Tuning the Carter Thermoquad Carburetor - TQ Dyno Dial Up!

Fired up, the TQ was adjusted for idle speed and mixture, and ran perfectly. The first pull netted us 206 hp at the rear wheels with the tach clocking 4,500 rpm. It seemed a little weak, and a check of the timing showed 25-degrees total.

James provided an acceptable explanation for the wildly maladjusted timing by claiming to having retarded it for a smog inspection, nearly always a sharp move to burn down hydrocarbon readings. We read the light, while James clocked the dizzy to a more appropriate setting of 36-degrees total. Power snapped up to 225 hp at 5,000 rpm. That was more like it. Wide-open throttle air/fuel ratio showed a rich reading, pegging mid-10s to low-11s on the scale. We already had the stock jetting: .092 primaries and .110 secondaries, about as lean as TQ jets come. Interestingly, power was already equal to the level reached by James' previous aftermarket carb. Since the carb already had what were considered quite lean jets installed at both ends, we looked in other areas to trim the fuel curve before cracking the carb open for a jet or rod change.

The secondary mixture of a TQ carb is affected by the wide-open position of the secondary air door, as this varies the pressure drop on the discharge nozzles. The general base setting here is .500 inch as measured from the rear of the choke horn to the edge of the air door held wide open. There is some leeway here to set the door at greater wide-open position to lean the mixture.

Bending the ear on the air door where it meets the stop, adjusts the air door wide-open position. We opened the door to .550 inch and found the curve began considerably leaner, but ultimately returned to about the same ratio by the top of the pull, just into the 11s. Power was not improved.

At WOT, we were fatter at high rpm and leaner at low rpm than we felt would be optimal. Getting more fuel at the front of the curve would be easy to adjust with rod/jet and air door position. We wanted to get the top-end of the curve close by other means before jetting, since the secondary jets were already the leanest commonly available size, at .110. Our next move was adjusting the float level. The float level has a significant affect on mixture, and though we had set this carb to the 2 7/32-inch level listed for stock application, a lower 1-inch spec was common for earlier carbs. Changing the float level to 1 inch leaned the mixture across the board, giving us a much trimmer mixture at high rpm, reading just under 12:1 at the top of the curve. With the lower float level, we were getting closer up top, but close to critically lean at lower rpm. To compensate, the air door wide-open position was returned to a normal .500-inch setting. With these changes, we had our best pull yet, recording 228 hp at 5,100 rpm.

It is commonly believed the secondary is so disproportionately large, that WOT mixture is negligibly affected by what is going on in the primary. To put that theory to the test, the auxiliary air bleed block-off plate was removed. Interestingly, we found the mixture was leaned out substantially throughout the WOT pull, with our high rpm ratio now moving to the mid-11s, about one-half a point leaner. Power was up to 226.4 hp at 5,100 rpm. The lower rpm mixture was affected even more, leaning the mixture 1 1/2-2 points, into the 14:1 range from the 12:1 range. We were a little too lean at the beginning of the pull and still fatter than we wanted to be at the top of the curve. Toying with the air door wide-open position setting further, to .585-inch, resulted in an even leaner condition down low, and about the same up top. The conclusion here was that air door position primarily trims the front end of the fuel curve.

We wanted to trim the WOT fuel curve further, taking even more fuel from the top. We pulled the metering rods and swapped from 1996 rods to 1966 size. This rod is mildly richer at cruise, but substantially leaner at full throttle (see metering rod chart). The rod change had the desired effect, recording a ratio of 12.5:1 up top. Power was up again with the change to the tune of 229 hp. While tuning for an optimal full-throttle fuel-curve may seem complicated, add in driveability considerations and transient response, and the tuning process becomes infinitely more complex. We determined that an even leaner WOT setting would help peak output, but running the car at part throttle showed it was running at the ragged edge of a lean miss. We needed more fuel at cruise, and were looking to lean WOT still further. A jet change alone would make the curve fatter or leaner everywhere, so here's where a jet/rod combination change can really dial in the mixture. We determined that upping the jet from the present .092 to a .095, while making a rod change from the present 1966 rods to a pair of 2145's would dial in about 7 percent more jet area at cruise, and reduce the WOT jet area by 6.7 percent. This move was aimed at reducing the fuel at WOT, while getting us out of a lean cruise condition. It worked on both counts, smoothing the part throttle driveability under light load, and posting a sizzling 233 hp at 5,100 rpm.