Mountfield 454 HP and Sp baffled

[Roger hamlett]

Recently taken in 2 mountfield 454 mowers one sp the other HP, within 1 day of each other.
Non runners both 2015 models with the Rm45 engine ohv. We mainly fix Briggs, sthil, John deere and are other local repair shop fixed the mountfields and older cylinder mowers, but they have sold up and moved, hence we are getting a few mountfields to look at.
We know are way around small engines but these have us stumped hence the post, I know the carburettors and engines are not great, Chinese ( pretending to be Honda, I wish). Done the usual cleaned stripped carburettors left them to soak overnight before reassemble, new plugs, new air filters, good spark from plugs and good compression. Won't start, so reset valve lash and gap as recommend by mountfield, just to make sure timing spot on, ( inlet valve 0.10-0.15, exhaust 0.15 -0.20) still no joy. Noticed that the exhaust valve on both engines were clicking just before TDC on the compression stroke, not sure if this a common thing on the newer mountfield engines, anyway we striped one down thinking it may have been a faultly compression lever on the cam shaft. No joy, perfect not broken moving and working as it should, today did a leak down test and no problems. So anybody any thoughts, replace carburettor!!! They are really poor quality....

 

[Burnerman]

I don’t think there is a problem with the valve timing, although the exhaust seems to be a little advanced it is usual.
The carburettors are copies, right enough, but they arent too bad and need to be cleaned and blown out with compressed air just like any other. Is the choke mechanism working?
John

 

[Roger hamlett]

Yes, carburettor dried with compressed air and choke on both machines working, thanks for reply.

 

[Burnerman]

Was there any crap or traces of water /rust in the float bowl?
When the main jet is unscrewed, the emulsion tube sits above. Tiny drillings all the way round, they must all be clear.
If there is a seperate pilot jet, its probably beneath a phillips screw under the idle adjustment screw that has to be removed to access it.
John

 

[Diver Fred]

Had similar myself this year...

Was going to say - drain the float chamber (which worked for me) but you have stripped the carb. The only other suggestion I have is the 'choke' function working properly (actually does something when the lever is pushed into Choke position).

 

[Roger hamlett]

Thanks again, these carbs only have a fixed pilot jet which you can't remove, main jet and emulsion tube stripped and cleaned in a ultra sonic machine overnight, we have made some progress can now get them to run on choke, but once you try to reduce speed they die, I'm convinced the carbs need replacing...

 

[Burnerman]

The slow running jet isn't removable, as you say but it is possible to blast compressed air through it.
Also, the slow running jet gets its air supply from small hole to the side of the main venturi so do check that.
If its running on choke, then its either:
The float needle is sticking , dirty or jammed (assuming there's a good supply of fuel from the tank);
The main jet is blocked or the emulsion tube above is likewise.....you cant really put the emulsion tube in the wrong way, but if you do everything is to hell.
Does fuel run freely from the tank - there is usually a gauze filter fitted to the outlet.
John

 

[Roger hamlett]

Thanks for quick reply, I agree its definitely a fuel supply issue, carbs have been stripped at least 3 times each cleaned, blasted with compressed air.
- main jet is lovely and clean and so is the emulsion tube all the holes are clean, see day light through them
Have blasted compressed air through the slow running jet, put a new float needle in one of the carbs although original looked fine. Float needle appears to be moving up and down when you press the the spring around it. Fuel from tank running fine. The only thing we haven't tried Is replacing the needle seat, unfortunately on these carbs it looks like it is built into the carb and can't be replaced and the seat is not rubber it is a type of alloy with rubber compressed around it. Still very frustrated, so bit the bullet and ordered a new carb, can't afford to spend any more time stripping and cleaning these carbs.

 

[Burnerman]

Thanks for the update Roger and do let us know how things go.
Replacement carbs are around the £25 mark so that’s not too bad.
John