 Std oil.

Nessuno said:

Yes, I know that. But it still doesn't answer the question.

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ANYONE WHO FIXIES ENGIINES FOR A LIVING WILL TELL YOU THE CARB FLOAT IS NOT WORKING PROPERLY. OVER FULL WITH GAS WILL DILUTE THE OIL, DIRTY AIR FILTER WILL DILUTE THE OIL. i WILL NOT SPEND TIME TELLING YOU HOW, JUST KNOW IT DOES.

VRR.DYNDNS>BIZ said:

ANYONE WHO FIXIES ENGIINES FOR A LIVING WILL TELL YOU THE CARB FLOAT IS NOT WORKING PROPERLY. OVER FULL WITH GAS WILL DILUTE THE OIL, DIRTY AIR FILTER WILL DILUTE THE OIL. i WILL NOT SPEND TIME TELLING YOU HOW, JUST KNOW IT DOES.

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Hey loud mouth,

Might want to read the entire post before spouting off. His carb is below. His carb doesn't have a float.

slomo said:

Hey loud mouth,

Might want to read the entire post before spouting off. His carb is below. His carb doesn't have a float.

View attachment 58403

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Thank you very much. I've been drinking a little bit today watching my fantasy football team go down the tubes and I was about to jump down the throats of a couple of posters who didn't read that I don't have a gravity fed system into a float bowl. LOL

Nessuno said:

Thank you very much. I've been drinking a little bit today watching my fantasy football team go down the tubes and I was about to jump down the throats of a couple of posters who didn't read that I don't have a gravity fed system into a float bowl. LOL

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We take care of our own on here LOL.