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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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I hate to be a smart ass, but you need to run the correct sized jets for your mods - it's up to the builder to re-calibrate the carb.
Generally, the pilot jet does not need to be changed and if it is, usually never more than one size. If the engine idles smoothly without loading up and dying and the fuel screw is set between 1 and 4 turns out from lightly seated, it's good. If you have to turn the fuel screw more than 4 turns out, then increase the pilot jet one size (go to a 40), if you have to turn the fuel screw more than 4 turns with a 40, then go to a 42. I have never seen any engine modifications using a stock carb that requires a pilot jet larger than #42.
With the changes to the intake you describe, I would suggest starting with a #160 main jet and adjust either up or down for proper full throttle operation. Too big it will blubber like the choke is on - too lean it will surge or spark knock. You should be using 93 or better octane fuel, but probably no higher than 97 and the fuel needs to be fresh. I recommend VP Racing Small Engine Fuel (SEF) - it's 94 octane, no lead and no alcohol. It's $20 a gallon in my neck of the woods. I can buy from NAPA and Big R farm supply or direct from VP Racing. Buying direct cost me a $50 hazardous materials shipping charge or I can drive over to Terre Haute, Indiana to pick it up and avoid the hazmat charge. Your nearest distribution center is in Newark, Delaware.
NOTE: your jetting requirement will change with altitude or fuel composition or fuel quality also.
Last edited by Here 2 help; 08-10-2018 at 02:57 PM.