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 Gas Engine Carb 101 
Gasser carbs are simple and very effective when put together CORRECTLY.. I found two this week that were "fixed" by the owner, who then had to send the engine in because didn't work... There are two covers on the carb, one cast and one stamped..The cast cover is on the side with the pump diaphragm 2 small flappers that work from the pulse from the crankcase...The pulse comes into the carb through a small hole in the cylinder, through the gasket, through the insulator block, through another gasket, and into the small hole in the carb casting..ANY interruption in this series of holes will cause the pulse to be lost and the pump will not work.. The other cover is stamped steel and has 1 or more holes in it..The purpose of the hole/s it to let atmospheric pressure operate the other diaphragm and let fuel into the carb through the inlet needle..The fuel is in the cavity under the diaphragm and is admitted to the venturi of the carb by suction across the holes in the venturi...There is one hole for the high speed needle and usually three for the low speed needle..The low speed needle CAN be opened up far enough to run the engine with the high speed needle almost closed, but will be WAY too rich for good idle and transition...If you set both needles 2 turns open first, then set the high speed needle, you will be assured that the low speed needle is not controlling the high speed mixture.. There is a small lever that operates the rubber tipped inlet needle under the inlet diaphragm..The lever is NOT an adjustment for richer or leaner mixture, its only function is to control the admission of fuel..The normal setting for this lever is flush to the casting next to it.. The diaphragms usually last a long time, but with old age get stiff..If you can hold the inlet diaphragm at the edges and it crinkles at all in the middle, it's too old and should be replaced..A carb rebuild kit costs about $5.00.. The two I found this week that were "fixed' had the inlet diaphragm UNDER the gasket, keeping the inlet needle open all the time..Can't work that way...The gasket under the stamped cover goes OVER the diaphragm...The gasket on the other side goes UNDER the diaphragm... There's a fine screen under the cast cover that keeps debris from clogging the inlet needle hole..It's a major cause of carb problems...It's easy to clean, just blow it out and replace it... Last, carb needles are there because NO manufacturer can predict how YOUR engine will be set up...They are made to be ADJUSTED when you start up your engine.. If you don't take the few minutes it requires to get the needles set right you'll never get the best performance from the engine... Updated October 1, 2004 |
