# 12 hp Briggs and Stratton riding mower engine trouble



## spiderdan

I have been trying to fix my grandmas riding lawn mower. My uncle was using it when it stalled and it hasn't started since. I'll try to explain everything to the best of my ability but I'm only 16 so my knowledge on the topic is very minimal. I appreciate your patience. Unfortunately, I don't have any more information on the engine at this time than that it is a 12hp Briggs and Stratton. The carburetor also sports the Briggs and Stratton logo. The engine is turning over correctly and it is getting a spark. I also know that the fuel and air filters are not the problem. I know that fuel is getting to the carburetor so I took apart (at least as apart as I could get it). I removed it from the engine and removed the float bowl. Both the float and the main jet (at least I think that's what it is) that comes up the middle of the carburetor seem to be functioning. I also blew compressed air through the whole thing. Nothing seems to help. I can't even get the engine to sputter. Another thing that's puzzling me, I noticed a tube attached near the air filter that led to a plate on the side of the engine block. I removed the plate and there were two metal rods with springs around them. There was some oil in the chamber but it didn't seem like it would fill with oil during normal operation. I initially thought these were valves but I didn't think lawn mower engines had valves. Perhaps it is chainsaw engines I'm thinking of. If this engine does have valves, perhaps those are malfunctioning? Anyway, I'm out of ideas. I'd like to avoid rebuilding the carburetor unless I know that will solve the problem as that seems to be complicated and I run the risk of making another problem. I also don't know that I would be capable of taking apart the block. I've reached a dead end and appreciate any suggestions.


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## 30yearTech

If it was running and just stalled, then I would not think you need to go too deep into the engine, at least not yet.

Those are the valves you are looking at, and the part you removed with the tube is the oil breather. Slowly rotate the engine around and you should see the valves operate.

Have you checked to see if you have any compression in the engine?


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## spiderdan

I'm not sure what you mean by compression in the engine, so to answer your question no. How can I go about checking for this. Also, if compression is the problem what would need to be done to resolve this issue? By the way, what is the purpose of the oil breather? I didn't think the oil needed to be in contact with air. Thank you for the reply!


30yearTech said:


> If it was running and just stalled, then I would not think you need to go too deep into the engine, at least not yet.
> 
> Those are the valves you are looking at, and the part you removed with the tube is the oil breather. Slowly rotate the engine around and you should see the valves operate.
> 
> Have you checked to see if you have any compression in the engine?


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## 30yearTech

http://www.hobbytalk.com/bbs1/attachment.php?attachmentid=62960&stc=1&d=1216339138
Pg76.pdf

This may help a little with your understanding of how this engine operates.

The oil breather is there to vent pressure from the crankcase and produce a slight vacuum when the engine is running, it's connected to the carburetor so that any oil vapor that may escape is drawn back into the engine through the carburetor intake. Some engines simply vent it back to the atmosphere.

A simple compression test for which no tools are required is to spin the engine over at cranking speed, then watch the flywheel on the engine as it slows to a stop, if it stops abruptly or bounces back in the opposite direction then there should be sufficient compression for the engine to start and run. The best way to check for proper compression is with a leak down test, but this requires special testing tools.

There is a wealth of information available in the FAQ section of the Briggs and Stratton website.


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## spiderdan

Thanks for the information 30yearTech. I checked out the Briggs and Stratton website. If compression is the issue, do you expect that I'll need new valves, gaskets etc., or will it most likely just be a jam? Also, I'm not sure I have the proper tools to remove the valves, such as a valve spring compressor. I think the guide on the Briggs and Stratton website also mentions a precise measuring instrument. Will I need to drain the oil from the mower to get to the valves? Thanks again for all the advice!


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## 30yearTech

If it is a compression issue there may be several different things that it could be.

It could be a stuck valve, bad valve seat, foreign object caught in the valve, carbon build up, and may not be the valves at all. You do not need to drain the engine oil to work on the valves, but you do need special tools to reinstall the valves if you take them out, and you should get an automotive feeler gauge to check the valve clearances even if you don't take them out.


