very rough idle

Roger,  Thanks for the spec measurement but you confused me with the last statement about:  "if it was 10mm or 11mm this wouldn't be an issue".  For my clarification - Do you mean that the nominal 10.5 mm dimension you gave also works within the range of 10 to 11mm?

Hi Larry

It sounds like you have a very old engine and perhaps some of the components are just no longer any good.  Such items would include the carburetor parts including the carburetor sockets.  These are 5 year life regardless of hours running.  The rubber bits time out, they get hard and fail just by sitting for so long.  The list of parts that require change at the 5 year time is found in the Line Maintenance manual, latest edition.  Roger is correct in that you need to get a good balance done also.  Remember that this engine was made with a max life in years intended at no more than 12 I believe, the time limits depend on your serial and that list is found in the Line manual also. 

The leaks on the carb vent lines may also be that the needles are not seating correctly.  This is as pointed out perhaps just some debris that is working its way out after sitting for such a long time.  The entire engine really needs a good inspection as there are many parts that might have corrosion internally after that long a time.  

Cheers

I'm surprised no one noted the 8 psi fuel pressure reported by the OP. AFIK, this is too high and will allow gas to push past the needle valve. The "fix" to lower the fuel level in the bowl, is quite consistent with having too much pressure; Doing so, up to a point, will cause the float to push harder on the valve, but will likely increase the chance of power interruption, especially in turbulence.

I agree with what Wizard says. Rubber parts just get old and dry and don't function like they do when within their normal life span. They can leak and crack. These rubber parts can be internal or external. Have you had a class on carb overhaul and some of the finer points.Things like the butterfly with the center punch direction, or the choke shaft center punch mark direction, float armature measurement. Things like this all play a part in smooth operation.Have you replaced the rubber parts externally like the rubber sockets or flushed the fuel hose system. Do you have an old out dated fuel pump?

The .5mm is so small that you can find many people can't adjust it that fine and or they get out of adjustment from time wearing on the needle valve or people taking the bowl off and tweaking the armature. Like many parts there are tolerances high and low.A half mm for the fuel armature and fuel level is pretty small.

Jeffery mentioned a post I evidently missed. If you have an 8 psi fuel pressure that is too high. Max is 7.2 psi and most peoples norms are around 3.7 - 5.5 psi depending on several other factors.

With the first high reading I was investigating the possibility of adding a pressure regulator to bring it down as a handful of builders I'm aware of have done.  The other option was to get a new fuel pump but this all drove me to get a decent pressure gauge.  On measuring the fuel pressure again with a better gauge the pressure turned out to be in the range of 4.0-4.5 psi with no pulsations. This reading was within the spec limit, which surprised me a little given the symptoms I was seeing.  My next task is to finish the pneumatic sync of the carbs.