Blackened spark plug each time the lawnmower is started

[Mowland]

Dear Experts. I am a complete novice in lawnmower maintenance, I was kindly given an old Hayter 41 petrol lawnmower of which recently it has started to play up. First the engine never started. Replaced with a new spark plug. It then started but only ran for about 2-3 mins max and cut off automatically. It restarted well but kept cutting out. So I have stripped the carb and cleaned it thoroughly but problem still persists. I then replaced with a new air filter and new carb. Now the engine starts and can stay running for as long as I want it to. However, if I cut off the engine and tried to restart, it failed. Upon checking the spark plug, it covers with black soot and if I clean it, I can restart but only once. This does not look right and I should not need to clean the spark plug each time I want to restart the engine. I read from various sources that this is to do with the excessive fuel intake that causes the blackened spark plug (?) If so, is there a way I could manually adjust the amount of fuel intake please?Or am I looking at the wrong place? I have attached a photo of the lawnmower and the throttle. I appreciate your knowledge and kind assistance. Thank you in advance.

[spudulike]

Black plug = running rich.

Mower carbs are pretty basic and most rely on the main jet for fuel delivery and have little in the way of adjustment so....I would say that your choke may be sticking on partially which would make it run rich.

I don't know how your choke functions but some have a heat activated solenoid, some have a manual choke lever, some operate off the throttle and some just have a primer bulb (No choke).

You will need to whip off the air filter housing and see how the flap nearest the outer part of the carb is operated. Having done this, at cold and in the starting setting, it should be fully cold. Warm the machine up and when fully warm, it should be fully open. If it isn't, it is either set wrong or whatever opens and closes it has failed.

BTW, typical carb issues are blocked main jet, emulsion tube holes blocked, crap in the float bowl or leaking needle valve. The OEM carb will generally be better quality than cheap Chinese.

[pleasant]

That engine is fitted with a manually operated choke via the throttle lever on the upper handle. The choke should 'flick off' once the throttle lever is moved back to the max revs position via that spring to the upper left of the pic. The plastic it is attached to on one end of the spring is the choke butterfly. You should be able to observe that plastic 'lever' flicking off when you move the throttle control off the choke position. If it doesn't move, then the choke is staying on whilst in use and will run rich which will cause the plug to foul.

 

If not sure it is flicking off when the throttle lever is moved away from the choke position, simply remove the air filter outer cover and the filter itself and observe the choke butterfly through the intake hole inside the filter casing. If the butterfly doesn't move when the choke is taken off, then that is no doubt your issue...however in 25 years on working with Briggs engines like yours, I would be surprised. Yours is an early model hayter and there should a small slotted screw to the front of the carb that will control the fuel richness. It is actually on the front of the carb underneath that pic of the governor assembly. A small long flat bladed screwdriver is what your need to adjust the fuel flow via that screw. Anti clockwise with richen it up, clockwise will lean it off. Do a quarter turn at a time. If someone before has over richened it then you will get a fouled plug, but them may have done that to overcome another issue with the carb such as a hunting issue. Below is a pic of the type of carb, and it is the brass screw you should be looking for.....it's only the early carbs that had this....yours may not have one, in which case it is a fixed jet carb and as you have changed stuff I would suggest already then I would be scratching my head without physically seeing it or hearing it running.

[Steven P]

Was giving the lawnmower its annual clean (makes me feel better for ignoring maintenance the rest for the year).

 

Anyway, remembering this thread, took the spark plug out and likewise it's covered in soot. Tomorrow might do the same again - todays was £100 B&Q job, tomorrows is Mountfield so 2 ends of the scale in terms of quality.

 

Anyway, a thought, these mowers (4 stroke) are stopped by stalling the engine - release handle, brake goes on, stalls engine - reasonably unique to mowers, most other engines are stopped by cutting the electricity supply to the spark plug? Not sure if that accounts for soot on the spark plug? - if still fuel in the chamber it might ignite (maybe not explode) but without the engine moving, no exhaust cycle, soot is retained in the engine ?? Just a thought though I am not an expert, it jut struck me as odd that mine is the same condition

[spudulike]

The plug colour takes a while to form. Black colour may just be a dirty air filter, big mower and small garden where it isn't warming up, the mower may not be revving properly.

I had issues with my Atco Royale yesterday, turned out to be water in the petrol....that's ethanol for you, all running well now.

[pleasant]

1 hour ago, Steven P said:

Was giving the lawnmower its annual clean (makes me feel better for ignoring maintenance the rest for the year).

 

Anyway, remembering this thread, took the spark plug out and likewise it's covered in soot. Tomorrow might do the same again - todays was £100 B&Q job, tomorrows is Mountfield so 2 ends of the scale in terms of quality.

 

Anyway, a thought, these mowers (4 stroke) are stopped by stalling the engine - release handle, brake goes on, stalls engine - reasonably unique to mowers, most other engines are stopped by cutting the electricity supply to the spark plug? Not sure if that accounts for soot on the spark plug? - if still fuel in the chamber it might ignite (maybe not explode) but without the engine moving, no exhaust cycle, soot is retained in the engine ?? Just a thought though I am not an expert, it jut struck me as odd that mine is the same condition

Releasing the opc lever (operator presence control) does not 'stall' the engine. It operates the same way as a petrol car, by cutting out the the ignition circuit to the spark plug. Stalling the engine is done by artificially richening the fuel by effectively putting the choke on an already warm or hot engine.....not good for any engine. 2 or 4 stroke. 

 

As and aside.....it would have to be a pretty awful b&q job, if you consider a modern mountfield a step up as they are shite nowadays

 

Edited Sunday at 19:42 by pleasant

[Steven P]

B&Q jobbie is about 10 years old, Mountfield about 20 - there is a noticeable increase in quality

[pleasant]

Like i said....thats because a 20  year old mountfield isnt a modern mountfield 

[Muddy42]

As above, check the tuning. Keep that old hayter going, I have one and its built well and the only issues have been fuel / carb related and easily solved.