(mower) Mechanical gods of STW help

Mountfield HP414 with the RS100 engine is “hunting” constantly with and without load (i.e. Cutting or just idling on concrete).

Initial research and previous similar issue said dirt in carb or old fuel.

Not to do things by halves and because the parts were cheap I have….

1. Stripped down and rebuilt the carb and in doing so replaced all the relevant gaskets. It has gone back together correctly and it doesn’t have adjustable jets so I’ve not cocked that up as far as i can tell.
2. New foam air filter (soaked in oil first as per instructions)
3. Changed the fuel hose
Replaced the spark plug (ensuring correct gapping first)
4. Drained fuel from lowest point in system and replaced the fuel with fresh unleaded from BP
5. Checked the spacing of the magneto armature to the fly wheel that ‘feeds’ the spark plug.
6. Oil level is ok (could possibly benefit from a change).

This is a ‘governor’ type mower so no manual throttle control or choke and I can see the governor mechanism bouncing during running.

The carb is a side mounted one.

What am I missing?!

I guess the magneto and HT lead could be defective? But that’s £35 so I’m reluctant to order that on the off chance unless I can prove it’s a likely problem first.

I forgot to order a fuel filter so that has NOT been changed yet.

What obvious thing am I missing or has it just had it’s day (6 years old and been plenty used).

I hate scrapping stuff (seems so wasteful) so I’d rather fix than replace but it isn’t worth paying someone else the labour.

Check governor arm and springs are clear of grass and dirt. Moving freely and the springs are in the right holes. Also check carb gaskets sealed properly when rebuilding.

Change the fuel filter too

Be careful how much you spend. I just went to buy a new mower and the shop (Action Leisure) had a few second hand machines in. I bought next mountfield up from yours, self propelled, serviced with new blade and three month warranty for £60.

My guess would be the governor mechanism / spring. You could test this by holding the throttle manually in a fixed position.

Thanks.

I will double check the gaskets but fairly sure they were assembled carefully and surfaces cleaned first.
I also covered the intake and cleaned all the linkages while the carb was off so I don’t think there is a debris or gunge problem.

MattOAB exactly my thinking the parts I bought were less than £20 and that’s getting towards my limit on a well abused old mower. This exercise is partly also about learning more about mower engines on this old mower before we ultimately upgrade so I know how to look after the inevitable new one better (this one has been a bit neglected if I am honest).

I will give the governor springs and fuel filter a try if they are only a few £

The governor mechanism itself is a pneumatic one I think. I will try a steady throttle on that later today by holding the link. If it runs smooth with steady throttle and load I guess that means a governor issue (as in the bit that twists the linkage in the engine) or something that affects it’s operation. BUT If it still misfires on a held steady throttle then I am looking at or for something else.

One other silly question would a damaged blade cause this through being unbalanced? The blade looks ok but it does gobble up the odd pine cone during mowing.

The blade won’t be the issue.

One other thing to try is slightly increasing the engine speed (by adjusting the governor arm) seem to recall my mower hunted a bit at certain speed settings.

The blade won’t be your problem…. odd pine cone? You should see what I uses mowers to cut/Hoover up! Shame you forgot fuel filter as that would’ve been first thing I would’ve replaced once everything drained and cleaned

Shame you forgot fuel filter as that would’ve been first thing I would’ve replaced once everything drained and cleaned

Yep felt a bit silly when I realised – order in haste redo the job at leisure… 😳