I’m running an O-320-A3A with an MA-4SPA 10-3678-32 carb. The engine runs smooth (except below 750 rpm; I attribute that to the wood prop). I can’t help but feel I am running lean due to the mixture control having very little travel before an RPM drop is observed. I did validate the mixture control is actuating the lever to both stops.
On the ground at approximately 1200 RPM I did not observe any RPM rise before a drop. In the air at 8500’ and WOT I observed approximately 50-70 dF EGT rise on the three cylinders for which I’m instrumented; I can’t say if this is peak or not, merely that’s when RPM visibly dropped off. RPM rise is either not there or imperceptible on my tach. The one CHT probe I have is on #4 and sees up to 370-380 during a long 100 MPH climb and around 300-320 during cruise.
I’m not instrumented for fuel flow. These temperatures at with ambient temperatures about 55-60 on the ground.
I will be installing a JPI soon so I’ll have an idea of what all four cylinders are doing plus have fuel flow information before making any changes. The lack of mixture authority is what got my attention though. I saw the posts here about RVs running lean and did some research, including calling MSA. They said the 3678-12 is the richer carb for my engine compared to the -32 I have installed. Yet, I saw posts here from the past year from others who called them and were told the opposite.
My theory is that this is one of two things - either my carb is running lean or I have one cylinder running very lean and causing an RPM drop off while the others are still rich.
Any thoughts or other avenues to explore here?
On the ground at approximately 1200 RPM I did not observe any RPM rise before a drop. In the air at 8500’ and WOT I observed approximately 50-70 dF EGT rise on the three cylinders for which I’m instrumented; I can’t say if this is peak or not, merely that’s when RPM visibly dropped off. RPM rise is either not there or imperceptible on my tach. The one CHT probe I have is on #4 and sees up to 370-380 during a long 100 MPH climb and around 300-320 during cruise.
I’m not instrumented for fuel flow. These temperatures at with ambient temperatures about 55-60 on the ground.
I will be installing a JPI soon so I’ll have an idea of what all four cylinders are doing plus have fuel flow information before making any changes. The lack of mixture authority is what got my attention though. I saw the posts here about RVs running lean and did some research, including calling MSA. They said the 3678-12 is the richer carb for my engine compared to the -32 I have installed. Yet, I saw posts here from the past year from others who called them and were told the opposite.
My theory is that this is one of two things - either my carb is running lean or I have one cylinder running very lean and causing an RPM drop off while the others are still rich.
Any thoughts or other avenues to explore here?