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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, 30/05/2017

Euthanasia is a bit drastic and shouldn't be necessary!

For a start just try opening the main jet quarter of a turn but if that doesn't work , Dismantle and thoroughly clean the carb with a solvent cleaner, or, if you know anyone with an ultrasonic cleaner try  a half an hour  hot clean.

Glencmp Tue, 30/05/2017

Thanks Wristpin, OK Stripped Carb, Blew it through with Air, still the same...

Ok, There is only one screw on top of the Carb which Im assuming is the Air Screw, which is set at about 3/4 a turn out, so a tiny bit in and I cant get the old girl to stop surging, a bit out it stops surging but I cant get it to idle without the blades spinning, So im thinking there must be a way to set these up, so does anyone have a carb similar and hows yours set up?

The tiny spring to the left of the carb is on hole 2

The Bigger spring to the right of the Carb is on Hole 2

Any suggestions?

Thanks in anticipation


wristpin Tue, 30/05/2017

If the carb is varnished / gummed blowing through is not good enough.  From what you are saying it sounds as though you have a late model carb without an adjustable main jet . From behind, on the right hand side of the float chamber, is there a blanking screw? If so remove it and then with a good fitting parallel ended screwdriver, unscrew the fixed jet and give all a good blast with carb cleaner  - may solve your problem . Take it that your "tiny" spring is on the left of the carb hooked around the throttle spindle and anchored to a tab held in place by one of the carb bowl retaining  screws. Its purpose is to "damp" the action of the governor on the throttle spindle - or not! 


This is the later Zenith with no adjustable main jet.…

Glencmp Tue, 30/05/2017

Thanks Wristpin, The PDF will help, im going to take it apart again but Ill use Ethyl Methal Ketone to dissolve the Varnish, but I may need a little extra help with how the jets come out etc, don't want to knack it at this stage :) However Im a bit concerned at the slow running tube, I dont seem to have that, the PDF looks the same apart from the fact the where the carb meets the body, mine is oval shaped but I cant see any difference apart from this slow running Tube

wristpin Wed, 31/05/2017

When you've released the float chamber the slow running tube may remain in position in its left hand edge; all you will see is a small tube protruding a couple of mm above the gasket face. It may be quite difficult to remove and you may have to grip it with pliers so to avoid damaging it find a twist drill or panel pin that is a sliding fit into it to avoid it being crushed. Heating the carb bowl with a hot air gun or immersing it in boiling water may help free off the tube. The calibrated part of the tube is at the bottom so along as you don't crush the top you wont upset anything..


ADDED some pix. Gasket omitted !



Glencmp Wed, 31/05/2017

Wristpin, hats just excellent :) When I first took the carb apart it must have dropped out, luckily I have a Zenith Carb from an old mower and hey presto there is a Slow Running Tube :) so popped it in and then tried to reset everything, but its still not running just right, so I know it needs tweeking but is there a formula to start with?

Get the Mower started and adjusted the Air until the Mower was running at its fastest point, then started to take the Throttle down to the little Dot. The Mower is not responsive at all and it takes a few moments for the Mower to either Accelerate or Decelerate, matter of fact it takes ages for it to settle and I can just about to get it to idle without the Blades spinning but it slowly peters to a stop.

So I know its something Ive done wrong, but as I said the Carb will be cleaned with MEK and then put back together, but has anyone put together a little sort of guide to Tune the Carb? Its like the Little Screw that with the Spring as is hovers around before it touches the body of the Carb and shuts down the mower, where on earth is it meant to sit? and if its too close to the body of the carb can it cause a problem as well?

My Mates Father in Law used to buy, sell and fix mowers, he would pop one of these on the bench, and this thing hadn't turned a Cog in years but he could set them up rough, pop some petrol in it, give it a pull and it would start, 2 mins later the mower would sound like it had been cutting grass for years, and he did it all from Ear and Touch. He's sadly not with us now but I did try to pick up as much as I could, but obviously not enough :) He'll be killing himself laughing at me now :)

wristpin Thu, 01/06/2017

Yes, I remember old boys setting up tripple SU carbs listening to the suck using a bit of hose pipe!

