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#1 JoshBrown OFFLINE  

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Posted March 30, 2011 - 12:52 AM

I picked up this little unit today for a song and have been trying to get it to run. This is not it but close.imagesmtdred.jpg It is a mid 80's Mtd Lawn Tractor. The model # tag was pretty wore off I could make them out if I really had to but it would be hard. It has a 11hp Briggs and stratton engine and a 38" deck. The PO said it had electrical problems but use to run good he seemed honest so I am kind of baffled. I unhooked the ground on the magneto and turned it over and had good spark. The thing that has me baffled is that I can not get it to hit with a shot of carb cleaner in the carb but has good spark.:confuse: It also has a lot of blow back through the carb it is like it is breathing in and breathing out. I took the head off and everything seems good the valves are opening and closing when they should and the magnet is right before the magneto on top dead center so appears to be timed. I have no idea what to do?:confuse: I dont know how to really check the time or how to check valve clearances so I am kind of lost. My skills are kind of basic. It also seems like there is a lot of air coming out of the breather of the valve cover I don't think this is normal. I thought about going to the scrap yard and picking up an 11 hp that looks good and switching them out. But I would really like to fix this one. It is the only way that I am going to learn anything. So anybody got any ideas of what it could be???

#2 NUTNDUN OFFLINE  

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Posted March 30, 2011 - 05:11 AM

Did you happen to check compression? If now I would check compression on it first. If it is low on compression that could be part of the blow back problem. To check the valve clearances I am assuming there is a valve cover on it, if you remove the valve cover you will see where the pushrod and valve stem meet. You can measure the clearance between the push rod and the valve stem while the valve is closed and the other one is open. You want to make sure the push rod is off of the cam lobe. I can type up a better description once I get home from work if nobody else replies.
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#3 Gtractor ONLINE  

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Posted March 30, 2011 - 05:38 AM

I'm not up on vertical engines but if it has a compression release it will blow just a little out of the carb. Make sure there is not any carbon under the valves that could hold them open. If it has a compression release you should be able to see the intake valve stay VERY slightly open as you spin the engine over. This only happens at cranking speed and is overridden when the engine starts. Other than that I can only think of one more thing and that is SOME modern carb cleaner are NOT flammable. If you have that environmentally friendly stuff it won't burn. I'm not trying to be funny, just make sure it says flammable on the can. I know someone here will help you figure this out.
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#4 DanO OFFLINE  

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Posted March 30, 2011 - 06:37 AM

Have you looked closely at the Shear Keyway Through & UNDER the Flywheel ? If it's even just slightly worn the timing can be off enough to not make it fire when it should. and create a lot of other simptoms that can lead you down the wrong path. $$$$$$$$ Changing things that don't need changing.
These Keys sometimes don't actually Shear BUT Twist a bit and look allright from the top side with the flywheel on but get out of wack under neath just enough to throw it offf. I know it sounds simple but it's undoubtedly the "Weakest Link" and Very Important to check . I've experienced this long ago and now it's one of the first things I allways check.....Sometimes .a New .65 Shear Key may save the day!
I may be wrong but it's worth checking out.

This Pic of a badly worn key shows what I'm talking about.

This video is a Good Example of what happens when you have a Bad Key in a GT with a Briggs Engine. It will NEVER Start....Notice the the Blow-By, as you mentioned you have with yours...
Good Luck

Attached Thumbnails

  • key1.jpg

Edited by DanO, March 30, 2011 - 07:55 AM.

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#5 JoshBrown OFFLINE  

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Posted March 30, 2011 - 10:10 AM

Did you happen to check compression? If now I would check compression on it first. If it is low on compression that could be part of the blow back problem. To check the valve clearances I am assuming there is a valve cover on it, if you remove the valve cover you will see where the pushrod and valve stem meet. You can measure the clearance between the push rod and the valve stem while the valve is closed and the other one is open. You want to make sure the push rod is off of the cam lobe. I can type up a better description once I get home from work if nobody else replies.

I didn't check the compression with a gauge but it felt pretty good by turning the flywheel. I will pick up a gauge and check. It kind of makes sense that low compression might be the problem it really seems like the air is going through the crank case and coming out the hole in the valve cover that is connected to a tube with the carb. I am assuming that it needs rebuilt then. The cylinder wall was really smooth I wonder of just a new set of rings would fix it or do you think it would need machined?

#6 JoshBrown OFFLINE  

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Posted March 30, 2011 - 10:15 AM

I'm not up on vertical engines but if it has a compression release it will blow just a little out of the carb. Make sure there is not any carbon under the valves that could hold them open. If it has a compression release you should be able to see the intake valve stay VERY slightly open as you spin the engine over. This only happens at cranking speed and is overridden when the engine starts. Other than that I can only think of one more thing and that is SOME modern carb cleaner are NOT flammable. If you have that environmentally friendly stuff it won't burn. I'm not trying to be funny, just make sure it says flammable on the can. I know someone here will help you figure this out.

No good tip but I am sure my cleaner is flammable. I have read about the compression release and I think that this is way more than that. When I took the head off I took a little sandpaper and went around each valve and cleaned up a little they seem to shut tight. Both valves will close down on a piece of paper and you cant pull it out without tearing. I am leaning toward the low compression.

