Hi Mel,

You could have a couple of issues, you may be sucking air from the pump and its not an issue untill your revs go up, or there could be an air leak else where, I doubt if changing the pump valves and springs will help, but I have been known to be wrong sometimes. Or there is something as simple as a pin hole in your fuel line somewhere.

Pumps can fail at any time, and having a new engine is no gaurantee of it not happening.
Run the engine and check for air bubbles in the pump to carb line, if there is and there are no bubbles going into the pump from the tank, replace the pump assembly, no if's and no buts, replace the assembly.
Or you can fit two pieces of fuel line to the pump and have each one going into seperate containers with some fuel in them and spin the motor over and watch for bubbles, if there is bubbles replace the pump assembly, you may have to prime the system if the pump is dry or weak.

If the pump is good then also check for air bubbles from the carb to the black injector.

You can test the pump valves to see if they are working, you need a syringe and a piece of fuel tube for this, attach the syringe to one side of the pump and pull on the syringe and push on the syringe, you should have a lot of resistance one way and none the other, then do the other side of the pump and you should have the same thing but opposite, if there is no resistance at all then look at replaceing the valves and springs, but there still can be a leak issue here also. I actually have a assembly that is no good but I cannot find the problem with it.

Other wise you could have a air leak else where and its a case of finding it.

I feel you could do better with your prop choices, try running a 16.5 x 12W APC which should drop your RPM's down also, try for about 8300, your 17 x 12 is more suited to the 170, I only run the 17 x 12 and 17 x 13 on our modified 160's and stock 170's, but, I don't know how the enviroment affects your engines over there, you may be right in your selection.

Will Crossman
DOWN UNDER