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Collection, Preservation and Display of Old Lawn Mowers

not idling... revs go up, revs go down...

So, my Webb Diplomat 430E (without the E, no battery) now has fresh oil (30W) and a new air filter (lightly oiled) from e-bay (see other thread for details of that part).

But the motor does a strange thing...

When warm it will 'idle', but it does what on a 4-cylinder 'bike engine, I'd desribe as 'hunting'.

The revs rise and fall back, repeatedly.

(This is a sypmtom of a multicylinder with unbalanced carbs, clearly not a possibility in this case.

I confess I don't claim to understand the DellOrto carb on this machine; carbs should have NEEDLES in my experience, I've seen the 'seconday choke like device (engine side of the carb) that actsto restrict the mixture flow, so that's how it controls the thing without a needle valve).

Anyway, when cold, it won't idle at all.

It starts, revs up, revs down, stops...

Watching it closely, I observe that the weird linkage thingy the top of the carb (two pivoty bits linked by a flimsy bar) is activating 

'in time to the music'.

So, I strongly suspect that this is the cause.

What are these things ? And why is it doing what it's doing ?

I can take a picture if needed, now that I've worked out the system (!)

Thanks in advance,




wristpin Thu, 01/08/2013

What you have described is hunting. It can be caused by incorrect governor settings but is far more likely to be the result of dirt in the carburettor or contaminated fuel .  If applying a little choke smooths it out that will confirm the diagnosis. Once upon a time one could tune out the problem by adjusting main and idle jets. Now due to emission regs carbs are non adjustable or if there is a slow running adjustment it is restricted or even hidden behind a blanking plug.

Basically the float controls the fuel level in the carb and in the centre stack (into which the bowl nut screws) there will be a fixed or removable calibrated main jet/ emulsion tube with very fine drillings in it.I think that your Dellorto one will be removable but you will need a good fitting parallel sided screw driver to get it out without chewing it up  - one chance or a lot of agro!

In addition to the emission regs we have ethanol added to pump fuel which is shortening shelf life of fuel and leading to gumming of carbs! Google it and there will be plenty of reading.

If giving the carb a good internal clean with a proprietary product doesn't solve your problem it may be worth finding a mower or motor cycle repair shop with an ultrasonic cleaning tank . Particularly since the introduction of ethanol blended fuel a u/s tank has become an essential bit of kit as in addition to what you can see there will be passages and drillings etc.hidden behind welch plugs and out of sight and access.


hortimech Fri, 02/08/2013

Hi, the problem reported was well known and if cleaning the carb doesn't fix it, there used to be a kit to replace the fixed main jet with an adjustable one, but if you can locate one GOK smiley

blambo Fri, 02/08/2013

I certainly gave the carb a good cleaning out, it didn't start before I did that.

I had the jet out, it was blocked.

I used loads of carb-cleaner thought it was very clean.


It's NOT cured by putting the choke on.

wristpin Fri, 02/08/2013

So what happens with the choke (full?) on? Does the engine choke itself to a standstill? If not there's something very odd going on! Does not just a little choke make any difference?

Just a thought, is the carb bowl nut "hollow" with a well inside it. If so make sure that it is cleaned right out. I've just unearthed an old Dellorto that probably came off a later Suffolk/Atco/Qualcast/Webb and it has got a slow running adjustment jet/screw under a plastic plug but I can't find an exact one for your model. May be worth having a look.

With the engine running at full throttle try holding the carb  throttle shaft in the flat out  position to stop it hunting. If the engine runs smoothly at full revs it basically tells you that the carb main jet  is capable of delivering sufficient fuel to sustain those revs and hunting at part throttle or idle suggests a partial restriction of the slow running/progression jet system which is where the ultrasonic cleaning may be of benefit.

Although experience suggests that it is a fuel problem it may be worth checking that the governor spring (tensioned by the throttle lever/cable) is exerting enough force to resist the closing action of the governor mechanism. If there is a series of holes on the gov arm try putting the spring into one where it will exert more pull . However , as Hortimech says those engines were known for carb problems - I've got one of those adj main jet kits and one day it will get me out of trouble!

hillsider Fri, 02/08/2013

The symptoms described certainly suggest a problem with the idling jet either blocked or incorrectly adjusted as has been suggested. Perhaps a photo of your carb would help us to identify what you are dealing with, I too have found the idling screws of Atco and Qualcast mowers fitted with this engine hidden behind a plastic plug.

blambo Sat, 03/08/2013

wristpin Sat, 03/08/2013

That brass slot headed screw just above the Dellorto logo is the idle/slow running jet and is "spring loaded" to hold its position. It is often possible to use it to tune out erratic running but if it makes no difference you almost certainly have a blockage in the idle circuit of the carb.

hortimech Sun, 04/08/2013

What you could try is, start the engine then hold the carb throttle bracket on tickover and try to adjust the slow run jet and tickover over screw to get a 'nice' tickover without the cylinder turning, you might have to have several attempts at this and if cannot get it tickover then the carb is blocked.

If you can get it to tickover but it will not then run at high revs, then either the main jet is still blocked or you need the adjustable main jet kit.

The reason the engine hunts is: when the engine starts it tries to run on the main jet, but if it is blocked or the jet size is too small then the engine runs too lean and starves out, the governor senses this and shuts the carb down to tickover, the engine picks up and the governor then opens the throttle again but the carb still cannot give the engine the fuel it requires so the governor shuts it down again and round & round it goes.

Another thing you could check, is the fuel coming out of the tank fast enough, are there any blockages?