Back to the matter in hand. Perhaps we should go back to the basic causes of hunting. A governor problem, too little fuel or too much air - ie a weak mixture. So you are hunting down the possibility of too little fuel but while you are at it check both  the carb to manifold and the manifold to cylinder block sealing. A straight edge across the manifold flanges to check for warping and if they are true a smear of heavy grease across the gaskets should sort any ingress of air there. No need for any fancy gasket sealants although I do like the smell of Wellseal. 

One area that we've not touched upon is the slow running mixture and once you've got the missing tube sorted it would be as well to look at what goes on under the vertical slow running adjuster screw where fuel meets air and the resultant mixture goes into the manifold via a tiny hole. Way back and only once, dirt in that mixing chamber and a blockage of that hole gave me the run around for more time that I will own up to. Suffice it to say the the engine ran well on a different carb so it had to be........!

After a bit of a fiddle and with a torch directed down the adjusting screw orifice, here is said hole! The little white dot at 5 o'clock at the back of the manifold.

I have been know to say that if you want a sweet running engine there's no point in fiddling round with ignition and carburettion unless the valves are good . Proper valve sealing and correct clearances really are the starting point for a good result  but that's another chapter for another day.


Glencmp Sun, 11/06/2017

So, just a little update :) no change :) I did take the carb apart again and flushed and steeped in MEK but its made no difference at all, so really the only difference from last year is that I forgot to add the Petrol Stabilizer for the Winter but all the Petrol normally evaporates anyway but Im still thinking there might be something a miss with the carb but im quite stumped now. The Mower is kep in a Shed that has a Dehumidifier in it but the humidity can still get high in the Winter but Im not thinking high enough to cause any electrical problems??

So the Mower is running not surging now, but its idle is still not the best, but maybe it just needs to cut the grass a little bit more? 

wristpin Sun, 11/06/2017

all the Petrol normally evaporates anyway but Im still thinking there might be something a miss with the carb but im quite stumped now.

It's the lighter elements that evaporate and what is left behind that causes the trouble. Dismantle, then hot ultrasonic clean and reassemble checking for and eliminating any air leaks. Any chance of borrowing a known good carb as a control?  If the idling is still rough I would be inclined to re-seat and re-gap the valves .

Glencmp Fri, 18/08/2017

Thanks Wristpin :) SO the old girl is still surging but Ive put up with it for the time being, but I decided to splash out and get new Gaskets, just in case, so while fitting the new Gaskets, I was trying my best to remove the Dampener spring when... Aye! Youve guessed, it pinged off into orbit!! I have hunted high and low and I can find it anywhere, so is there anywhere I can buy one that you know off?

Ive hunted ebay etc but maybe im asking for the wrong thing, but I cant find one anywhere, would anyone know where to find one, except on my foor :)



wristpin Sun, 20/08/2017

I suspect that the spring you are referring to is the one that loops around the extension to the throttle butterfly spindle and anchors to a little plate held by one of the float chamber screws.

if so, the part number is F016L08694

try Jon Cruse at the Hailsham  Mower Centre.

Glencmp Thu, 24/08/2017

Guys :) Thank You for your help :) Ill give the Mower Shop a call once I get back from Scotland, I'll buy 2 or 3 Springs just in case :) :) 

Glencmp Thu, 07/09/2017

I'm a Broken man, No springs in stock, the world has ended!! (Well not yet, unless China has anything to do with it)

Are there any other places to try? The nice bloke did say, try in a few weeks, so fingers crossed

wristpin Thu, 07/09/2017

A lot of those engines did not have that damper spring and functioned quite happily  so a bit of systematic trouble shooting experimentation may solve your problem. First make sure that the carburettor is totally clean. Then turn your attention to the air vane governor. Make sure that it's free to move on it's pivot pin and not binding. Then take a bit of wire and bend it to match the bend in the vane; that will be your reference to return the vane to the original shape if it all goes wrong. Then experiment to bend the vane closer or further from th flywheel to change the effect of the air pressure from the flywheel find.

You may, just may, find the " sweet spot" where the hunting is eliminated.

