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crockhamtown Sun, 28/08/2016

The mowers I've had and my present modern one have either had a button under the carburetor or a bulb to charge it up with petrol. The Briggs & Stratton 3.5HP Hayterette I've just inherited has neither.

Does anyone know why this is and do I have to do anything to charge it up or is it unnecessary?

crockhamtown

 

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wristpin Sun, 28/08/2016

The chances are that it has an automatic choke operated by a diaphragm and spring sandwiched between  the carburettor and fuel tank.

With the engine not running , confirm this by removing the air filter and you should see the choke butterfly completely obstructing the carburettor venturi. If it is  at an angle and only partly closing the venturi it is out of adjustment and starting may be difficult. 

The other option ( but unlikely on a BS 3.5 ) is that it has a mechanically operated choke operated by moving the throttle lever past full speed . Again, with the air cleaner removed,  see if moving the throttle lever to past full speed and back to stop moves the choke butterfly.

crockhamtown Tue, 30/08/2016

Thanks wristspin.

I think your first idea is correct.  

1. When viewing the carb after taking the filter off and without the engine running, the butterfly completely obstructs the venturi. When I operate the throttle fully off to fully on, there is still no butterfly movement. 

The throttle seems to have natural resting places at 1/3rd and 2/3rds of the way down which I take to be the idling and choke positions. When starting the engine on choke position the butterfly opens and stays open and when I push the throttle all the way down the engine goes faster. The butterfly remains open when I pull back to idle and closes when pulled all the way back to off.

Further points:

2. Whatever the engine speed, the revs continuously speed up and slow down.

3. It seems good practice to start with a full tank of petrol and keep the petrol top air holes clear.

 

wristpin Tue, 30/08/2016

From your description it sounds as though the automatic choke is working correctly.

The fluctuation in speed that you describe is known as hunting. It can be caused by a governor fault but more likely by a weak fuel mixture causing the governor to react to try to achieve the governed speed while the engine is being starved of fuel. If the machine has stood for some time gum may have formed in some of the small passages in the carburettor restricting the passage of fuel.

Your machine is of an age where , unlike modern emission compliant engines, the fuel mixture may be set   with  the adjustable main jet - a slot head screw protruding from the side of the carburettor body. With the engine thoroughly warmed up, try turning that screw anti-clockwise, an eighth of a turn at a time to see if it will smooth out the revs. If  that fails it may be necessary to strip and clean the carburettor.