Written by Ivan Bailey
F. Avery Tepper

This is the second installment of the TR6 upgrades - Click here for Part One

For this series affordable bolt-on projects were selected to improve the low and mid-range performance of my 1972 TR6. Our last Dyno results indicated that the engine was being starved for fuel at higher RPM and the factory B1AF profile metering needles were at the end of their possible adjustment to provide the additional fuel demanded. During the earlier Dynotuning session, the operator commented on how the rear of the car seemed to move around on the rolling road. So in addition to changing the carburetor needles we asked, what could we modify tor educe the movements of the rear while strapped to the Dyno?

With these two goals on the table,needle change and suspension improvements,we began our upgrades.

Needle Profile Change

The factory needle profile is targeted to provide a balance of performance and fuel consumption. In our modified TR6—although used as an occasional commuter—miles per gallon can take a back seat to smiles per mile. We began by starting and running the car to operating temperature before making any changes. After determining that everything was operating properly we then made the single change, replacing the B1AF needles in the carburetors with new B1AQ needles obtained from Kai at Wishbone Classics.

Figure 1 identifies the differences in diameters of the needle profiles. (We originally planned to evaluate the B1AR needle but decided to try the B1AQ needles first.) Using the chart below, you can easily see that at the last 5/8thof an inch, the B1AQ needles are much more aggressive, however, still provide for decent fuel consumption with non-spirited daily driving.

We worked on the carburetors individually and began by removing the top cover; figures 2 and 3. We then proceeded to carefully slide out the damper assembly, taking caution with the rubber diaphragm which is easily damaged. To remove the needle, we used a small screwdriver and a Stromberg mixture adjusting tool, (part number: 386-310 from Moss Motors or The Roadster Factory p/n: CAT1).

I unscrewed the needle using the mixture adjusting tool, turning it counter-clockwise, and lowering the needle until enough of the needle carrier was exposed so it could be grabbed. Once the needle was lowered enough,the locking screw was removed with a small flat-bladed screw driver.

The locking screw, figure 5, binds the needle into the carrier by locking it via a small channel in the needle carrier to prevent it from turning while being adjusted. Installation of the new needle is just the reverse. When reinstalling the set screw, be sure it is positioned to insert into the channel.

Reassembly is the reverse. Be sure that the tab on the rubber diaphragm is aligned with the machined slot on the top of the carburetor body, figure 6. Once reassembled, our new needle was adjusted by making full clockwise turns until it stopped. We then made 1 and ½ turns counter-clockwise. This established our starting point to begin tuning. (Excellent instructions on tuning the Zenith-Stromberg carbs are available at http://www.buckeyetriumphs.org). The engine was restarted after we completed the second carburetor. Initially, the engine ran poorly and idled very low. Additional adjustment of the needle height, (Turning the screw clockwise raises the needle to enrich the mixture. Turn the screw counterclockwise to lean out the mixture),brought us back to a solid running engine; we waited until our Dynotuning session to make further adjustments to the timing.

Differential Stabilizer Installation

The TR6 rear suspension is a complicated affair which includes sliding splines, numerous u-joints and four rubber mounts that secure the rear differential to the chassis.

It is possible to upgrade the four O.E. rubber mounts to a harder material, polyurethane for instance. Although polyurethane mounts have many advantages over the stock rubber, including less twist in the mount,driver reviews indicate that these mounts resulted in a firmer and maybe even a harsher ride.

It seems this has been an issue with racers for some time and a solution has been developed in the UK which uses an alloy cup to stabilize the stock rubber mount. Alexander Racing Enterprises is offering these components in a kit available on their website. The kit includes two machined alloy cups and mounting hardware, figure 7. These alloy cups replace the large steel washers, figure 8, on rear differential mounts. These replacement cups are made to support the standard rubber mounts to prevent lateral movement.

Alexander Racing Enterprises also claims, “That by holding the differential in place also reduces stress on the front differential mounts, which is a stress area that frequently requires difficult repair work.”

With another Dyno run scheduled, we decided it would be a useful experiment to see if an improvement could be observed by beefing up the rear differential mounts with these alloy cups.

After removing the nyloc nut that secures the factory washers, you get a good view of the rubber that is going to be “wrapped” by the new alloy cup, figure 9. Remember, if you are installing new rubber differential mounts be sure to grease the mounting pin where it slides in through the rubber mount.

After removing the nyloc nut that secures the factory washers, you get a good view of the rubber that is going to be “wrapped” by the new alloy cup, figure 9. Remember, if you are installing new rubber differential mounts be sure to grease the mounting pin where it slides in through the rubber mount.

Completed installation took less than 10 minutes.

DynoTuning Results

Debaise Bros Custom Cycles and Hot Rods in Meriden, CT were scheduled to run or Dyno session. On arrival we backed the car into position on the rolling road. In short order the car was strapped down and was ready to begin testing. The first test run with the new needles provided numbers of 98 rwhp and 104 of torque. These numbers were dismal and way below our final and best Dynotuning results obtained during our previous winter session. As we leaned out the mixture, the air fuel ratio improved and the engine began to make more power. In tuning the engine we strove to keep the air\fuel mixture between 12.5:1 and 12.75:1. Our previous Dyno runs using the B1AF needles resulted in horse power and torque figures of 103.76 and 122.73 with an air\fuel mixture of just over 13.6:1. Our mixture was almost perfect from 3500 RPM on up,and much better than the B1AF needles.

In conclusion, our original plan was to make the change to the BA1Q needles and evaluate their performance. After consulting with Kai at Wishbone Classics who noted: “Overall, while I think there is some improvement to be found by further refining the AFR, I don't think you're going to find too much more. 12.3:1 is reasonably close to a max power AFR (12.5 typically being considered the max power AFR….We can also view the rich AFR with the Stag B1AQs this way. The Stag produced about 140bhp with those needles on dual ZS175s. They're rich at the top end on your engine because you don't have the additional air flow to make them work. That pretty much sets it in stone that your engine simply doesn't have the breathing capacity to go any further, which we sort of knew earlier on from what I knew about your cam (not enough lift, and possibly too much duration/overlap). If I recall correctly, your cam is about 290 degrees duration and I can't recall the lift other than it was relatively low. I've just reviewed the flow bench figures you gave me from your head work. There is enough flow there to get past 150hp, and probably past 160hp.”

We agree with his assessment of our tuning data and don’t think there is much more power to find at the top end without additional improvements in the engine. It is obvious that the long-duration Kent camshaft in the car does not provide us with enough lift. The modification to the rear differential mounts eliminated any unwanted movements from the rear and seemed to make some difference; the car hooked up nicely and ran straight and true without any movement on the DynoJet roller.

Therefore, before we can get the carburetor optimally tuned, we will have to increase the lift. Stay tuned for the winter 2012 modifications; we hope to install the new headers that are sitting in the garage waiting to be powder coated, and explore ways to increase the lift.

But for now, fall has now returned to New England and the weather is just right to be out and about enjoying the autumn leaves.