Afterthought -  Now you'll tell us that it's a plastic vane!


Thinking about it, if when the machine is running you can restrain the governor arm / throttle spindle from moving and the hunting stops the cause is almost certainly a governor issue - weak spring or compromised air flow on the vane. If, however, with the throttle spindle held steady the hunting continues,  it is a carburettor issue.

Glencmp Sun, 10/09/2017

Thanks Wristpin, so I've blown out the carb, renewed the gas kits, started the mower, surging like mad, grabbed the thing that's flying back and forth connected to the Weather vane thing, and the surging stopped, now if my memory serves me correctly it is plastic, this is bad news right?

the mower actually sits ok when you grab that thing, idles just fine, but let it go and it goes mad, Would it help if I posted a couple of pictures?



wristpin Sun, 10/09/2017

The fact that the surging is controlled by stabilising the governor suggests that is not a carburettor problem  but a governor one. It would be nice if we could come up with a scheme to replace the missing  anti surge spring. It would seem that maybe  it was introduced in conjunction with alterations to the air vane governor ; may even be as a remedial measure for problems associated with the introduction of the plastic vane. An uninformed and may be erroneous guess is that maybe the plastic vane is lighter and has less inertia than the metal ones , and as a result has less self damping.

Nothing wrong with the plastic vane per se but I suspect that it will not lend itself to trial and error adjustment.  I've got more than a dozen Suffolk  engines , either on machines or as spares but not one has a carb with that extended throttle shaft and damper spring but I'll give the matter some thought.


hortimech Sun, 10/09/2017

If you can post a few photos it might help, I don't remember any Suffolks surging in the way you say yours does, even ones with plastic air vanes. An air vane governor works by the flow of air from the flywheel trying shut the carb down to a tickover and the governor spring trying to pull it to flat out. Where they balance at is the governed rpm, if it surges, there has to be a reason for it, possibly  incorrectly set jets.  

Glencmp Thu, 28/09/2017

This is how the Mower is currently set up, it seems to be once it get warm the surging gets worse!? I have tried to save the pictures as large as possible so you can zoom in, just in case Ive been a Clot and put sothing where its not meant to be :)

Current set up

Current Set Up

wristpin Thu, 28/09/2017

That looks ok but a couple of suggestions . Try moving the governor spring back one hole in the arm. May interfere with the tick over setting but worth a try.  Also. Difficult to see in the image but is the governor rod rubbing against the side of the vane just to the rear of where it hooks in . If it is, put a slight bow in it so that it clears the vane and it all moves freely.

Something else you can try , when it starts hunting move the choke lever a few degrees towards closed . If it smooths it out  it suggests a weak mixture even though everything done so far has suggested otherwise.  I'm still of the opinion that Suffolk fitted that throttle spindle damper for a reason!!

hortimech Thu, 28/09/2017

There doesn't seem to be anything mechanically wrong, so it is probably a carb problem. I think this is happening, the engine starts, the spring pulls the carb to full throttle, there is a partial blockage in the main jet, so it cannot supply the required amount of fuel and the engine runs lean. the engine stalls and revs drop off. The engine now runs on the slow run jet, which, if it is adjusted correctly, the engine picks up again. The engine will now try to run at the governed speed again, but cannot, so around we go again, and again, ad infinitum

The A98 engine didn't last long, it was soon replaced by the A118, but even in its short life, it went through several changes, the main one being to electronic ignition.

I would start by cleaning the carb again, check for air leaks and if all else fails try refacing the valves.


Glencmp Sat, 30/09/2017

Thanks Guys, I really appreciate your time on this, I look over all your suggestions and report back, if I can just ask, Ill look at the main jet last but can you oint out which one it is and how to remove it, but Ill do everything else first and that will be my last fix.


Thanks once again

Really appreciate it :)



wristpin Sat, 30/09/2017

In the image in your post 22 there is a horizontal brass screw head together with a vertical one above the choke lever. The main jet lurks behind the horizontal one and itself unscrews.

Everything that Hortimech says is tue and valid but I thought that we had been there before and found that there was no hunting with the throttle held open to eliminate governor reaction   - that would suggest no fuel restriction. 

