As some of you may recall from previous postings, I began to experience engine roughness approximately three months ago, at engine hour 16.
The roughness showed itself at altitudes above 4,000’, and was significant enough that one saw the vibration on the panel, and felt it in one’s butt.
Compression and injector checks were negative. A visit by the TCM rep, and an upward adjustment in fuel pressures provided temporary relief, but the problems surfaced again two hours later. Next, the fuel pump and metering unit were replaced, followed by compliance with TCM Sid 97-3. Again, vibration! This time the vibration manifested itself any time the power was reduced to a cruise setting, at all altitudes above 3,000’. Talk about frustrating.
Despite carefully adjusting metered and non-metered pressures consistent with the Sid, I decided to make another attempt at adjusting fuel pressures, this time using very precise numbers provided by Chris Dixon.
At 600 rpm, unmetered pressure was set to 7.2 psi, metered pressure 3.5. Idle cut off should provide a 25-50 rpm rise.
At 2650 rpm, unmetered pressure was set to 21.5, metered to 14.2.
Each adjustment required going back to the other settings and fine tuning.
The results worked, and my three months of frustration ended. The engine is still not as smooth as I’d like at cruise, but certainly acceptable.
What can be learned, other than TCM has “issues” with this engine and its aneroid? Simply this: one can comply with the Sid 97-3, and still have problems. The required adjustments are far more exacting than the TCM bulletins. I hope they amend it with better information. Remember, the numbers I used are for Madison, Wisconsin, field elevation 868’.
I think that my early problems were caused by running too lean, as evidenced by minimal temp rises no matter what I did with the mixture, followed by being too rich after pump replacement. Now, like Goldilocks, I hope this will be just right.