If you are working with a new carb and having issues with engine not wanting to operate at high rpm something to be aware of. Needle valves and seats are made in several sizes and for a fuel pump, different pressures, and for gravity feed. I ordered my carb from Amazon, it is a mikuni VM28 but other manufactures and sizes of carbs are open to this problem. When i was doing initial testing temporary tank was mounted up close to wing level, approximately 32″ above carb, engine appeared to operate close to normal. Now with my actual tank in place mounted where it belongs if you operate engine above 1700 rpm it runs for a few seconds and dies. It easily restarts but if you go to full throttle when it gets to 1800 rpm it dies, at 1600 rpm it does not run study but will drop off and pick back up. This is the condition during taxi tests. I had enough RPM to get tail up but not to get in air.
A critical step in any new installation or change in fuel lines or carb if s a fuel flow test. It is easy to do, shut off fuel, remove drain plug from bottom of carb and open fuel valve and drain into a measure cup for one minute. First test shows in one minute 4 ounces of fuel, 4oz. * 60 minutes / 16 oz gets to pounds / 6.5 pounds of fuel per gallon gets to 2.3 gallons per hour, gph, Not enough flow, flow should be at least 2 times maxim flow of engine at full power or for a 42 hp engine min. of 5 to 6 gph. We took carb apart and found a small red spec of plastic in needle valve, thought this is it but flow did not increase, We removed the needle valve completely and tested fuel flow, 10 oz of fuel in 1 minute, 5.76 gph, this quickly shows problem is related to needle valve. For reference ORV, the red LEXL, has an identical fuel tank set up, fuel valve, fuel filter, distance between bottom of tank and carb as B69A but it has a VM26-606 carb with fuel flow of 4.8 gph. Steve Kiplinger told me about the two types of jets long ago so we investigated the VM28 carb and find the needle valve installed is for a fuel pump installation, so I have new needle valve and seat on order.
crude in needle valve, but removing thus did not fix problem