If there is a weak mixture issue, running with the choke partially closed should counteract it and either prove or  disprove the theory!  

Glencmp Tue, 08/05/2018

So we are a few months on since I last posted, and I did my yearly ritual, The Petrol in the tank, I added a Stabilizer and some Redex, ran it through and left it. So this year, added fresh Petrol, Started the ole girl, and off she went, she did Hunt a little bit but I gave the mixture screw a wee tweek and it ran a little better, but It has a new problem which might now be the source problem.

If you move or turn the mower it starts to splutter and will cut out, that tells me it might be a float issue? The mower now instead of Hunting, the RPM is rising and falling quite a bit slower What might the Diagnosis be?

Thanks for any help :)  

wristpin Tue, 08/05/2018

Sounds like erratic fuel level or supply. Give the float a shake and listen for fuel sloshing about inside. If nothing heard, drop it into a mug of just boiled water and watch for bubbles.

Pull the fuel pipe off the tank tap and check for constant flow for half a minute. Any faltering, check breather hole in cap and filter on tap. all well there, drop the float chamber and repeat the flow test. If that's ok, remove all jets etc and thoroughly clean the carb. 

Glencmp Tue, 08/05/2018

Wristpin, your're a STAR :) Ill check once I get home. Its a shame as this old girl runs like a wee clock when running but its just not happy at th e moment, but we'll get to the bottom of it sooner rather than later :) 

I'll let you know the out come :) 



sidevalve@44 Fri, 11/05/2018

Might be worth checking to see if the throttle spindle is a sloppy fit in the body. I recently bought an ATCO 14" with the same engine, that behaved just like yours. I noticed the spindle could be rocked in the body so, was obviously sucking air in and compromising the mixture. I obtained a NOS carb via the internet and it now runs really well.  

Glencmp Mon, 14/05/2018

Hi Wristpin & Sidevalve :) 

So Ive had a dig this weekend, So the Throttle Spindle seems nice and tight, not a wobble to be seen, I pulled of the pipe to see how the petrol flows and it pours out the pipe no problem at all, so Im going to have a look at the float, drop it in to a mug of Hot water to see if its having a fit, apart from that im stumped, :) I wish you guiys were closer to Home :) 

Glencmp Wed, 06/06/2018

So a further update. Checked float and its fine, I lost the Slow Running Tube (Again!) Replaced it, and the mower is back to the surging, it will now only keep going if you push the little priming button several times. So Im really stumped with this. Im going to replace the carb, hopefully with something working and we'll try the process again.

Ill keep this old carb to see if I can make headway with it, but a replacement seems to be the answer.



wristpin Wed, 06/06/2018

Pressing the little button on top of the carb pushes the float down and admits more fuel - raises the fuel level in the float chamber. If that cures the problem it is telling you that the engine is running lean. Presumably you have tried opening the main jet a tad.?  Also if closing the choke a fraction smooths out the hunting it is another indication of a fuel problem.

I'm fairly sure that the cure will be a really thorough clean of the carb - both the main and idle passages.

I will dig out and post a schematic of the Zenith carb which shows the rout that the fuel takes for both running and idling . This will show whT has to be clean.





Glencmp Wed, 06/06/2018

Ahhhhhhhhh! Wrist pin, now you've hit something there, I may have read this in a pervious post, but I did start to close the choke and it did star to run better, and after about a third of the way it started to labour and then chose on it cut out.

so I should be turning the screw anti-clockwise to enrich the mixture?

I have blown through all the jets and everything seems to be clear, so I'll try and rich things up instead of being gentle to see how it goes, failing that... I'll cry a bit :)


wristpin Wed, 06/06/2018

I will send the schematic but the PC has been busy doing a two and a half hour update to the satnav - the joys of slow rural internet!

Blowing through jets is a start but a badly contaminated carb can  require a bit more in depth cleaning. Have a read of the instructions for setting / adjusting the jets and see where that gets you.


