Mountfield SP470 over-revving after service

Hi All
I have a Mountfield SP470 and have just serviced it with new spark plug/diaphram/gasket/air filter. I took the carburettor off and cleaned it completely. New fresh fuel was also added. I also replaced the two Governor Springs (see attached pic).
Upon starting it up (primarily to warm up the oil before changing), the engine over-revved significantly. I checked the Governor Springs and both seem to be correctly installed. I tried bending the tab in both directions to see if that would help but to no avail. As far as I am aware, this indicates some issue with the Governor springs (the smaller one I suspect) but there is nothing obvious.

Does anyone have any thoughts? The engine starts fine so the fuel delivery and spark are working so I do not think the Carburettor and diaphram are to blame but I am at a loss on what to try next.

Any thoughts greatly appreciated!

If not seeing thing wrong it looks like the o-ring at the intake manifold is out of place. I think I see the black o-ring sticking out. This would cause vacuum air leak.


Now if it is only over revving at full speed setting then you just need to adjust the speed by bending the tab on the main governor (heavier larger spring).

The small spring mutilated and streatched to put more tension on the governor. The large one it hooked wrong. It goes inside the loop on the gov link. The small one looks like something someone got out of the spring assortment at ACE HARDWARE. Take the small one off and hook the large one correctly and see how it runs then. Some of these engines did not have the small one. If it did it hooked in the loop on the large spring. Make sure the throttle is moving freely and not binding. These engines had an air vane governor and some times bind up due to rust on the pivot bolt.You can adjust the top no load speed by bending the tab on the throttle control. Good luck.

Many thanks for these very constructive suggestions. I will put this into action and report back. Very much appreciated!

steve900 said:

Many thanks for these very constructive suggestions. I will put this into action and report back. Very much appreciated!

Click to expand...

Hi All

Reseated the Governor springs and the Carburettor o-ring seating but to no avail! Still over-revving massively.

Are you using OEM supplied springs or after market kit? It takes very little difference between factory and aftermarket to cause grief. You may have to pre-stretch your springs slightly to get sufficient slack to let governor hold engine speed to spec if you cannot get speed down with screw or bending tabs...

To me looks like one too many springs, installed wrong, over stretched and wrong hole as others said.

Reduce spring tension to lower revs.