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Collection, Preservation and Display of Old Lawn Mowers

Suffolk Punch 43L Zenith Carb Surging

Hi Everyone, Brilliant Site :) 

I have a loved Suffolk Punch 43L, with a Zenith Carb, but when I took the Dear old machine out for its first cut its was surging? now I usually add Petrol Cleaner and Stabilizer to the fuel but this has not helped this year, any advice on what it might be, as this old girl is way to good to send to Lawn Mower Heaven :)






wristpin Tue, 19/06/2018

Other than  very old brass floats the needle is fixed. You’ve either been sold a dud or it’s been damaged in transit. 

I’m guessing that your carb is not totally clean. They are such a simple carb that while U/S cleaning is useful it’s not essential as all the critical places can be reached with the thin tube fitted to the nozzle of a can of carb cleaner or, whisper it ever so quietly,a piece of thin soft wire. The critical areas are, the main jet behind the blanking screw on the right side of the float bowl. The slow running tube that fit into the right side of the float bowl - remove it , check that it is clear and that the bottom of the housing that it sits in is clear. Finally remove the slow running adjuster screw and check that the tiny hole, from where it meets the feed from the slow running tube , is clear into the inlet manifold.

If all of the above are clear , you have covered all the critical areas, unless the throttle spindle is so worn that air is being drawn in around it and completely unbalancing the mixture!!




Glencmp Mon, 13/05/2019

Hi Guys :) Happy New Year :) So this is my yearly pilgrimage to catch up and see how everyone is doing and also to give you an update on this old girl :)

So I have managed to get a carb, roughly set it up and I can now get the old girl to idle with the choke off :) :) ;) So happy :) But the idle is not great, it wont idle very low, the blades are still turning.

Im going to check the air filter and give it a clean but is there anything else that might assist to get the idle a bit more settled? I have cleaned this carb through with Carb cleaner and nothing seems obstructed in any way its as clean as a whistle?

Any help as always I really appreciate ;) 

wristpin Mon, 13/05/2019

Assuming that your current float is plastic, the needle is rigid.

Mixture screw . Looking back over your previously posted images I see that you have a comparatively late  carb with a fixed main jet. The slotted head screw just to the left of the choke lever is, I believe, a blanking screw over a fuel gallery that has a screwed in “fixed “ main jet behind it. It could be that it is partially blocked.  If there is a vertical screw with a spring on the top of the carb it will be a slow running  mixture adjustment and have a minimal effect on full throttle operation.


Further to the above I've found an IPL for the fixed jet carb as used on the A98 aluminium engine. You will see that the carb bowl is  listed as just one part number with no reference to the visible blanking screw or the jet behind it - odd.…

2nd EDIT

sorry, I got interrupted before finishing the post.  You will see a second slot head screw above the choke lever. That appears to also give access to the fuel gallery. If you remove both screws and , if possible, the jet and give the whole area a good blast out with carb cleaner, you may well solve your running problem.



Glencmp Tue, 14/05/2019

Hi Wristpin, You are a star :)

So I have managed to get the old girl to idle, but not very well. I removed the plug and its full of oil!

Now truthfully I cant remember what oil is in this, so is there a recommended oil for it?

The Plug is covered in oil, so ive turned back to the Pin with the spring back to 1 turn but its not happy, so Ive opened it up to about 1 3/4 turns but the plug is just black, Ive taken it out 3 times to clean it. 

When the old girl is running you can see smoke but its difficult to determine what colour?

SO Ive added a link and a couple of pics :)

So could it be the wrong oil?

Would one of the oil seals on the piston be gone?

Anything else you could think off?

Thanks again guys 



wristpin Tue, 14/05/2019

Not sure about this full of oil . Are you saying that the engine cylinder is full of oil or just that the spark plug is oiled up? Unless you’ve had the machine A over T, there shouldn't be any free oil up there - unless of course you have an exceedingly worn engine ( worn piston rings and cylinder bore) or have grossly over filled it.

When  you have had it running, albeit surging, was it belching white smoke?