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## spiderdan

Hopefully I wont have to remove the valves, I have a feeling that's wishful thinking though. If this were carbon buildup, wouldn't this have happened gradually rather than all of the sudden? If the problem isn't the valves I would assume it would most likely be a bad seal near the ?cylinder head? (the part the covers the piston and valves) or the spark plug, is this correct? Once again, I really appreciate all the help.


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## 30yearTech

Well I have seen a piece of carbon build up, chip off and get under a valve, so it's possible, but you won't know if that it without an inspection.

The first thing I would try is a new spark plug, also do the basic compression test. Make sure you have spark to the plug as well.

If and when you take the cylinder head off, you will need a new gasket (seal) before you reinstall the head.


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## spiderdan

Hopefully it is just a piece of carbon build up, that would probably be easy enough to fix. I checked the spark and it was fine. I also tried a new plug. I probably won't get a chance to look at it again until early next week, Monday maybe. I'll be sure to post results. Thanks!


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## Albionwood

I think you're getting way too far ahead, worrying about valves. There are simple diagnostic steps you should go through before tearing into stuff.

You've already done the first: check for spark. So if the spark is fine, the next thing to check is fuel - is any getting into the cylinder? Try starting the engine, allowing it to turn over several times, then remove the spark plug and inspect it. If it's dry, and doesn't smell strongly of gasoline, then maybe fuel isn't getting through the carb. At this point I would try starting fluid, or you can add a half teaspoon of gasoline directly to the cylinder through the spark plug hole; reinstall the plug and try starting again. If it starts, then dies, you have a fuel-supply problem - gasoline isn't getting from the carb to the cylinder. 

There are three or four possible reasons for that: Gas tank cap vent plugged; fuel line plugged; fuel filter plugged; carburetor dirty. Find the fuel line coming from the gas tank, and pull it off the carburetor nipple. If gas doesn't flow out, then it's the fuel line or fuel filter. If it drips unsteadily, maybe it's the gas tank cap vent. If gas flows out steadily, the problem is in the carb, so the next step would be taking that off and disassembling it for a thorough cleaning. Only if that fails would I start looking at the intake valve. 

It's very common for a small bit of gunk to get into the float valve or the main jet on these carburetors and stop the flow of fuel. Even a very tiny particle can do it.

If fuel is getting to the cylinder but it still won't start, then maybe the gas is bad (water in it) or maybe the plug isn't sparking after all. Or something is blocking the air intake, so the gas can't get mixed with air to ignite; but usually if that's the case it will at least cough before dying.


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## spiderdan

Thanks for your response Albionwood. I have already tried disconnecting the carb and fuel flowed through the filter quite well. I actually drained the whole gas tank like this with the fuel cap on (through some of the draining process, I took it off at some point to check progress.) This makes me think the cap isn't the problem. I also tried starting it with the cap off. As for the carb, the float valve seemed fine and the main jet was not clogged, but I am no expert on carburetors. Its possible I missed something. Are there any other jets than the main? Also, I ran compressed air through the carburetor intake and it was expelled from the hole where the float valve was. I'm assuming this means it is clear and when the valve is open fuel just comes out around it?


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## Albionwood

OK, you've eliminated all problems upstream from the carburetor, but it is still a possible suspect. Compressed air can get through where fuel can't, because the gas does not have much pressure behind it. If the float valve sticks just a little bit it can be enough to prevent fuel flow. I've had to disassemble and clean a carb two or three times, each time believing it was pristine, before it would work. The B&S book on "Small Engine Care & Repair" shows how to do this in good detail. There might be something online as well.

How did the engine behave before it died? Was it "hunting and surging," going up and down in speed, or did it run just fine and then suddenly stop? "Hunting" is a typical symptom of a dirty carb or bad gas. But if it was running fine and suddenly quit and never coughed again, that might point to something more serious.

