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Briggs powered Sears.
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Discussion Starter · #1 ·
2 or so weeks ago my plow tractor busted the transaxle no reverse. then i got a 6 speed from my boss. then i got sick for 2 days then this last week i got to feeling well enough to go outside and do some work in it opened the 6 speed up regreased it and mixed some gear oil with the grease for extra lubrication. slapped the transaxle back together fixed a tire. so tuesday i went for a test run and the head on the 14.5 hp briggs blew the valve guides. oil everywhere out the exhaust found issue after i took the head off intake valve was covered in oil. so friday after i got paid went to scrap yard and found a 17.5 hp ohv briggs with broken rod. so i got my head for the 14.5hp. today i put it together with a new head gasket. now the engine doesnt respond to lesser throttle lol.it will run either wot or 3/4 even though the throttle position is all the way down. oh well atleast it runs. sounds really good too but i found out it sucks the gas more than before lol.
 

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SEARS TRACTOR COLLECTAH
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1,386 Posts
Probably the differant head dropped your compression ratio. That would make the engine want to run at full speed.
 

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Briggs powered Sears.
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2,844 Posts
Discussion Starter · #3 ·
thing is heads looked the same. the valves are same size only difference i seen was the fins stuck out further (had to cut the sheat metal a bit to make it fit). i also had to change carberator cuase the one i was using had needle seat issues and i didnt have money to put in new needle and seat so i stuck a carb on that was off a 12.5 hp. doesnt seem to be down on power or anything. just wont idle.
 

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SEARS TRACTOR COLLECTAH
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1,386 Posts
Are the 12.5hp & 14hp carbs the same? That 12hp carb may be getting overwhelmed by the bigger engine. I just checked the specs, The stroke on the 14hp & 17hp engines are the same, However the bore is larger on the 17hp, That would cause a compression decrease. - Brad
 

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Briggs powered Sears.
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2,844 Posts
Discussion Starter · #5 ·
oh well lol. i bought the 14.5 from same scrap yard i got my moneys worth out of it in work if it blows it blows. i have other engines for it :)
 

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Are the 12.5hp & 14hp carbs the same? That 12hp carb may be getting overwhelmed by the bigger engine. I just checked the specs, The stroke on the 14hp & 17hp engines are the same, However the bore is larger on the 17hp, That would cause a compression decrease. - Brad
By itself, bore doesn't affect compression ratio (CR).

For visualization and talking purposes, I'll assume we are talking about vertical cylinders with the crankshaft below (you could, of course, have horizontal cylinders, for example, and the principles are the same, but the directions are different).

CR is the ratio of the cylinder volume above the piston when the piston is at the bottom of it's stroke to the volume when the piston is at the top of the stroke. CR is a function of the stroke and the crankshaft centerline position relative to the cylinder height (assuming the cylinders are vertical).

If you had an engine that had a different stroke because the crank offsets from crankshaft centerline were different, but had the same crank centerline position, that would certainly result in a different compression ratio--longer strokes would produce higher CR. If you had the same crank offsets (meaning the same stroke), but moved the centerline up or down relative to the cylinder, that would also change the CR (up would increase CR, down would reduce it).

You could also change CR by changing the cylinder head to include more or less volume above the top of the cylinder. More volume would lower CR, less volume would raise it. One way to increase volume is to use a thicker head gasket.

But, everything else being equal, the bore wouldn't affect CR at all.
 

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SEARS TRACTOR COLLECTAH
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1,386 Posts
Jerry, I fully understand about what was said above. I'm not trying to start anything here, But if he put a cylinder head from a larger engine that had a larger bore, The combustion chamber in the cyl head on the larger engine has been casted or machined to acommodate the lager bore. When it was installed on the smaller engine, The piston wouldn't be able to fill the cylinder with as much volume as before, Because of the larger head. Thus decreasing volumetric effeciency and compression. Its like putting a chevy 350 cyl head on a smaller bore 305, The compression ratio will drop significantly. - Brad
 

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Jerry, I fully understand about what was said above. I'm not trying to start anything here, But if he put a cylinder head from a larger engine that had a larger bore, The combustion chamber in the cyl head on the larger engine has been casted or machined to acommodate the lager bore. When it was installed on the smaller engine, The piston wouldn't be able to fill the cylinder with as much volume as before, Because of the larger head. Thus decreasing volumetric effeciency and compression. Its like putting a chevy 350 cyl head on a smaller bore 305, The compression ratio will drop significantly. - Brad
BRAD,

I agree with that 100%. I see I missed the comment about replacing just the cylinder head with one from a larger bore engine. Of course, that's the same effect as a thicker head gasket: The same amount of extra volume at piston up and piston down, making the ratio of the two volumes smaller.

What I read:00000060: :00000060:into the previous posts was replacing the engine with one having a larger bore, not just the head.
 

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Cranky Motorsports
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15,199 Posts
Sounds like you have been having "fun"
 
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