I think that the recommendation was for SAE 10/30 but I tend to put straight 30 in old engines.

Based on what you’ve told us, I’ve run out of ideas; perhaps it’s time to get a hands on engine man to have a look , it’s often easier when faced with the actual metal!





Glencmp Tue, 14/05/2019

Hi Wristpin, Not too sure if you had a chance to look at the small youtube video that I posted up, just to let you see how it runs and you might be able to identify the colour of the smoke.

As I wasnt sure how long iyt had been since it was changed I just changed it anyway fro 10w 40 but I can change this later for SAE 30.

So the ole girl is still lumpy but not quite the same smoke. 

The plug seems really oily. the Engine doesnt need any choke and first pull of it goes, so Ill take your advice and Ill take the top of and regrind the valves just in case.

Originally when you take the plug out its covered in burnt carbon when the machine has only beed running for about 5 mins, Ill take the plug out clean it, pop it back in and within 5 mins its covered in Black carbon. Is that just way too much fuel?



hortimech Tue, 14/05/2019

Drain the oil, refill with 1/2pt of good quality SAE30 oil and see if that helps with the oiling, but you may have to let it run to burn off any excess oil.

As for the tickover, you might just be looking in the wrong place ;-)

Those 17 inch Suffolk Super Punch mowers were prone to the engine platform deforming, which stopped the clutch from being parallel to the drum. Try slackening the three bolts that retain the spherical bash into the left hand frame, this may help. If it doesn't, try shimming the engine mount bolts to try and get the shafts level.

Glencmp Tue, 14/05/2019

Hi Hortimech, thanks for that, I was wondering if you saw the small youtube video? I think the smoke can be cured with the SAE 30, The will be done asap

When you say the left is that on the drive side or the opposite?

My only concern is that the plug is very wet and oily like there is a ton of fuel being poured in and Im not to sure how to stop that, as the plug gets really black from carbon very quickly.

As this is a second hand carb it might be knacked as well?

Thanks once again for all the help



hortimech Tue, 14/05/2019

On a lawnmower, left is always from behind looking forward, which is a good thing on a Super Punch, as which is the drive side ? You have the drive chain on the left and the drive belt on the right ;-)

Your carb is one of the last of the Zenith's, with a fixed main jet, so you cannot adjust it, but all is not lost. If you can find an earlier float bowl (with an adjustable main jet) you can interchange it with your existing one.


Chris G Tue, 14/05/2019

Just to add.. from the oil filler cap, from cold, you should not have much more oll level visable from the start of the oil level thread with mower on a flat surface. If its a lot higher than that then crank case pressure will be high forcing oil up potentially. Per Hortimech for the cost of some sae 30, get it warm and change the oil to the right level.

I've seen the youtube vid and the smoke is less likely to be about oil grade, possibly too much oil, valves or piston/ring wear - either way it did not sound too heathy.

Offers still open to strip & US your carb for postage, I have a spare engine as well which maybe worse than yours - not opened it up yet.



Glencmp Wed, 15/05/2019

Thanks Guys :)

Chris Much appreciated once again. Im going to change the oil to SAE 30 and then grind the valves hopefully this weekend. Ill check the 3 bolts at the clutch side and if this doesnt work I may then have to look for another engine ;) 

Glencmp Mon, 20/05/2019

Sooooo.... Its nearly but as good as SUCCESS :) 

It was painful but I got there, and sincerely thanks for all the help ;)

So I made 2 carbs into 1 roughly knowing what was good and what was bad between the two

 Took the head off reground the Valves, new Gasket and put everything back together, now that made a difference! but still not right but loads better

Took flywheel off and had to make a thingy to get the flywheel to move bit it worked well :) but discovered that its a sealed unit with no points so cleaned all the contacts on the fly wheel just to make sure.

Plug still dirty but not as bad

Fitted a fuel filter and another fresh fill of fuel

Checked spark plug and renewed it, and it was nearly Hey! Presto!

So everything I did made just a little difference but everything together made a huge difference :) Its now idling lower than it ever did and starts with no Choke :)

Thanks once again for all the help