Will a JD G carb work on a A

47 & later A on gas 38 HP @ 975 RPM & 321 cu in.Best modified HP @ 975 RPM = 52 HP & 91 cu ft/min airflow. 60 on gas 42 HP @ 975 RPM & 321cu in. 620/630 on gas 49HP @ 1125 & 302 cu in

47 & later G on dist 38HP @ 975 RPM & 413 cu in. Best modified HP @ 975 RPM = 67 HP & 105 cu ft/min airflow. 70G 50 HP @ 975 RPM & 380 cu in 720/730G 59HP @ 1125 RPM & 360 cu in. So what do the numbers say? The B series engines were built to the hilt at the factory & there isn't large/easy power/airflow increases left. The A series engines have some extra airflow potential avalable. They made as much power as a G with 78% of the displacement on fewer gallons of fuel per hour. Stock the A's flowed at least 85% of the amount of air a G did per revolution. At 975 RPM & with internal airflow modifications they can handle & need a carb rated at 100 CFM @ 1.5 inches hg drop. A big nut G carb PROPERLY calibrated works well. Even a stock A with the RPM's at 800 & above will make more power with the big nut G carb. Lugged to 600RPM or less the stock "A" or common "G" sized carb on the "A" engine will usually make more torque. But the idea when pulling is not to get lugged that low,but if lugging happens the smaller carb will keep going & the big nut carb may puddle fuel & stall at the end of the track. The G's were way underbuilt with a restrictive extremely high velocity/turbulance/high temp intake system & low compression to burn low grade fuels. Lots of power avalable if the intake system,ports & exhaust are freed up & compression added. Look at the HP jump with the later versions. A stock 520 will will walk all over a stock G due to airflow/combustion efficiency. An over rich mixture will allow a very slow/ smooth idle as there is lots of fuel vapour avalable to burn with that extra fuel.Or that big nut G carb on the "A" may also have a well set/tuned idle circuit to assist fuel metering/vapourization. Just like setting an oxy/acetylene torch. The lean mixture is harder to light. However lean burns clean,hot & efficient, not red,cool & sooty. Many operators with chronic fouled plugs & high ring wear would have fewer problems if the coolant was at 180F to 200F and the mixtures were leaned to just above miss fire.

With a 413 CID 4-stroke engine, the cylinder will suck through the carb every other revolution. So the cylinders fill at half the 975 RPM rate, or 487.5 times a minute. A cubic foot has 1728 cubic inches. 487.5 fills per minute x 413 CI = 201,337.5 cubic inches per minute. Divide this by 1728 to convert it to cubic feet, and you get 116 CFM. Valve timing, venturi pressure drop, and other factors don't allow for an absolute cylinder filling at standard barametric pressure, so 100 CFM is a pretty decent rate. Hope this helps some. Cam