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Carburetion, compression and ignition all make an internal spark combustion engine run. If one of these lag behind, power will be decreased dramatically. When checking for loss of power, make sure to check the following things:
ü Carburetion is when an adequate amount of air and fuel enters an engine smoothly. A carburetor with the venturi removed (straight-through throttle bore) will help an engine with a larger (oversize) intake valve to produce more power. But a bored out carburetor may not work so well on an engine with a smaller, stock size intake valve because there's simply not enough engine vacuum (or high velocity of incoming air) upon acceleration to draw the fuel from the main nozzle.
ü Check the ignition timing. Chances are, if an engine is losing power, the timing isn't advanced enough. Check for a worn points lobe on the camshaft, too.
ü Compression is when the air/fuel mixture is adequately pressurized on the compression stroke. The secret to gaining more horsepower and torque is increase the compression ratio and combustion chamber pressure. Popping the piston out of the cylinder approximately .020" (NOTE: due to various manufacturing tolerances with OEM parts, not all pistons come flush with the top of the block at TDC. Therefore, the piston may not pop out of the cylinder a full .020" after offsetting the bearing in the rod) and milling the [stock] cylinder head .050" will increase the power output dramatically.
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Discussion Starter #2
Another way to gain more power

Another way to gain more power from the high rpm is to install a special camshaft along with larger diameter valves, stiffer valve springs and porting/polishing the intake and exhaust runners.
 

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Discussion Starter #3
How To Get Maximum Horsepower and Torque from a "Basically" Stock Engine:

Approximately 48% more horsepower and torque can be produced from a basically "stock" single cylinder flathead Kohler engine. This means that approximately 15hp can be produced from a 10hp, 17hp from a 12hp, 20hp from a 14hp and 23hp from a 16hp governed engine at 4,000 rpm! And even more power can be produced above 4,000 rpm! (Be sure to install a billet steel flywheel and connecting rod scatter shields whenever running an engine above 4,000 rpm though.) The compression ratio must be increased in order to increase the power output.

Add 1 point to the compression ratio when popping the piston out of the cylinder .020" (NOTE: due to various manufacturing tolerances with OEM parts, not all pistons come flush with the top of the block at TDC. Therefore, the piston may not pop out of the cylinder a full .020" after offsetting the bearing in the rod) plus milling of the stock (OEM) head .050". After the modifications have been made, the increase in power will definitely be noticeable. Rather if it's used for pulling competitively or just for yard work, a "built to the max" stock engine will perform much better with this modifications

1)First, completely disassemble the engine and then...
• Being the connecting rod will need to be fitted with a bearing insert, you can kill two birds with one stone by having the rod bored .020" offset (toward the dipper). This will pop the piston out of the cylinder approximately .020" (NOTE: due to various manufacturing tolerances with OEM parts, not all pistons come flush with the top of the block at TDC. Therefore, the piston may not pop out of the cylinder a full .020" after offsetting the bearing) and the rod will have a protective bearing. It'll cost a lot less to have it done this way, too.

2)Only on the 10hp through 16hp flathead Kohler engines, mill the stock cylinder head .050" to increase the compression. Remove just the "ridge" that mates with the head gasket. You'll be removing about .050". This is when approximately .050" of metal is removed from the raised gasket mating surface. To guarantee that the head is absolutely parallel, this must be done on a milling machine with a flycutter and not on a sanding disc. (Use a sanding disc only if the head is warped.) Milling of the head will increase the compression ratio about 3/4 of a point, and depending on engine size, resulting in about 1-3 more horsepower. Reworking the valves so they'll clear the head may also need to be done. In most cases, there should be adequate clearance once the head is shaved. Always check the clearance when the valves are at full lift before "shaving" a head! NOTE: Do not mill the head to increase the compression on the 7hp and 8hp Kohler engines. Because on these heads, the clearance is already very close between the valves and head. You can have the head resurfaced on a sanding disc if it's warped though.

3)If your engine doesn't smoke (blue smoke) and burn oil, then it's okay to reuse the same piston and rings. Because there's probably not that much wear on them. When reinstalling a used piston/rings though, oil the rings and install the piston with the ring gaps set 180º apart. But if it does smoke and use oil, it's best to have the cylinder measured for wear and perhaps bored oversize, or if it's within tolerance, just install a new piston and rings assembly. And you're going to do just yard work with your tractor, you can just use an OEM piston assembly. But if you're going to pull competitively, it'll be better to use a Mahle piston. They're made for the newer Kohler Magnum engines. They're manufactured in Germany and made of tougher material than the older Kohler pistons. They hold up to heat better and wear longer, too. And they weigh the same as a Kohler piston assembly. Mahle pistons have the same compression height as the K-series pistons. Mahle pistons just last longer under severe conditions, that's the only difference. I don't have any part numbers for Mahle pistons. You'll have to check with your local Kohler dealer for part numbers. Tell the counter person you need it for a Magnum engine.


4)Due to the increase in the compression ratio, which places more strain on the connecting rod, you'll definitely need a stronger rod, but only if your (12hp or 14hp) engine doesn't already have one. To tell if your engine has a stronger 16hp rod, it'll be of one solid color. The "two color" rods (light gray at the wrist pin and dark gray at the crank pin) are too weak for this purpose. The 16hp rod is much stronger than most 12 or 14hp rods. And the 18hp OHV engine Alcoa connecting rod is even stronger yet. And as far I've seen, most of the 14hp and 16hp rods are the same. Although some 14hp's (and 12hp's) came from the factory with the weaker "two color" rod. But some 12hp's also came from the factory with the stronger 16hp rod. And ALL 18hp rods are made of forged aluminum alloy. They're the strongest of any Kohler rods. If you're going to run your engine no faster than 4,000 rpm, a 16hp rod should suit your engine just fine. By the way, the new 16hp rod is designed for the new-style 16hp piston, but it'll work with any 12hp or 14hp piston with no problems. Also, all 16hp rods come with a long oil dipper. So if you're using a flat bottom oil pan, you'll need to cut half of the dipper off with a hacksaw and smooth up the rough edges with a flat file. Be sure to sharpen the edge of the dipper with the file so it'll slice through the oil with less resistance. It'll still provide plenty of lubrication, too. (A "blunt" dipper has a tendency to rob horsepower.) And it's safe to reuse a stock rod in a 10hp engine. They're strong rods. And being the engine is going to be operated at a maximum of 4,000 rpm, there's no need for a rod bearing insert

5)Port and polish the intake and exhaust ports. Grind (and smooth) the intake port so it's the same diameter as the carburetor throttle bore. Just smooth up the exhaust port and remove any rough casting slag.

6)Perform a quality valve job. Undercut and swirl-polish the intake valve head and grind a 30º angle on the face of the intake valve and seat only so more air/fuel will enter the combustion chamber. And for the exhaust to exit the combustion chamber quicker, undercut and grind a 45º angle on the exhaust valve head

7)For up to 4,000 rpm, you can either re-use the stock OEM valve springs, or to reduce "valve chatter" and to insure proper valve action, install new OEM springs. And it's safe to re-use the OEM retainers and keepers. To reduce valve float and loss of engine power above 4,000 rpm, use the "lightweight" valve springs
 
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