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Collection, Preservation and Display of Old Lawn Mowers

Suffolk Punch 35 Series 5 - Zenith Type 13 carb problems


I have just acquired a Qualcast Suffolk Punch 14" cut to go alongside my 20" heavy beast.  His & hers lawn-mowers lets call it; although my missus isn't too happy at my latest acquisition.

Anyhow, I gave it a bit of a blow off and service. I did all the sensible things like put a modicum of oil in as it didn't have anything in there, inspected the carb bowl and cleaned some dust out of it - plus totally removed the stale fuel and put in fresh petrol.

The machine has compression and to my amazement fired up. However she runs for 50 seconds and then fritters out. I go away for 3 minutes and the carb is peeing out fuel from the gasket. Mop up the petrol and give it another start, it runs for a minute and then shuts down again

There is a plastic paddle near the starter cord that seems to provide some governance on the engine with a metal bar and spring. When it has run for a bit it moves something on the right of the carb and it is then it falters.

Attached are some pictures, hopefully someone can give me some tuning tips. Hopefully I'm not missing any parts.

Zenith Carb




wristpin Sun, 28/05/2017

Some systematic trouble shooting is required - bit at a time. Starting with the carburettor , the flooding is due to the float needle valve not sealing - dirt or wear - and allowing uncontrolled fuel flow into the carb. The starting point is a thorough clean of the whole fuel system starting with the tank and working through to the carb followed by an inspection of the float and needle.

Your engine has an air vane governor . For that to work correctly the area under the shroud that carries the recoil starter should be clear of all debris, so take it off and have a good clean up. While there, note that the plastic vane (governor vane) is connected to the throttle spindle with a rigid rod, so moving the vane opens and shuts the throttle butterfly valve. At some point along the rod there is a loop to which is attached a spring (the governor spring.) The other end of the spring is attached to a lever that is operated by the throttle cable.

The operation is (should be) that the cable opens the throttle via the spring and rod but once running the air movement generated by the flywheel fins acts on the governor vane and attempts to counteract the pull of the spring and shut the throttle. When the two are in equilibrium that is the governed speed. 

timsp8 Tue, 30/05/2017

Brilliant advice. In my failed attempts I watched the air vane push back the butterfly valve. So that is actuating..

The spring associated with the governor bar does seem a little loose irrespective of what hole you thread it through.

Maybe time for a new spring here to get the balance right? Anyhow I'll take your good advice and start with the carb clean and float testing in a mug of boiling water.

wristpin Tue, 30/05/2017

With the handle bar lever in the slow position and the engine not running the governor spring will be slack.

timsp8 Fri, 16/06/2017

Ultimately, I found a slightly bent pin on the top of the float that I sharpened up again. The flooding has stopped. Thanks for the advice! The Zenith also got a new gasket!

The air vane is correctly operating. But that whole equalibrium thing you say.. I don't think the spring that links at the mid governor bar provides much resistance and when the air vane flaps back, it is a sure tale some seconds after for a stall!

Any advice available on replacing this long spring & sourcing another one?

wristpin Fri, 16/06/2017

Difficult to diagnose over the air but that governor spring looks OK so I wouldn't bother about trying to source a replacement. Unfortunately your version of the Zenith carb has a fixed main jet , otherwise I would suggest richening the mixture on one with an adjustable jet, so can only suggest making absolutely sure that the jet (under the blanking screw) is clean and unobstructed..