Carb testing

[Old2stroke]

So you have taken a carb apart, carefully cleaned it, checked the control lever setting and put it back together with a correctly installed new kit, why not do some simple tests to make sure its basic functions are working before sticking it back in the saw? Many with a lot of carb experience will say this is a waste of time "rebuild it right and it will run right". Anyway, if you like testing.
The obvious test is to pressurize the fuel inlet port with about 8psi to make sure the inlet valve is seating and there shouldn't be any flooding issues. Next, if the carb has a port to connect a primer bulb, plug it (piece of tubing with a nail in the open end), wedge the throttle open and with pressure still on the inlet port, bring a shop vac nozzle up to the throttle end of the carb and verify that the pressure drops, this indicates that the main diaphragm is responding to intake vacuum like it should (never do this if there is fuel in the carb). If the carb has a separate primer bulb, there will be check valves in the carb (probably 2) and they can be checked by plugging the fuel inlet port and and sucking and blowing on the primer port. When sucking, the port should hold a hard vacuum and when blowing, you should be able to blow into the carb with small resistance. Bear in mind, check valves that pass a static test can still act funny in a running saw and if the carb only has a main nozzle check valve (no primer), it is difficult to test that check valve and also, there is no easy way to test the fuel delivery of the fuel pump section.
Set the throttle stop screw so that the throttle plate isn't completely closed, set the L and H screws 1 and 1/2 turns out and it should provide a good starting point to do the tuning once the engine is running.

 

[heimannm]

If the metering needle bleeds off slowly, try putting a little fuel in the line as "wetting" the needle & seat will often make a good seal and verify that the needle is in fact is seating. On many carburetors you can activate the metering diaphragm through the vent hole to unseat the metering needle and verify it seats again.

Depending on the design of the carburetor it may be possible to check the high speed check valve using the "straw" from a can of WD40, carburetor cleaner, etc. Remove the H jet screw completely and do your best to seal the straw where the H needle seats and alternately apply pressure and vacuum (blow and suck). If the check valve is working the carburetor should pass pressure and hold a vacuum.

Mark

 

[Old2stroke]

On many carburetors you can activate the metering diaphragm through the vent hole to unseat the metering needle and verify it seats again.

This is a good feature and on many carbs where the vent hole is in the wrong place, I usually drill another one in the centre of the cover plate to enable this.