Try this: Remove the wire from the spark plug. Take off the air filter and filter housing. Cover the air intake with your hand, and manually turn the engine flywheel through at least one complete rotation. You should feel resistance as compression builds in the chamber, then it suddenly gets easier to turn as the exhaust valve opens; and at one point when the intake valve opens you should feel suction on the intake tube.

If there's no suction on the intake at any time, then the intake valve probably isn't operating. If you don't feel compression building and releasing, then maybe the valves aren't working (one or both is stuck open or leaking slightly), or there's a leak (blown head gasket or cracked head), or worse (piston connecting rod broken). I'm not expert enough to help with any of that - maybe somebody else here can step in.


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## spiderdan

Thanks for following up albionwood. As I said, my uncle was using it when this issue arose so I can not tell you first hand what happened. According to him, it was just fine but suddenly stopped. No sputtering or anything. I will give your suggestions a try.


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## spiderdan

I got a chance to look at the engine today and will post my results. Using 30yeartech's method of compression testing the flywheel did not "bounce back" the first couple times I tried this test but started bouncing back after the third try. Using Albionwood's compression testing method I noticed slight increases and decreases in resistance but never noticed any sucking. While I had the air filter off I did notice some liquid under it. I thought it was gas but I suppose it could be oil from the oil breather. Anyway, I decided to take the cylinder head off since I have nothing to loose. I found that both valves were operating properly, the piston, however, did not move. Since the engine would crank over I would assume I have a broken piston rod, crankshaft, or something in that assembly. I imagine this would be difficult/expensive to replace. Even if I could replace the broken part, isn't it possible that the piston seized up or something to that extent and caused the problem in the first place? I did notice some broken carbon deposits around the piston, I don't think this would be enough to restrict it's movement, however. It's a shame they won't change their oil periodically, then they could avoid problems like this. Perhaps they will learn from this. How difficult would this be to fix? Would I need special tools? I have a feeling this is probably out of my league now but perhaps it's not as difficult as I would imagine. Thanks to both of you for all the help!


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## 30yearTech

A broken connecting rod can be replaced in most cases with standard hand tools. The engine would need to come off of the mower and any drive pulley's removed from the crankshaft, and then the oil sump removed from the crankcase on the engine. This will give you access inside where the connecting rod is. The crankshaft journal where the rod attaches needs to be inspected for possible damage, if the crankshaft is damaged or if there is any other internal damage as a result of the rod breaking, a short block may be in order.


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## justin3

In alot of cases with the series engine you have, they are known to have leaky carburators in which case the carb will continually leak fuel into the engine even when it is off. The fuel gets into the oil and dilutes the lubricating property of the oil resulting in engine damage like you have. If you smell any gasoline mixed in with the oil you will now know how the engine blew.


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## spiderdan

You make this sound so easy 30yeartech, but I'm nor sure that this will be practical, for me, to fix. Removing the engine at my grandma's is out of the question as she doesn't have the workspace, tools, and it would be at least a days work for me. This would mean getting a truck and taking the whole mower to my house and then back again once the problem is fixed. Even if I could get the connecting rod replaced, that doesn't rule out the possibility of other problems. Unfortunately, when it comes to routine maintenance like changing the oil and filters, this lawn mower has been abused. I think that the breakage probably had something to do with the oil. Perhaps it was a lack thereof or perhaps it was gas leaking in as justin3 suggested. It could be dirty oil as well. This is also a very old lawn mower. My uncle had to spend about forty dollars on a part for the blade deck last month. A friend of my grandma's got a new riding mower and offered to sell my grandma her old one at a good price. If this doesn't work out then maybe I will try to contact one of the local lawn mower service places. If I get lucky they might even let me watch them replace the part, but even they couldn't fix a seized piston and I don't think this lawnmower is worth a small block. As much as I would like to see the inner workings of this engine, this project has already caused me to fall behind on other things. I'm going to have to call it as it is and let this lawn mower be replaced. If I have a change in heart I will report back but I don't foresee that happening. I appreciate the great help this community has given me. Especially 30yeartech. Thanks to you I know more about a 4 cycle engine than I ever have before. Thanks for your suggestions and ideas as well albionwood and justin3. If I ever have other engine issues, I will know where to turn to.


