Is there something I'm missing? I think the oil in the intake is a blow-by problem, but what should I do about it?

[bshr49]

This is my first post, I have pretty much zero experience with small engine repair, so bear with me... A little backstory: This is my in-law's mower; they got it brand new at the beginning of this year's season. It's a Craftsman M320 with a Briggs engine (104M02-0223-F1). A few months ago, they asked me to look at it because it wouldn't start. I discovered that it had been way overfilled with oil and was hydrolocked. I cleared all the oil out; cleaned the carb; still no start. I replaced the carb and got it running again. I only ran it for 30 minutes and didn't see any signs of oil being sucked into the intake, so I thought it was good.

Fast-forward to yesterday, he tells me its not starting again. The deck is covered in oil again, the air filter and plug are soaked, and there's a pool of oil in the intake. The oil looks thin, milky, and has a slight gas smell to it. I know it wasn't overfilled with oil this time, he wouldn't have had any to add to it, I made sure of that. Coming from a hobbyist automotive background, the cylinder leak down test seems to be good (a hair over 2%) , but the compression seems way low (55 psi in 3 compression strokes), which tells me that the rings are shot. See more here.

What is the professionals' consensus?
1: Tear it down and rebuild it (it seems to be an aluminum block, so it's probably trash).
2: Get a new carb and get it running again to hopefully make it through the rest of the season.
3: Replace the entire engine.
4: Buy them an electric mower.

I'm not afraid to rebuild it and am not terribly concerned with longevity for reasons I won't get into, but keeping in mind low stock of push mowers at local stores this late in the season and the cost of a rebuild, what would you do?

 

[bertsmobile1]

A very common problem particularly with walk behinds
The float valve in the carb is not shutting off the fuel completely
The small leak is not enough to cause the engine to flood while using it but enough to allow the cylinder to fill with fuel while sitting in the shed.
EPA regulations prohibit carb vents / overflows to drain to the atmosphere so it is internal thus an overflowing carb fills the engine with oil.
I have problems with these newer carbs and just replace them with known good ones from an after market parts supplier who guarantees the carbs are good .
There are a lot of defective ones out there on ebay / amazon
You can pull it apart & clean it
When inverted the weight of the float alone should hold back 7 psi .
I pump it up then leave it for 1/2 hour or so.
If the pressure drops I ether replace the float needle ( &/or seat ) or the entire carb .

 

[Richard Milhous]

I have push that blows 50 psi, at best. Runs, smokes, cuts. Rings ain't your problem.

EDIT: Actually I did have to disconnect the crankcase vent on that mower to keep it from blowing too much oil into the intake. It starts fine though.

If the oil was overfilled, could it have popped a seal?

 

[bshr49]

A very common problem particularly with walk behinds
The float valve in the carb is not shutting off the fuel completely
The small leak is not enough to cause the engine to flood while using it but enough to allow the cylinder to fill with fuel while sitting in the shed.
EPA regulations prohibit carb vents / overflows to drain to the atmosphere so it is internal thus an overflowing carb fills the engine with oil.
I have problems with these newer carbs and just replace them with known good ones from an after market parts supplier who guarantees the carbs are good .
There are a lot of defective ones out there on ebay / amazon
You can pull it apart & clean it
When inverted the weight of the float alone should hold back 7 psi .
I pump it up then leave it for 1/2 hour or so.
If the pressure drops I ether replace the float needle ( &/or seat ) or the entire carb .

Previously, I had a carb on order from Jack's Small Engines, but after a couple of weeks of being on B/O and getting tired of taking our mower over every weekend, I ended up just getting one off Amazon. Maybe I just got a bad one, like you say. I didn't have any luck trying to disassemble/clean the original one, so I'll probably just replace it. What's a reputable online source for good parts?

How is oil getting back into the intake, enough to soak the filter and leave a puddle in the bottom of the filter housing? I can understand how it happened the first time when he filled it to the top of the oil fill tube, it would have simply drained through the breather tube, but how is it happening now? I'd hate to replace the carb just to have it happen again.

