ORIGINAL: DarZeelon
Einar,
Many carburettors, also from highly reputable companies, like OS and Rossi, have a tendency to run too rich in the mid-range, even if the idle and the high speed settings are perfect. The (wrong) taper on the idle needle is the culprit.
To somehow make the engine more tractable in the mid-range, it may be necessary to make the idle mixture setting leaner than you really want it. So much so that it may occasionally affect the idle to full power transition.
For Rossi there is a carburettor made by MVVS, that made this problem go away. For Irvine, I have not yet heard of a solution.
The problem is not a widely known one, so it must be minor.
I have an Irvine 61 ABC, 61 ringed, 46 ABC and two 53 ABCs. They are all wonderful engines but they all had the rich mid range throttle problem. When I got my first Irvine (the 61 ABC) back in the early 90s I tried everything to solve the problem including sending the engine back for service. The service depot said it was fine but that was not the case.
I must admit that I’m very particular about how my engines run so I may be noticing something that others do not. In the olden days this type of problem was resolved by filing a very small notch at the top opening of the throttle barrel to create an air bleed in the throttle chamber. The air bleed leans out the low speed and mid range mixture and can be tuned to control only the mid range by using the low speed needle valve.
Rather than filing the throttle barrel I just drilled a very, very small air bleed hole that goes from the front of the carb body and enters the throttle chamber about even with the center of the needle bar that is in the center of the carb. The hole is positioned so that it is not covered by the throttle barrel from low idle to about 3/4th throttle. I used a 1/16” hole on the 61s and 1/32” on the 46 and 53s. The best approach is to start out with a very small hole first and the make it larger if necessary.
The attached image shows the air bleed on the 61. The hole is drilled at a downward angle so it enters the chamber at the center line of the needle valve bar in the center of the carb. You will be amazed how much better the Irvine engines run with the air bleed! The carb must be removed from the engine when drilling so that the drilling shavings can be cleaned out of the carb.