McCulloch Mower Hunts

[ipbr21054]

Hi,
I purchased a cheap petrol lawnmower.
McCulloch M40 450
I already had the exact same item but for £20 & very good condition i just decided to buy it.

It was cheap as the owner could not start it but i fitted a new plug & now starts no problem.
One thing i have noticed is that 9/10 when i start it i have noticed it hunts ?
Not sure of correct name but sounds a bit like somebody is pressing down thr throttle pedal as if on a race track.
I have noticed also it seems to run faster / louder then my other of which is exactly the same.

I thought i was hearing things so i started it & confirm mine is much quieter.

So my questions are.
Why is it hunting ?
Why is it running faster & louder than mine ?

I did pop off the air filter cover to find say 3/4 of the foam filter missing,would this be the cause.

Thanks

 

[Nige F]

So my questions are.
Why is it hunting ?
Why is it running faster & louder than mine ?

I did pop off the air filter cover to find say 3/4 of the foam filter missing,would this be the cause.

Thanks

Yes, very likely

 

[ipbr21054]

Ok great.

Do these have a tickover screw that you know of ?
Im looking for a mnual pdf etc to check it out but very hard time doing so

 

[Burnerman]

Take the air cleaner box off completely, the tickover screw is underneath. At the same time, see how the engine runs and if the hunting has gone.
The great issue with these engines is the gasket between the carb and the fuel tank.....it becomes unstable with time and is responsible for the fuel to be pumped up into the carb. Its a good idea to replace it with the genuine Briggs part.
The carb is held with 5 screws and if you remove it youll find a dip pipe with a gauze filter in the end.
John

 

[ipbr21054]

Hi,
So tomorrow remove air filter housing completely.
Start the mower & if it doesn't hunt any more then the new filter which is in the post to me will fix the problem.

Should it still hunt with the air filter off then I need to fit these 2 items.
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/253255508683

Does that sound correct.

 

[Burnerman]

Yep, thats a good move.
This carburettor is the offspring from the Briggs Pulsa Jet carb of old.....basically it uses crankcase decompression to pump petrol from the tank up to a secondary chamber just below the tank top face, where normal induction vacuum can take over. With the carb off you’ll see what I mean.
Observe a steel pipe with a rubber elbow that connects to the carb - this must be air leak free.
This nylon carb is very simple, but all drillings must be clean.....use a carb cleaning aerosol for this.
Note, your purchase includes a gasket and a thin diaphragm.....do take care that you get them the right way up, i.e diaphragm to the tank, gasket on top. The U shaped flaps are actually valves that allow fuel through but not back again.
John

 

[ipbr21054]

Cheers will report back tomorrow.

On a side note I want to spray my other mowers deck.
Without oil filling the bore like I've done before when I leaned it up at the back & down at the front which way do I lean it but this time of the sideways axis.
I assume then it's just a matter of one nut & blade off then say 4 bolts and lift engine off ?

 

[Burnerman]

If you must tip this machine on its side, tip it to the left as you would operate it, but back on the handles is better.
Of course if you drain the oil and fuel out, you can tip the machine right over and rest it on the engine!
The blade, usually, has 1 centre bolt 14mm and the engine 3 or 4 bolts.....I guess this is a push machine, not self propelled.
The blade bolt may have a dished washer which acts as a spring to press down hard on to the blade boss. If it has, note which way up it is.
John

 

[ipbr21054]

Yes self propelled.
So lean the mower back so spark plug now up in the air seems the best option.

Thanks.

 

[ipbr21054]

Afternoon,
So ive taken off the air filter box completly & just started it from cold.
Brake bar held to handle using tape so i could video it.
As you can see from this short clip it still hunts.

https://www.theremotedoctor.co.uk/forums/MOWER.mp4

 

[Burnerman]

This doesn’t surprise me.....what’s happening is this:
The engine is slightly starved of fuel, so the motor starts, runs up to speed, gets low on fuel and dies.
The governor opens the throttle full, the motor recovers, runs up to speed and dies once more, so the cycle repeats.
Replacing that diaphragm set and cleaning the carb should restore things to normal.
Clean the outside of the carb with cleaner before you start!
John

 

[ipbr21054]

Thanks i will order that gasket etc now

 

[ipbr21054]

Afternoon,
Today the gasket kit has arrived.
I have just fitted the gasket & diaphagram
I also installed a new spark plug & air filter "oiled slightly".

First pull the engine started & run but was still hunting.
I assumed this was as fuel had been emptied etc & i was waiting for air to be removed.
Still hunting.

Second pull seemed much better.
Third pull fine with no hunting.
Forth pull hunting again.

5 phillips screws for carb/tank tight.
Black O ring & white plastic ring fitted to carb & pushed onto engine etc.
Black rubber elbow breather hose fitted.

Also as mentioned before it seems to run louder than my other mower which is the same.
How can i turn down the tickover as i didnt see a screw etc.

Can you check if this is correct filter as i need to push it into the case whilst doing up that long screw on the top.

https://www.ebay.co.uk/itm/Air-Filt...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

 

[ipbr21054]

I have found the issue.
It is one of the springs on the carb.

I was moving the boomerang plate on the carb & noticed the engine run better.
I swapped the spring from my other mower & runs without hunting & revs are much lower

 

[Burnerman]

That is the correct filter - it compresses into the can when you put the lid on.
I wonder if that governor spring had been tampered with by someone attempting to put things right....dead easy to stretch it!
At least you're sorted!
John