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Collection, Preservation and Display of Old Lawn Mowers

Zenith Type-13 -- Main Running Screw

I posted a thread a few months ago about my new Atco B14. The blades have been sharpened and ithis along with the terrific weight in the rollers (compared to modern roller mowers) means it leaves a beautiful cut. I am happy with that.

Unfortunately I've struggled to get it to idle very nicely and I'm not sure the advice I've been given on the carb set up is right.

Currently the main adjustment jet is turned out 3/4 of a turn and the slow running jet 1 whole turn.I can easily start the mower with the choke closed which I then need to keep half open within a few seconds of starting. After this point, fulling opening the choke results in the engine dying as does keeping it fully closed.

My first thoughts were that more fuel needs to be introduced into the mixture but opening up the main screw further results in what I believe is too rich a mix (engine splutters on acceleration, exhaust smokes and engine dies).

Does anybody have a picture of the main running screw/needle? I'm exploring the possibility that mine may have been damaged. The end is flat with a surface diameter of around 1-2mm -- I was expecting a sharp pointed tip but of course may be wrong with no reference picture.


wristpin Thu, 11/07/2019

From your description the screw is ok.

if you can get the engine running on half or three quarter throttle . Screw the adjuster in, bit at a time until the engine starts to go weak and note the position. Then unscrew it , again in small increments, until it starts to “ fluff” / run heavy, and note the position - then wind it in to the mid point between weak and heavy. It’s important to move tha screw in small increments and let the engine settle each time.

Once you’ve achieved satisfactory “ fast running” then do a similar process on the slow running adjuster , starting with three quarters of a turn undone and making very small adjustments. If you are unable to achieve satisfactory slow running , check for air leaks and work right through the slow running system from the tube in the carb bowl up into the mixing chamber below the adjusting screw and finally the tiny hole from the mixing chamber into the back of the manifold.







sawleyman Thu, 11/07/2019

Thank you very much for your detailed response. I hesitated before posting this as I know the carb has been covered extensively on this site, but I struggled to find a picture of the screw as well as an agreed setting for the screws.

Good to know the screw is probably okay. I have been careful to feel for contact when tightening it.

Now you mention the throttle, I have been opening it only very slightly before starting. Would having the throttle open be detrimental to achieving correct fuel/air mixture settings? I remember when getting the mower it would start without throttle even touched.

Thank you for the pointers on air leaks and checks. The gaskets are flat and clean and all screws secure, but it may be worth another strip and clean. 

wristpin Thu, 11/07/2019

For a cold start, full choke and half throttle . If the main jet is set right you should be able to take it off choke almost immediately.. Let it warm up and then tweak the main jet to the mid point between weak and rich. I do that at nearly full throttle.

Better pic of the screw taken with a proper camera!

sawleyman Thu, 11/07/2019

Understood -- thank you. So up until now I had only set the throttle at minimum while making adjustments, but will follow your advice. I have stripped the carb twice now and it was very clean inside and no visible debris. As well as this the fuel from the tank is free-flowing and consistent, the plug is new and air filter sponge clean.

I'll get something put onto YouTube for anybody interested when I've managed to achieve a nice idle.

sawleyman Thu, 11/07/2019

Thanks too for the macro shot of the screw -- very consistent with my own so that alleviates the concerns over the condition of mine.

sawleyman Fri, 12/07/2019

Here is a very short video of me starting the mower, idling it briefly and also engaging the cylinder. I had to be quick as it had started raining...

The idle is alright for now, certainly better than it was although I'm not happy with the loud noise the cylinder makes as it rotates -- I've oiled the mechanisms and ensured the two blades just make contact. Does anybody have a spare minute to have a listen to it?

wristpin Fri, 12/07/2019

Not too bad, will soften with some grass running through it but try backing off both adjuster screw just a gnat’s. ( imperial gnat, of course).

In your original post you say that the blades have been sharpened - did that include refacing the bottom blade , or , if it’s a new bottom blade, was it skimmed? With a reground cylinder and a skimmed bottom blade  it’s possible to get good results from a virtually contactless setting which , I understand, is the in thing with professional green keepers etc.


sawleyman Mon, 15/07/2019

But to how many thou of an inch should this gnat be? I jest -- I spent a good 30 minutes making slow and minor adjustments until the 'grinding' noise was reduced. Then testing the cut with a clean peice of paper resulted in even cutting across the cylinder.

The the disagreement of some on here, I resharpened the cylinder blades myself using one of those sharpening strips that fix onto the bottom blade. I couldn't sharpen the bottom blade though so took it to a lawn mower man in Derby and he did this for me. It was then down to me to get the seating of the cylinder right which evidently I'm still learning!

Have to say a huge thank you for your help and (from reading other posts on this site) your huge welath of knowledge.