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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?
#1
B
bshr49
B
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?
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?
#2
B
bertsmobile1
B
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 .
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 .
#3
R
Richard Milhous
R
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?
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?
#4
B
bshr49
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.
B
bshr49
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?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 .
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.
#5
B
bertsmobile1
B
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
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
#6
R
Richard Milhous
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.
R
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.
#7
B
bertsmobile1
B
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 .
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 .
#8
B
bshr49
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: .
B
bshr49
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.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 .
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: .
#9
Scrubcadet10
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
#10
winmod21
winmod21
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.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.
#11
B
bshr49
B
bshr49
I think it's done for. There's got to be something wrong with the cylinder somewhere in the stroke. Crankcase is full of pressure and the oil is contaminated just getting it warmed up. I still can't figure out why it would be doing that and seem to pass a leakage test. Start of compression stroke.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
#12
Scrubcadet10
Scrubcadet10
Could be a faulty crankcase breather valve.
#13
B
bertsmobile1
B
bertsmobile1
Nothing inside your engine makes a 100% seal over time
The rings only seal tight when the piston is moving and even then there is a gap between the ends.
I went to the effort of writing a full description of the mechanism in my previous post
If you want to know then read it
If you don't then ignore it
The forum works a lot better read from your browser than as messages on a phone.
The rings only seal tight when the piston is moving and even then there is a gap between the ends.
I went to the effort of writing a full description of the mechanism in my previous post
If you want to know then read it
If you don't then ignore it
The forum works a lot better read from your browser than as messages on a phone.
#14
R
Richard Milhous
R
Richard Milhous
So does everything else.The forum works a lot better read from your browser than as messages on a phone.
#15
B
bshr49
2. I'm going to have to disagree that rings only seal when they're moving. They may seal better in motion, but they still seal when they're not.
3. I know it takes time/effort to respond to questions on a forum, especially when it's a question that could easily be answered with a quick search. All of my previous searching has not led me to an answer, which is why I joined here.
4/5. I read your posts and understand what you're saying. Perhaps it's that I don't know how much crankcase pressure is to be expected from a single cylinder with a relatively small small crankcase vs. an automotive engine that would have a larger volume available/cylinder. While running, the pressure is enough to make the oil cap jump like an exhaust flapper and spray oil if the cap is just sitting on top of the fill tube.
6. I am on a browser on this forum. I should be getting email notifications (that I would see on my phone) when a comment/reply is posted because I want to stay engaged and avoid ghosting my own thread, but I'm not getting any emails.
It still seems like the oil is getting contaminated with fuel a lot faster than it should be. The links in my last post were after about 5 minutes of run time after replacing the carburetor and draining/refilling the oil. The picture in the post before that (oil draining out of the filler tube) was what the oil looked like before I started working on it this time. I haven't drained/refilled the oil multiple times as Scrubcadet10 has suggested, but I wouldn't think a small amount of still-contaminated oil would make the oil look like that within 5 minutes.
If you think I'm ignoring your comments/advice, I'm not, I'm just trying to understand the root cause of the problem to see best how to fix it, or if it's even worth it. If the oil "mist" is instead a "fog", the WHY is what I want to know. I thought I might have a leg up having prior automotive experience, but apparently no. Sorry if I'm frustrating you, thank you for your time.
B
bshr49
1. I know that the cylinder is not going to be 100% sealed. Perhaps in my "hobbyist automotive background", I should have mentioned that I've rebuilt engines before. I am familiar with how an ICE works, ring gap, and that some amount of blow-by is always going to be present.Nothing inside your engine makes a 100% seal over time
The rings only seal tight when the piston is moving and even then there is a gap between the ends.
I went to the effort of writing a full description of the mechanism in my previous post
If you want to know then read it
If you don't then ignore it
The forum works a lot better read from your browser than as messages on a phone.
2. I'm going to have to disagree that rings only seal when they're moving. They may seal better in motion, but they still seal when they're not.
3. I know it takes time/effort to respond to questions on a forum, especially when it's a question that could easily be answered with a quick search. All of my previous searching has not led me to an answer, which is why I joined here.
4/5. I read your posts and understand what you're saying. Perhaps it's that I don't know how much crankcase pressure is to be expected from a single cylinder with a relatively small small crankcase vs. an automotive engine that would have a larger volume available/cylinder. While running, the pressure is enough to make the oil cap jump like an exhaust flapper and spray oil if the cap is just sitting on top of the fill tube.
6. I am on a browser on this forum. I should be getting email notifications (that I would see on my phone) when a comment/reply is posted because I want to stay engaged and avoid ghosting my own thread, but I'm not getting any emails.
