Carb Balancing issue
Have just balanced the Carbs on a 912 ULS installed in a BushCat. All balanced up nicely at 3500 and 1900 - needed some adjustment but now all good. I had the balance tube disconnected and long tubing into the cockpit and connected to the 2 Vac Gauges. At idle, if I pull the right manifold tube off the gauge and leave open to atmosphere, it causes the engine to almost die as you would expect and you have to quickly put your finger over the end of the tube to prevent it from dying but doing the same with the Left causes the engine to pick up significantly in RPM when left open!!
The only explanation I have for this is that the Left Carb must be running excessively rich and by opening the tube to atmosphere is sucking in more air allowing a better leaner mixture? It doesn't blow any black fuel smoke from the exhaust and seems to run quite smoothly so this doesn't make much sense.
I was just going to check and adjust the mixtures but unfortunately to do this on the BushCat installation it is basically impossible due to a Stainless Steel Heat Shield attached under the Carbs. You would have to remove both Carbs to check/adjust it and I just can't believe it could be that far out - the engine and aircraft are only 2 years old and 55 hours TIS. Surely it was set properly at installation. The 'only' way to be able to tune the carbs would be to drill holes in the heat shield plates under the mixture screws and even that would be very difficult to achieve in-situ.
The idle balance had to be adjusted only a tiny amount from the nominal settings and I would have thought that a mixture that rich on one side would have affected both the idle speed and balance significantly?
It runs fine and smooth but it really has me intrigued as to why the difference between sides??
Any ideas?
