Mountfield HP470 power varying after servicing

[Iain741]

I've just serviced my Mountfield HP470 mower. I've replaced the blade, oil change, new air filter, new spark plug, new fuel primer, cleaned the carburetor (used brake cleaner spray), new diaphragm & gasket...but it's not running right.

To be honest, it's pretty similar to how it was before serving it!

any thoughts on what's up there?

 

[Burnerman]

Did you manage to remove the main jet, and the emulsion tube which is above it?
I presume the engine is surging - i.e the revs are going up and down?
John

 

[Iain741]

Hi John...is that this bit? (See photo) I removed the tube and just sprayed cleaner everywhere else, including inside it. Would you disassemble it more?

 

[Burnerman]

Yikes, humble apologies, your machine must have the side valve engine.
That pipe shown is the dip pipe which pumps fuel from the tank up to the carb.
The big issue with this set up is the gasket between the carb and tank - it’s vital that it’s in good condition.
Are you happy that you have the gasket and diaphragm assembled in the right order?
Other than that, this carb responds well to cleaning but compressed air helps a lot.
One more thing, if your engine has a rubber pipe connection at the carb, this takes a pulse from the crankcase to pump fuel. That rubber connection needs to be tight.
John

 

[Iain741]

Thanks mate, yep pretty sure I've got the gasket & diaphragm on right. (Thin bit on the tank side). There is a rubber pipe elbow connecting through carb to the mower, I'll have a look at that.

The other thing I wasn't sure about was the governor springs...should those move much ? Mine seem pretty slack and don't seem like they do much at all! Video here:

 

[Burnerman]

That's a bit of a museum piece, but you can see the rubber elbow on the carb there. Air leaks around here interfere with the engine's ability to pump fuel up from the tank.
I guess the filter on the end of that dip pipe was clean? Early models had carbs with two pipes.....you haven't replaced the carb have you?
The governor springs and levers will only come into play when the engine is actually running .......don't worry about the springs too much, it's the levers that do the work.
As for that swivelling plate, I tend to lock it in one place or another as it doesn't seem to do much!
John

 

[Iain741]

Ha, yeah I've definitely had my money's worth out of it!! No the carb hasn't been replaced. End of the dip pipe was clean. The rubber elbow does seem a bit slack, I'll maybe try it with a tie wrap securing it on each end

 

[Gasguru]

What petrol are you running it on? Is it old and contaminated?
My 30 year old Hayter with a Briggs engine refused run on the new E10..it would gradually reduce revs and die under load.
Running with 99 octane and it was instantly back to normal. (AFAIK has only 5% or no ethanol).