If you are supposed to adjust that pilot screw with the carburetor installed, I suspect you are meant to do it either from above by using a universal joint on a socket extension, with a screwdriver bit attached:
[Linked Image]

or from in front with one or other of these:
[Linked Image]


If the engine over-revs that can't have anything to do with the idle (pilot) system - on a 4 stroke it sounds like a governor problem, not a carburetor one. Interesting that the other engine had a second governor spring fitted, as if it had governor trouble and the tenant made an improvement. Also, I notice the GV150 has an air vane governor, very much like a second and later generation Briggs one, but with a complicated and fraught-looking vane pivot. It sort of all comes together if there is a friction or binding problem in the governor vane or linkage. The vane pivot looks like a good candidate for causing trouble when its really old and corroded:
[Linked Image]

If you want to look at pages from the illustrated parts list, it is available (free) here:
http://www.ereplacementparts.com/ho...ine-parts-c-37657_37658_38010_38030.html


On a different subject, why not clean up your dirty carburetor anyway, since you have another engine?

Last edited by grumpy; 23/10/12 11:18 AM. Reason: Add detail