Mountfield sp414 (rs100 engine) only runs for a few seconds

[Stringman_uk]

Hi all
I have had the above lawnmower for 7 years now and it seems that every year I get a problem with it.
This year its the return of the problem; runs when primed for a few seconds but then stops.
After looking at various posts and fiddling I realised i had lost the brass jet. So I bought a new carb for it and have just fitted that with new gaskets ( only thing i kept was the fuel pipe from tank to filter)
Still the same old problem. starts and runs for a few seconds and cuts out.
From what I can see there are two ways of adjusting the carb the brass needle jet and the black plastic thumb screw ( which seems to adjust the speed) I set both to middle, no joy and adjusted to no avail. I even took the brass jet out. It ran for longer( maybe 20seconds) and seemed to be hunting but still cut out.
I have removed the airfilter, cleaned the plug. Nothing seems to work.

Any ideas before I just give up

Thanks in advance

 

[Burnerman]

The plastic screw only adjusts the idle speed, if there is a brass screw with a spring behind it then that’s the idle mixture and is usually set 11/4 turns out from fully in.
I’d check the fuel flow from the tank, it should flow freely from the pipe.
When the engine starts to fail, what happens when you press the primer bulb gently?
A pic or two of the carb would be good if you can, there’s been a few engines fitted to this one!
John

 

[Stringman_uk]

The plastic screw only adjusts the idle speed, if there is a brass screw with a spring behind it then that’s the idle mixture and is usually set 11/4 turns out from fully in.
I’d check the fuel flow from the tank, it should flow freely from the pipe.
When the engine starts to fail, what happens when you press the primer bulb gently?
A pic or two of the carb would be good if you can, there’s been a few engines fitted to this one!
John

Many thanks for the reply John
I will check all that tomorrow. I did think that the brass screw only took 11 turns from fully in before it fell out!! but I will check.
Do you want pics of old carb or new one? (old one may well be an ebay special ) the new one is https://www.ebay.co.uk/itm/394441438007

But I will also check that i have used the correct gaskets etc

Thanks

 

[Burnerman]

That’s about right for the mixture screw......remember, 11/4 turns out from fully in!
Any pics would be great, but please be aware the filter shown on the eBay picture isn’t suitable for you and will stop the fuel flow so don’t use it.
Interested to follow this up!
John

 

[JohnD]

Does the petrol filler cap have a tiny hole, and is it clogged?

Is there a petrol filter, and is it clogged?

Any sign of rust in the tank?

 

[Stringman_uk]

Does the petrol filler cap have a tiny hole, and is it clogged?

Is there a petrol filter, and is it clogged?

Any sign of rust in the tank?

Not sure but I losened off the filler cap, the filter is the wrong type, would hope there is no rust as its a plastic tank lol

 

[Stringman_uk]

That’s about right for the mixture screw......remember, 11/4 turns out from fully in!
Any pics would be great, but please be aware the filter shown on the eBay picture isn’t suitable for you and will stop the fuel flow so don’t use it.
Interested to follow this up!
John

Sorry for late reply but I am in the middle of a summer house build so This gets bumped down.
I have ordered a new fuel pipe with correct filter so will try that , along with your other suggestions and report back

 

[Stringman_uk]

SO the latest update!
Having fitted a new fuel pipe with the correct filter I started it up.
Well it runs- boy does it run.
If it had a rev limiter it would be bouncing off it!!
I have tried the speed adjuster screw but it makes no difference.
I tried the mixture screw 11 1/4 out and then screwed it all the way in - no difference.
The only thing I can think of is that I may have put the gaskets on in the wrong order.
I have ordered replacement gaskets.
Any Thoughts?

 

[JohnD]

Running weak.

Best not to run it for long.

Is there an air filter?

 

[Burnerman]

I think the governor rods have kept the carb on full throttle.......have you got them in the right place?
Check that the throttle fulcrum can be moved by hand.
N.B The mixture screw only works on idle - it is bypassed when the engine is running fast!
John

 

[Stringman_uk]

Running weak.

Best not to run it for long.

Is there an air filter?

I originally ran it with no filter or cover (just to try) and it ran for 2 seconds then cut out.
When i put the filter and the cover on it runs ( just a bit too fast)
funnily enough it starts ( when warm) with no bulb pushes ( which it didnt do before)
But when the collector box gets full and the base starts to get clogged the engine slows down ( and will eventually stall) but if you tilt the more back it picks up speed again.

 

[Mike13]

It is "One and a quarter turns" from shut, not eleven and a quarter turns, that will cause big problems with the mixture

1 1/4 turns, and NOT 11/4 turns.

 

[Burnerman]

The primer bulb is only really used for a cold start, but maybe a small push will help if the engine has been still for say a couple of hours.
Do check that you can easily move the governor rods by hand with the engine off.
John

 

[Stringman_uk]

It is "One and a quarter turns" from shut, not eleven and a quarter turns, that will cause big problems with the mixture

1 1/4 turns, and NOT 11/4 turns.

It is "One and a quarter turns" from shut, not eleven and a quarter turns, that will cause big problems with the mixture

1 1/4 turns, and NOT 11/4 turns.

No wonder it nearly fell out lol
Thanks for spotting that!!!!

 

[Stringman_uk]

Thanks for all the replies.
It put away at the moment as I am desperately trying to finish the summerhouse.
But hopefully over the weekend I will sort the jet out and fiddle with the govenor rod

Thanks all