ZENITH CARB schematic and setting instructions.…

Chris G Wed, 06/06/2018

Glen, no idea where you are on the planet, but if you want the carb ultrasonic cleaned I could do it for you - if wristpin provides the diags that is :-)

Glencmp Fri, 08/06/2018

Thanks Wristpin, That will help loads, Ill print these off this weekend and have a look

Hi Chris, Ill have a dig at this too see how I get on, but I may take you up on the offer as the Carb on the outside is really clean, so I may just get it cleaned anyway, Chris how much would you charge for this? Im in Durham so I guess I would have to post it to you ;)

Thanks Guys, your complete stars :) 




Chris G Fri, 08/06/2018

Glen, I would do it for postage :-) . See how you get on but pop me a PM to share address if you want it done.

Glencmp Sun, 10/06/2018

Hi Chris, Thanks for that, Ill PM Soon ;) 

Apologies for all the replys, No Idea what happeded, I tried to update but it kept saying ERROR so I thought nothing was going up, so I logged in on my PC to see loads of reply from me!! and I cant delete them?? No idea whats happened there!!

OK, So I thought I had great news, but not quite, Took the carb apart again, cleaned it again, blew through all the nooks and Crannies, put it all back together but... I did it slightly wrong! The spring that connects the swivel that connects to the Throttle cable, I connected it to the little anchor point on the back of the big plastic fin, so Started the mower and it took off!!! Revving like hell, grabbed the little rod and pushed the little arm towards the carb pulling the big plastic fin and it settled down and idled very low!!!! so I thought Id cracked it, all I needed to do was sort the revving!! then checked the pictures, discovered I connected it wrong, put it back the right way and Im back to where I am :( So I was thinking, am I a Spring missing? what should be connected to the little anchor point at the back of the plastic thing?

So the ole girl wont idle, I can get it quite low but the Blades keep spinning, so same again its workable but, I now know that its capable of very low idle speed, so it seems I can get it all to work properly in stages but not alltogether.

Any chance you could have a scan at my pictures to see if Im missing anything and its connected the right way?

Thanks for all your help so far



Glencmp Mon, 11/06/2018

Im so Embarrassed! and I cant delete them!!! 

Different note... Ive ordered a new Float, just in case

Glencmp Tue, 12/06/2018

Hi Wristpin, Ive used the schematic to the best of my tiny knowledge, as I thought I had to cracked but alas no, But a very handy Diagram none the less, so thanks for that :) 



wristpin Tue, 12/06/2018

A few more thoughts. Make sure that the gasket between the carb and the cylinder block is intact and not allowing air to be drawn in.

Going back to an earlier suggestion, when it’s hunting, have you tried closing the choke a fraction? If so, what is the result. Just another thought, you are running it with the air filter in place?????


Glencmp Thu, 14/06/2018

Hi Wristpin, Ill double check the choke thing :) Its a brand new gasket, and yes there is a filet in place, but Ill pop the new float in this weekend to see how it goes, but do you think im missing a spring? because of the little anchor point at the back of the [plastic fin?

wristpin Thu, 14/06/2018

No, you’re not missing anything from there but you could try moving the spring in your post 20 to the middle hole of the throttle relay lever - the one where the cable connects..

On an air vane Governor set up such as you have, hunting or surging is usually caused by too little fuel,  ingress of air diluting the correct amount of fuel or a weak/ incorrect Governor spring causing over sensitive throttle adjustment . There are other things but I think that one or more of the above is the root of your problem.

Glencmp Tue, 19/06/2018

Thanks WRISTPIN :) so this is a 2 part reply

So Ive got a new Float, but the pin in the middle moves around, is that right? So I had Petrol pouring out of the float bowl at one point! but I moved the pin around and got it running. The float I took out, The pin was fixed, Im lost here as the new float is brand new.

Secondly, Wristpin, look like your Bang on the Money. Got the old Girl running but I slowly started to close the Choke and the throttle and I then started to open up the mixture screw. I have the Choke 2/3 Closed, the Mixture screw is now wide open and nearly ready to come out, it will only have about a turn left. But the old girl is idling quite low. SO Im going to order another set of Gaskets and Speak to Chris to see if he could Ultrasonically clean the carb.