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Tractor Talk Discussion Forum

small engines

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joe in Pa

04-25-2005 14:37:31




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could someone tell me how to go about adjusting the carburator on a 3.5 HP lawn mower. a friend gave me a mower that he said had worked, then his kid was screwing with it. the carb has 2 screws for adjustment. one is larger then the other. now both are turned in all the way. if I spray a little gas in the air filter it starts right up but then shuts right off.




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Bradley

04-26-2005 06:30:31




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 Re: small engines in reply to joe in Pa, 04-25-2005 14:37:31  
check the float also take off the bowl underneath and make sure is in good shape no cracks or nothin then put it back on



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msb

04-25-2005 14:42:06




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 Re: small engines in reply to joe in Pa, 04-25-2005 14:37:31  
Try setting the small jet at 3/4 turn open and the large jet at 1 1/2 turn open.That should let you get it started.Adjust the large jet at wide open throttle splitting the difference between too lean and too rich.Then set the small jet at slow idle the same way.Then repeat the adjustments to fine tune the settings.



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jdemaris

04-25-2005 19:57:34




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 Re: small engines in reply to msb, 04-25-2005 14:42:06  
Don't know if he's got a Walbro, Tecumseh carb, Briggs, Carter, or whatever - but just want to add one thing to what you said. The low speed idle mixture screw also serves as the "accelerator mixture" screw - same purpose as an accelerator pump on an automotive carburetor. So, it should be set initially to let the engine idle at it's best speed when warm (not hot). But then, you try reving the engine up - from low idle to high speed - just like stomping on the gas pedal of a car. If it hesitates and leans out, it has to be opened a bit. So, it's actually to be set with a compromise between best idle and best acceleration. Regardless, it should still wind up 3/4 - 1 1/2 turns as you mentioned. Also - sounds silly but I've seen it happen several times. If the carb is an older Walbro - you can't adjust the idle-mixture screw with the carb. upside down. What happens is - sometimes the carb. gets taken off to clean - and then gets held upside down during assembly. With the Walbro, there is a floating idle jet tube inside the carb idle circuit - and if you screw in the idle screw while upside down - it will smash the tube and ruin the carb.

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