[toastedcrumpet]

Can anyone help me with a running problem with my Suffolk punch lawnmower. It only runs with the choke on and doesn't cough or splutter even when cutting the grass still on full choke. As soon as I try and alter the lever to the normal running position it falters and stops. I have had the carb off a couple of times and thoroughly cleaned it, fitted new gaskets and am now out of ideas for what else to try.

[smud6ie]

As you have probably guessed your engine is running with too weak a mixture which means a jet is blocked , there is an air leak or possibly  poor compression.
An air leak can be from the inlet manifold or a poorly seating valve.
If you are certain you have cleaned all jets,emulsion tubes etc then the manifold flange may be distorted.
First try squirting some oil around the joint  to see if the engine puffs blue smoke
If you lay a sheet of fine wet and dry paper on a piece of glass and wipe the flange across it you will see if there is a hollow in it ,if there is,continue to wipe it until a flat even finish.
smud6ie

[R Tallentire]

I’m not all that well up on Suffolk models but if it has the Qualcast A114 engine with aluminium block and a Del-Orto carburettor, this is a well known problem when the machine has been laid up over winter. Qualcast tried to fix it with a different plug and finally fitted a different carburettor but if you have a Del-Orto you are stuck with it. The jets fir up and blowing them out doesn’t work, but clean the carb out anyway just in case the problem is dirt. Happily in mower applications the engine will start to run on half choke after an hour or so and will start to run without choke after about another hour. Rotovator owners were not so lucky as most had finished the plot after two hours so never got off choke.

[Gwiz]

The problem was so common place that there was an "After-market" adjustable jet kit made for the engine by pattern parts companies. You maybe lucky to find one on ebay. It cures the problem and allows the engine to be tuned.
I wish I had a fiver for every one of the kits I fitted in the past.