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## sledge1987

I have a 12 hp BS engine that I have recently re assembled after putting it all back togehter and firing it for the first time it started and ran for about 2 minutess sound good then dided and now it will not restart what is wrong its getting spark and fuel


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## Bandsmurf

Morning, I was just reading this and wanted to ask, I just got this 12 HP I/C Engine, replacing an 8 HP, and was wondering because this engine doesn't have as much resistance when turning the crank as the 8 HP does, is this good or bad, I could put a wrench on it and just sit there and turn it round and round with little resistance, however it does suck air in through the carb, and does push air back out through the exhaust, however turning the crank with a wrench is as far as I have been able to get because it came with a busted oil fill tube, the spark was broken in half, and the guy that owned it before, had just hard wired it and then cut the wires so I haven't been able to rig an electric start. I also have an 8 HP that tries to start when i am pull starting it but then quiets, it appears oil may have been coming through the head gasket, I'm guessing this could be a simple blown gasket causing a loss of compression correct? I as Spiderdan is am only 16, but have spent a long time working on Cars, Semi-Trucks so.


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## 30yearTech

Bandsmurf said:


> Morning, I was just reading this and wanted to ask, I just got this 12 HP I/C Engine, replacing an 8 HP, and was wondering because this engine doesn't have as much resistance when turning the crank as the 8 HP does, is this good or bad, I could put a wrench on it and just sit there and turn it round and round with little resistance, however it does suck air in through the carb, and does push air back out through the exhaust, however turning the crank with a wrench is as far as I have been able to get because it came with a busted oil fill tube, the spark was broken in half, and the guy that owned it before, had just hard wired it and then cut the wires so I haven't been able to rig an electric start. I also have an 8 HP that tries to start when i am pull starting it but then quiets, it appears oil may have been coming through the head gasket, I'm guessing this could be a simple blown gasket causing a loss of compression correct? I as Spiderdan is am only 16, but have spent a long time working on Cars, Semi-Trucks so.


It's pretty hard to tell just by turning the engine around by hand if it's good or not. Pulling at cranking speeds is a much better indicator. The 8 hp could be binding on the bearings making it seem harder to rotate, but may not have anything to do with compression.


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## Bandsmurf

Ah I see, Ill have to go out and set it up and put the the wrench on the end of it and spin it faster. I removed the Cylinder Head from 8 HP earlier and discovered that it does have a blown head gasket, so tommarrow I will hopefully be going to get my parts.

I gotta get a new spark plug, exhaust system, oil fill group, and drive-plate for the 12 HP (also someone rigged a pull start to the top of it, but I have been told you could dislocate a shoulder attempting to pull-start)

I also have to get a new starter gear, and a new head gasket for my 8 HP.

If you have any advise on how to remove the starter gear, I have so far been unable to remove the pin from the shaft. I also need some advise, if you have any on how to remove the rod from the carb for the 12HP, because so far I have been unable to.


73's 
Steven


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## paulho1966

*Same problem*

I have the same motor on a murray lawn tractor. My floods out with gas coming out the breather. I unhooked the gas and emptied the float but the gas keeps coming out the breather. I can get it to run for a few seconds but once its running the gas comes out and floods the motor. Any ideas. Thanks Paul


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## Bandsmurf

You probably need a new float bowl, my 8HP did that, I replaced the float on it and it ran fine.


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## Bandsmurf

I still haven't gotten the 12 HP running, and the 8HP wont start, but we are going to Indiana Sunday, and since my grandfather owns a trucking company he has a shop with almost ever tool imaginable, and we are gonna take it and overhaul it.


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