 

[bertsmobile1]

The engine is designed to toss oil everywhere inside the engine to lubricate itself.
It is called "splash lubrications"
The engine also has a vent so when the piston goes back towards the sump and compresses the air in the crankcase this pressure gets vented out the breather.
One of the reasons for this is take pressure off the seals & prevent them leaking or blowing .
Now the vent has to go back into the engine because the vented gasses will contain some oil mist .
In your case the "mist" is a dense fog

 

[Richard Milhous]

The engine is designed to toss oil everywhere inside the engine to lubricate itself.
It is called "splash lubrications"
The engine also has a vent so when the piston goes back towards the sump and compresses the air in the crankcase this pressure gets vented out the breather.
One of the reasons for this is take pressure off the seals & prevent them leaking or blowing .
Now the vent has to go back into the engine because the vented gasses will contain some oil mist .
In your case the "mist" is a dense fog

Sir, I must dispute. If the pressure to be relieved were caused by the descent of a piston, then the ascent of said piston would create a suction, resulting in (probably unfiltered) outside air being drawn into the crankcase. I believe that the pressure being relieved is that caused by blow-by, as in automobiles.

 

[bertsmobile1]

You are almost right
The engine is supposed to run with a slight vacuum in the crankcase.
This dates back to the days prior to oil seals when engines used slinger & scrolls to keep the oil inside
Almost no one bothered much about this till Rex Bunn was too lazy to remove the barrel from his B44 , countersink the bolt holes and put an O ring in there which had been the fix to preventing oil wicking up the barrel bolts & leaking out from under the head nuts .
Breathers work quite well on big singles up to around 4000 rpm ( what speeds do mowers run at ? ) after which the latency caused by the mass of the air causes most timed breathers to get out of phase and they start to work backwards.
Reed valve type breathers have a lag at the beginning because crankcase pressure has to overcome the spring tension of the reed
Flapper valves do the opposite and work best at very low speeds and get progressively worse as the speed increases.

The optium system is to make the crank case open and run the air counter current so the engine sucks through the rocker cover & blows from the crankcase .
Historic racers got from 0.5 to 2 Hp extra out of their engines after fitting a Bunn system , HD fits a modified one to some of their engines as does Royal Enfield .
However this makes impulse pumps redundant so don't think about bumping up the Hp of your mower .

 

[bshr49]

A very common problem particularly with walk behinds
The float valve in the carb is not shutting off the fuel completely
The small leak is not enough to cause the engine to flood while using it but enough to allow the cylinder to fill with fuel while sitting in the shed.
EPA regulations prohibit carb vents / overflows to drain to the atmosphere so it is internal thus an overflowing carb fills the engine with oil.
I have problems with these newer carbs and just replace them with known good ones from an after market parts supplier who guarantees the carbs are good .
There are a lot of defective ones out there on ebay / amazon
You can pull it apart & clean it
When inverted the weight of the float alone should hold back 7 psi .
I pump it up then leave it for 1/2 hour or so.
If the pressure drops I ether replace the float needle ( &/or seat ) or the entire carb .

Something must be wrong in my settings, I'm not getting notifications for my post. I went ahead and got a new carb from the Amazon B&S store, hopefully it's good (crossing my fingers), but most likely this will be the last season he will be able to use it.

If it is the needle just letting gas flow, I can see how it could leak into the cylinder and probably past the rings into the oil. I tried to attach a picture of what the oil looked like, but it was too large. Link here:

https://imgur.com/a/a2GKGuH

.

 

[Scrubcadet10]

Yep, that's gas in the oil.... when the needle fails, gas will go into anything lower than the tank. buy 2 or 3 20oz bottles of SAE 30 oil, drain the old, put in new, run it for a few minutes, drain it again, to be sure all the gas is out

 

[winmod21]

Previously, I had a carb on order from Jack's Small Engines, but after a couple of weeks of being on B/O and getting tired of taking our mower over every weekend, I ended up just getting one off Amazon. Maybe I just got a bad one, like you say. I didn't have any luck trying to disassemble/clean the original one, so I'll probably just replace it. What's a reputable online source for good parts?

How is oil getting back into the intake, enough to soak the filter and leave a puddle in the bottom of the filter housing? I can understand how it happened the first time when he filled it to the top of the oil fill tube, it would have simply drained through the breather tube, but how is it happening now? I'd hate to replace the carb just to have it happen again.

I have no expertise and/or advice, as the many experts do above, but just recently I did find a best price - for the carburetor and fuel pump and ignition module - I was looking for at: smallenginepartssuppliers .com ; oem parts w/quick delivery.