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Ok, it depends. no showboating, just facts. I have to sort of a phsycic here but here goes. Remove the carb bowl, disassemble the bowl, Main jet adjustment screw, and the metering stem. The metering stem is located in the center of the bowl. It is brass, use a 3/8 deep well socket and a flat tip screwdriver to remove the stem. Next, you need to drill out the bottom 2 holes in the stem. The size of the drill bit is .110. for the stem. Make sure you get out all the shavings from the stem. Next you need to drill out the main jet. use a slightly larger drill bit than the one you used fore the stem, .125 will do sufficently. BE CAREFULL when you drill the main jet, it is a shallow hole and you could break off the bit by hitting the side of the metering stem hole. That will give a stock SM plenty of fuel. Dependig on the compression pressure, A SM-450 venturi will work just fine, It somewhere around 7/8 to 15/16 when you measure the bore. When you bigger than that, with a stock engine, it will make good power, but it loses peak power when you lug it past the 540 pto rpm. Unless you are running 170psi or more, then you need a bigger venturi, to properly get the air fuel mixture correct, Look at the plugs, if you do not have access to the dyno. You will have to be able to work the engine pretty hard, then immediatly shut it down and check the plug color, it should be a coco brown color. Aftre reassembling the carb, You are able to set the fuel setting, the same way the owners manual expains it. OR, open up the throttle all the way, and back out the main jet screw. It should smoke, or create a rich condition, just turn the screw in until it smooths out, but, is not too lean. That is pretty much it for carbs, Then you need to look at the governor spring, that is the only part in the governor that actually changes the throttle responce, You need to be able to go past the recommended rpm rule you club allows and then back the rpms down, instead of using a stock spring, which does not have much spring tension in the first place, You could go to the hardware and get a stiffer spring. the spring needs to be 3/4dia. by 2-7/16 long, and .105 wire thickness. that is the exact measurements of the M-450 governor springs I offer for sale. Ignition timing.....
anywhere from 24-28degress advance, where ever the engine seems to make the most power, stock settings do not work. In most cases you advance the distributor while the engine is running, until it labors or drops about 100 rpm, then back it off until you split the difference or, goes up 50 rpm. It does not take much, but it is worth about 2-4 more hp more hp over stock specs. what I just explained, is the procedure I use when I dyno tune an engine. some results may vary due to compression pressure, fuel quality, and possibly worn parts. Any questions??? Feel free to ask me. Chads
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