#7 JoshBrown OFFLINE  

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Posted March 30, 2011 - 10:18 AM

Have you looked closely at the Shear Keyway Through & UNDER the Flywheel ? If it's even just slightly worn the timing can be off enough to not make it fire when it should. and create a lot of other simptoms that can lead you down the wrong path. $$$$$$$$ Changing things that don't need changing.
These Keys sometimes don't actually Shear BUT Twist a bit and look allright from the top side with the flywheel on but get out of wack under neath just enough to throw it offf. I know it sounds simple but it's undoubtedly the "Weakest Link" and Very Important to check . I've experienced this long ago and now it's one of the first things I allways check.....Sometimes .a New .65 Shear Key may save the day!
I may be wrong but it's worth checking out.

This Pic of a badly worn key shows what I'm talking about.

This video is a Good Example of what happens when you have a Bad Key in a GT with a Briggs Engine. It will NEVER Start....Notice the the Blow-By, as you mentioned you have with yours...
Good Luck
YouTube - Briggs 12.5-Bad Flywheel Key

I thought about the timing and the key looks ok but I tried to take off the flywheel anyways and check it but couldn't get it off. When I go to town today I will get a cheap puller and a compression gauge so I can check both.

#8 jhn9840 ONLINE  

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Posted March 30, 2011 - 10:44 AM

JoshBrown,

I am not familiar with your engine but if it has compression realease you will not be able to get a reliable compression reading with a compression gauge. You will need to do a leak down test if that is the case. For this you will need a leak down gage, and access to compressed air.

Amazon.com: OTC 5609 Cylinder Leakage Tester Kit: Automotive

Harbor Freight sells one in the 30-40 dollar range but could not find it on there web site for some reason.

jhn9840
John

#9 JoshBrown OFFLINE  

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Posted March 30, 2011 - 12:11 PM

JoshBrown,

I am not familiar with your engine but if it has compression realease you will not be able to get a reliable compression reading with a compression gauge. You will need to do a leak down test if that is the case. For this you will need a leak down gage, and access to compressed air.

Amazon.com: OTC 5609 Cylinder Leakage Tester Kit: Automotive

Harbor Freight sells one in the 30-40 dollar range but could not find it on there web site for some reason.

jhn9840
John

That stinks I just came back from town with a puller for the flywheel and a compression tester.

#10 JoshBrown OFFLINE  

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Posted March 30, 2011 - 01:32 PM

I pulled the flywheel everything looked good the key and keyways on crank and flywheel looked good. I am really leaning toward thinking that it just has leaking rings. The air is getting pushed past the rings and into the crankcase and out through the breather in the valve cover. I guess I am going to have to head to the scrap yard and try and find one that feels like it has good compression and swap. I could I guess rebuild but I need to fix this one as cheap as possible ( within reason). I was wondering if I could order like a set of oversized rings and put on it or if it is going to have to be machined. I had a small engine class in high school and think I could put them on if that would fix it. What do you think ring set or swap from scrap yard?

#11 NUTNDUN OFFLINE  

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Posted March 30, 2011 - 01:47 PM

I pulled the flywheel everything looked good the key and keyways on crank and flywheel looked good. I am really leaning toward thinking that it just has leaking rings. The air is getting pushed past the rings and into the crankcase and out through the breather in the valve cover. I guess I am going to have to head to the scrap yard and try and find one that feels like it has good compression and swap. I could I guess rebuild but I need to fix this one as cheap as possible ( within reason). I was wondering if I could order like a set of oversized rings and put on it or if it is going to have to be machined. I had a small engine class in high school and think I could put them on if that would fix it. What do you think ring set or swap from scrap yard?


Are you able to measure out of round bore? Can you feel any edges on the cylinder wall? If there is no edges on the cylinder wall you might be able to get a set of standard size rings and hone the cylinder. Just make sure it was never bored before or the standard rings won't fit. I can't see a set of rings costing more then $50. But you might be able to find an engine cheaper then that at the scrap yard.

#12 olcowhand ONLINE  

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Posted March 30, 2011 - 02:21 PM

Pushing past that much air, it may have seized/galled rings.

#13 DanO OFFLINE  

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Posted March 30, 2011 - 03:37 PM

I know most likely you thought of this , and again, like the Key, it sounds simple but I allways try the Less expensive things first before i condem an engine to major repairs....since you can't do a compression test...Another inexpensive thing to try is Replacing the Head Gasket Itself, I know you said you pulled the head and on some of the Briggs engines I've worked on, regardless of how GOOD the Head gasket Looked it was actually BAD, causing loss of compression And really should allways not be replaced once removed....Since it's an item that Should be changed anyway, there's no loss May wan't to just try changing the gasket, usually cost less than $10.......Just another 2 cents I'd thought I'd throw in the pot....

#14 JoshBrown OFFLINE  

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Posted March 30, 2011 - 04:49 PM

Are you able to measure out of round bore? Can you feel any edges on the cylinder wall? If there is no edges on the cylinder wall you might be able to get a set of standard size rings and hone the cylinder. Just make sure it was never bored before or the standard rings won't fit. I can't see a set of rings costing more then $50. But you might be able to find an engine cheaper then that at the scrap yard.


I can just feel a little rough spot about 1/4" inside the cylinder probably where the last ring is on TDC. I think I will head to the scrapyard and look around I think I remember seeing several different size single cylinder briggs. I hope I can find one with some decent compression and swap everything out. I would like to get this one going. My son likes it and said he want's to mow with it this year. It is good to hear that he wants to mow with an "old cute little mower" and not a new one. I will keep you posted as to what I find.

#15 Alc ONLINE  

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Posted March 31, 2011 - 07:55 AM

Josh , this is a long shot but could the exhaust be plugged up so bad that it won't pull air in and just blow back through the carb. ? I've had that on 2 strokes but never a 4 cycle .




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