It still seems like the oil is getting contaminated with fuel a lot faster than it should be. The links in my last post were after about 5 minutes of run time after replacing the carburetor and draining/refilling the oil. The picture in the post before that (oil draining out of the filler tube) was what the oil looked like before I started working on it this time. I haven't drained/refilled the oil multiple times as Scrubcadet10 has suggested, but I wouldn't think a small amount of still-contaminated oil would make the oil look like that within 5 minutes.
If you think I'm ignoring your comments/advice, I'm not, I'm just trying to understand the root cause of the problem to see best how to fix it, or if it's even worth it. If the oil "mist" is instead a "fog", the WHY is what I want to know. I thought I might have a leg up having prior automotive experience, but apparently no. Sorry if I'm frustrating you, thank you for your time.
#16
B
bertsmobile1
B
bertsmobile1
There is only 2 places where fuel can get into the engine
1) fuel pump
2) carburettor
leaking fuel pumps are easy to find because they drip fuel or have a diaphragm that can be seen.
If you want to see this happening then set up a cylinder head off with fuel dripping slowly into the cylinder
It will pass right through as if the piston was not there.
now pour the fuel into the cylinder and it will pool for a long time because for the fuel to get into the crankcase, the air has to get out and the petrol sitting on top of the piston prevents this happening very quickly.
If you really want to understand engine breathing then buy Rex's eBook and read it.
If you want the research numbers then go to the University of Auckland & buy the research papers.
Excessive blow by is a two way street and usually will cause carbon fouling of the plug
Fuel leaking usually does not do that .
1) fuel pump
2) carburettor
leaking fuel pumps are easy to find because they drip fuel or have a diaphragm that can be seen.
If you want to see this happening then set up a cylinder head off with fuel dripping slowly into the cylinder
It will pass right through as if the piston was not there.
now pour the fuel into the cylinder and it will pool for a long time because for the fuel to get into the crankcase, the air has to get out and the petrol sitting on top of the piston prevents this happening very quickly.
If you really want to understand engine breathing then buy Rex's eBook and read it.
If you want the research numbers then go to the University of Auckland & buy the research papers.
Excessive blow by is a two way street and usually will cause carbon fouling of the plug
Fuel leaking usually does not do that .
#17
H
hlw49
H
hlw49
Not gas that is water. Take a lighter to it and see if it burns. If so it is gas if not it is water and hold lighter to it long enough it should spatter and sputter. Don't burn up the dipstick though.
#18
B
bshr49
B
bshr49
If it was water cooled, I would agree and start looking at a blown head gasket. It does not ignite immediately, but will eventually produce a short-lived weak flame, but that could be the oil igniting, I don't know at what temperature that would start to happen. It does not show signs of boiling water out (I'm assuming that is what you meant by spattering and sputtering).Not gas that is water. Take a lighter to it and see if it burns. If so it is gas if not it is water and hold lighter to it long enough it should spatter and sputter. Don't burn up the dipstick though.
#19
B
bshr49
Edit: I forgot to add that once I checked the breather valve, I was going to fill the fuel tank and just let it sit for a week. If fuel is leaking through the carb (even though this one is brand new), it should cause the "oil" level to rise.
B
bshr49
It has only sat for 12 hours today after me getting it started. The plug is not carbon fouled as far as I can tell, it has some light grey deposits on it, not black, as I would expect with carbon fouling. I was going to take a look at the breather valve, too, and have run into my next problem, the holes to be used to loosen/pull the flywheel are not threaded.There is only 2 places where fuel can get into the engine
1) fuel pump
2) carburettor
leaking fuel pumps are easy to find because they drip fuel or have a diaphragm that can be seen.
If you want to see this happening then set up a cylinder head off with fuel dripping slowly into the cylinder
It will pass right through as if the piston was not there.
now pour the fuel into the cylinder and it will pool for a long time because for the fuel to get into the crankcase, the air has to get out and the petrol sitting on top of the piston prevents this happening very quickly.
If you really want to understand engine breathing then buy Rex's eBook and read it.
If you want the research numbers then go to the University of Auckland & buy the research papers.
Excessive blow by is a two way street and usually will cause carbon fouling of the plug
Fuel leaking usually does not do that .
Edit: I forgot to add that once I checked the breather valve, I was going to fill the fuel tank and just let it sit for a week. If fuel is leaking through the carb (even though this one is brand new), it should cause the "oil" level to rise.
#20
B
bshr49
B
bshr49
The valve does not sit flat against the block. Bad valve? Just guessing, but I would think it should normally stay in a closed position. Thinking back on everything now, if he completely filled the crankcase, there would have been no place for the oil to go except through the valve, likely with a lot of force, that could have easily bent it.Could be a faulty crankcase breather valve.
#21
Scrubcadet10
Scrubcadet10
If i remember right there is a specified gap, let me look through my service manuals.