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NoBozo · 03-07-2012, 03:37 PM

Forget the compression test for a moment. With old fashioned carburetters, the engine is cranked over to start the engine. The pistons go up and down and the valves open and close. When the piston goes down..air and fuel is "Sucked" into the cylinder through the intake valve at the correct time....each cylinder in turn. The air/fuel mixture is ignited at the correct time and you're off and running.

Back to the compression test: ALL the spark plugs have been removed. The only spark plug hole that is "covered" is the one with the compresssion guage in it. Turn the engine over with the starter (coil disconnected).

Here's the Falacy about inadvertantly flooding the cylinders with fuel: ALL the other cylinders are OPEN to atmosphere via wide OPEN spark plug holes. The piston going down will be sucking air IN through the ALL the open holes...therefore there will be no..or very little "Suction" at the carburetter throat capable of introducing fuel into the engine via the carburetter jets.

NB

PS: Fuel Injection "may" be another story..depending on the Type of fuel injection system AND it's computer..??..controls..

BTW: WOT is used more to defeat any "suction" capability AT the Carb venturi, than as a method to supply more air for the test. One cylinder doesn't need much air.

03-07-2012, 03:37 PM	#17
NoBozo Senior Member Join Date: Jun 2008 Location: Portsmouth. RI Posts: 2,231 Thanks: 400 Thanked 460 Times in 308 Posts	Try To Picture This Forget the compression test for a moment. With old fashioned carburetters, the engine is cranked over to start the engine. The pistons go up and down and the valves open and close. When the piston goes down..air and fuel is "Sucked" into the cylinder through the intake valve at the correct time....each cylinder in turn. The air/fuel mixture is ignited at the correct time and you're off and running. Back to the compression test: ALL the spark plugs have been removed. The only spark plug hole that is "covered" is the one with the compresssion guage in it. Turn the engine over with the starter (coil disconnected). Here's the Falacy about inadvertantly flooding the cylinders with fuel: ALL the other cylinders are OPEN to atmosphere via wide OPEN spark plug holes. The piston going down will be sucking air IN through the ALL the open holes...therefore there will be no..or very little "Suction" at the carburetter throat capable of introducing fuel into the engine via the carburetter jets. NB PS: Fuel Injection "may" be another story..depending on the Type of fuel injection system AND it's computer..??..controls.. BTW: WOT is used more to defeat any "suction" capability AT the Carb venturi, than as a method to supply more air for the test. One cylinder doesn't need much air.