Dana
“……………There must be something wrong with the carbs. I followed all the carb tuning sequences in the book and on this forum. I might have the floats set a little low, but I would not think that would be the cause of the problem………”
“………….While the “rich on the overrun” is interesting…………”
I can offer a particular scenario that may be at the root of your problem, it has certainly been the case in the past. I think you will also find it is not in any book, or this forum.
I believe your overrun enrichment has been caused by your attempt to tune the carburetters for acceleration, while the float chambers are under depression.
This can be because the overflow pipes are blocked or undersize. However it is usually because the internal serrated fibre washer S,U Part AUC1928 (Jaguar Part 1298) positioned between the float chamber lid and overflow pipe has been squashed, not fitted or there is only one washer on each carburetter.
That is to say that there should be TWO AUC 1928 stacked together on each carburetter. This arrangement was always stressed at the S.U service school. Unfortunately, as far as I know the Jaguar parts manuals were never modified, other operators like R-R, Bentley and Aston made sure that detail was contained in their parts manuals.
Those internal serrations on the AUC1928 are the only path for air to vent the float chambers, if they are crushed by overtightening that air path is further reduced resulting in fuel starvation. In some instances, particularly in North America dollar area, the special serrated washers have been replaced with normal fibre washers, blocking off the air supply. Remember, any owner, or mechanic will receive rebuilt carburetters in part disassembled form and that owner or mechanic will be the one to make the usual fatal error when they attach the overflow pipes. Unfortunately, they remain under the impression that their carburetters are beyond fault……after all they have had specialist attention.
On larger engines like the Jaguar XK the air restriction demands an extra serrated washer is fitted, or even a third washer. The advice at S.U, in the days when technicians knew how these carburetters worked, was to file a “flat” from the underside of the hexagon head on the float chamber cap nut, to form a “D” section. The “flat” section extending down the S.U Part AUC1867 (Jaguar Part 5832) cap nut to the inside of the float chamber lid. This provides a permanent air vent and I believe was a standard feature of Rally cars.
You are frankly wasting your time trying to tune any S.U carburetter without two of these washers on each carburetter and most certainly trying to obtain 14.7 ratios with any carburetter. Those units were never specified using that ratio when new. As someone previously stated they have to run rich on acceleration. Obtaining needle sizes and power output was not done by trying to adhere to 14.7 ratios but by forming power loops to obtain the best MEP and WMMP performance.
I hope that my explanation of a particular scenario may prove helpful, if not, it may explain to some other owner why their car is down on power.