The Old Lawnmower Club
Collection, Preservation and Display of Old Lawn Mowers
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Sv150 engine hunting

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steveteigh Tue, 04/10/2016

Hi all,  hope someone can help I have a mountfield rotary with a SV150 engine that constantly hunts

starts first time hot or cold engine hunts in high or low revs.

carb has been thoroughly cleaned via ultrasonic but still hunts.

there are no adjustment screws on the carb

does it need a new carb ?

Forums

hillsider Tue, 04/10/2016

Welcome to the OLC Forum,

Re your hunting SV150 engine it sounds very like the idle jet is blocked, was the jet removed when cleaning the carb? The idle jet is housed in a plastic plug located below the idle speed adjusting screw, the plug can be carefully extracted by using two screwdrivers to wriggle it out of the carb body. The jet is located in the bottom of the plastic plug and can be checked and cleared easily, do be careful though there is a very small O ring located on the bottom of the plastic plug take care not to damage it when reassembling and be careful to align the flat of the jet plug with the alloy carb body or the jet will not go fully home when you refit it.

If the carb has to be replaced they are fortunately not too expensive to replace.

steveteigh Tue, 04/10/2016

Hi thanks for that but I can't see any bung

theres just a hole with nothing there

will put some carb photos on

hillsider Tue, 04/10/2016

Now that is just plain sneaky! The carbs that I have serviced all had the jet inserted into the hole that you have pictured. I have stretched the image as far as I can and it certainly looks as though the bottom of the hole is a solid casting so no chance the jet has been lost and the engine would probably not run at all in that situation any way.

This leaves the problem of clearing the jet when there is little or no access to it. Stepping back a little you mentioned cleaning the carb in a U/S tank, when this was done did you remove the main jet and the emulsion tube from the carb before cleaning? This would allow better checks to be made of the  very small cross drillings in the tube and a more thorough cleaning action in the U/S cleaner. Also on some carbs it is possible to blast cleaner through the idle passage from an air inlet to one side of the air intake side of tbe carb.

Apart from not wishing to give up on clearing the problem you may have to accept that you are spending far more time and effort on it than the replacement cost.

steveteigh Tue, 04/10/2016

Yes I cleaned the main jet while out

cant think of anything other than to fit a new carb

hillsider Wed, 05/10/2016

Frustrating as may seem there are times when we have to bite the bullet and fit new. Sometimes you can pickle a carb in the U/S tank as many times as you like but still not clear the blockage especially in carbs such as these that are not exactly built with repairs in mind.

It is a sad fact that in a commercial dealership it simply would not be a viable repair to strip and clean this carburettor as the labour costs involved would be greater than the cost of fitting new. 

wristpin Wed, 05/10/2016

There were very cheap replacements on that auction site - presume still are.