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I recently rebuilt my SU carburetors and went with SM needles-i have a stock 240 engine (71) with headers and the plugs are black after less than 1000 miles-1 of them actually quit firing which made me pull them in the first place-anybody successfully running SM needles on a stock 240 engine? I've got Scott's video, and the SU's are tuned properly-as a matter of fact the car runs great until the plugs get fouled up from the rich mixture...just wondering if perhaps the SM needles dump too much fuel into a stock engine so that's why i wanted to know if anyone has a stock 240 with SM needles without mixture problems. Thanks for any help you can provide.
 

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its possible

Its possible that they dont work well with your motor. I have rebuilt SUs from Ztherapy with the SM needles, and my plugs are nice and tan.

My motor is not stock though.


They say the SMs start out lean, then get really rich, that may be your problem.

- T
 

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I have installed SM needles in several stock 240's with no problems. I'd bet your mixture is wrong. Either that or your carbs have leaky throttle shafts and they're fooling into thinking that they are adjusted properly. Have you checked everything for vacuum leaks? Are you sure that your jet nozzles are not worn or defective and thus allowing too much fuel? Another thing to double check is to insure the needles are installed at the right height with the "shoulder" right at the bottom of the piston. I don't like Scott's method in his video because if the jet nozzle isn't all the way up you'll wind up with the needle being installed too low. If you have your needle installed at the wrong height, you will definetly be running too rich/lean depending on which direction they're off.

One other thing to consider... Is your ignition system working properly? It's possible you're not producing a hot enough spark.
 

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try Vpower NGK plugs.4291.. they helped me.. and to set the needle.. unscrew it so it slides up and down in the piston.. then screw the mix nut all the way in(lean) then slowly lower the piston into the carb body.. the needle will push back into the piston until it no longer contacts the nozzle.. thats where you want your needle.. for stock applications and a little more.. think about it .. when the carb is all the way lean there shouldnt be any gas coming in..simple.. then unscrew mix nut 2-2-1/2 turns.. usually it ends up so that the shoulder of the needle is even with the raised part of the piston.. not the inner groove.. look through the carb throat if you can while you do this so you can make sure the pistons sliding and see where it stops.. you dont want it to move after that.. email if you have trouble
 
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