Small Engines (Lawn Mowers, etc.): McCulloch 28cc, weed wacker, brass tubes

Question
Eric,

I acquired what appears to be a new engine - a half finished conversion to model aircraft spec. McCulloch 28cc engine with a Walbro WT231A Carb.  I converted it from flyheel ignition to electronic, set at 28 degrees BTDC, as the flywheel was lightened to excess and I was worried about its integrity.

I'm unfamiliar with the carb - there are 2 brass tubes - I assume the larger one is the fuel inlet (near the diaphragm cover with the screw) and the smaller one seems to be a return? I have simply left this open...

The only way I can get it to fire (very briefly) is by injecting fuel into the cylinder and replacing the plug.  Cranking the engine gives 100 PSI but it doesn't seem to be pulling any fuel in - even squirting "easy-start" into the carb has little effect, seeming to tend to blow it away rather than suck it in!

The carb seems to be inverted from the position shown on an exploded diagram I've found of a weed wacker using this engine/carb but the pressure tapping feed has a semi-circular tract connecting both holes to the diaphragms so I don't think it would make any difference.


Any ideas?

Thanks in advance,

Martin

Answer
The hole you mention is the fuel pump pulse. The engine creates pulses and these pulses operate the fuel pump.

Below are links to service manuals:

http://www.walbro.com/techtips.aspx

http://wem.walbro.com/distributors/servicemanuals/WTseries.pdf

http://wem.walbro.com/distributors/servicemanuals/ServiceManual.pdf

Make sure the carburetor insulator gaskets allow the engine pulse to enter the carburetor fuel pump pulse hole on the carburetor.  Do you have the fuel line connected to the correct brass tube on the carburetor?

Eric