Tech Tips: Triumph TR6

[Editor's Note: We ran this story in the July 2017 issue of Classic Motorsports. Some information may be different today.]

Story by Listed Reference • Photography by The Staff of Classic Motorsports

The Triumph TR6 has been a cornerstone of the sports car world since its 1969 debut: strong lines paired with an equally strong engine. These days, however, the newest example is more than 40 years old– and might have an issue or two. Need help righting a few wrongs? We present four experts eager to share their knowledge.

Expert: Richard Good

Good Parts
(610) 777-4457

Extra horsepower is of no use if your TR6 doesn’t handle well under aggressive driving. A thorough rebuild and replacement of all suspension bushings, ball joints, tie-rod ends and trunnions, along with proper alignment, can do wonders for these cars.

Additional upgrades, like uprated bushing kits made using harder material, can offer a big improvement in handling with little sacrifice in ride quality. Solid aluminum steering rack mounts can provide better steering response by keeping the rack from sliding in its mounts.

If you want to go a little further, upgrade the front anti-roll bar and add a rear one with maybe some firmer springs. Finish off the job with some good tires, and your TR6 will handle like a sports car should.

Expert: Joe Curto

Joe Curto, Inc.
(718) 762-7878

Triumphs have long used Zenith Stromberg carburetors; the TR4A first had them in 1965. When U.S. emissions came into play in 1968, however, Zenith went to the dark side and came up with the CDSE type. These carbs were much maligned and disliked, but in truth they were pretty trouble-free.

The carburetor pistons aren’t adjustable, so the only way to change the fuel mixture is to lower the jet. The early cars from around 1967-’71 had the jet set below the bridge by about 0.085-0.095 inch. Unfortunately, with modern gas this setting does not provide enough fuel.

I find that setting the jet to the later dimension of 0.125 inch below the bridge helps to remove a lean surge at speed. You want to use professional tools to move the jets since you don’t want to distort the shape of the orifice. There’s also an adjustable jet conversion available that fits all the CDSE-type carburetors for both 0.100 and 0.090 jets.

The later, 1972-’76 carburetors have an adjustable metering needle. To identify a carb with an adjustable piston, look for two notches found across from each other on the steel tube that serves as the damper oil reservoir. This is where the special tool mounts.

The later carburetor pistons from an XJ6 Jag, V12 Jag, Lotus, Jensen, etc., will fit and are a common and relatively easy-to-obtain improvement. You’ll also have to either source adjustable versions of the metering needle or swap the adjuster tops of the old needles over to the original needles.

The old carburetor oil recommendation was ATF–auto trans oil. The TR250 uses a B2Y metering needle, and the TR6 uses a B1AF metering needle. The number is stamped on the top shank of the needle; to find it, pull it down against the spring. The TR250 needle is richer than the B1AF, so fitting them will again help on the lean surge issue. However, they’re usually nonadjustable, so you have to change the tops if using them on adjustable pistons.

Sometimes these cars get a high idle–1200-2000 rpm–which cannot be controlled by the idle screw. The usual suspect is a unit called the throttle bypass assembly, which is located on the righthand side of the carb when the unit is viewed with the air intake toward the rear. The throttle bypass assembly is an emissions control piece designed to keep the engine speeds high during shifting.

For test purposes, you can turn the adjusting screw counterclockwise–to do so, pop off the brass plug on the end of the unit. If this causes the rpm to drop to a controllable level, then the throttle bypass assembly needs to be rebuilt. Rebuild kits are available.

If the engine is running too lean, check out the temperature compensator. It can be found under a long plastic cover, just in front of the bypass assembly. This is also a do-not-touch item. It features a bimetallic strip that has a plunger at one end. When properly adjusted, this device opens and changes the “depression” in the throat of the carb under high-temperature conditions, dropping the piston and thus leaning out the mixture. Once the underhood temperatures return to normal, the temperature compensator returns to the closed position.

The temperature compensator commonly goes out of adjustment on its own, defaulting to the open position and causing a lean condition. Previous owners and mechanics have been known to mangle it and disable it.

A lean mixture can also be caused by an open idle trim screw. This is a horizontal brass screw found near the temperature compensator that’s only supposed to be open during the car’s first 1000 miles.

If you find yourself with a hard-starting car that runs very rich once the choke is off, it’s possible that the choke assembly has been disassembled and then reassembled incorrectly.

The choke unit is located on the left side of the carb. To check the choke’s operation, remove the unit and hold the mounting lugs in the 12 and 6 o’clock positions. The series of holes on the rotary disc should be in the 3 and 9 o’clock positions on the 6 o’clock side.

If the holes are located at the 12 o’clock position–aka the “on” position–then you’ll get a rich mixture even though the choke cable is in the closed position. A disc that’s corroded or not flat can also lead to rich running. On most Zeniths there’s a knurled screw on the choke housing: This is the summer/winter screw. A quarter-turn one way lengthens the screw; a quarter-turn the other way shortens it and thus changes the travel of the cam, which makes the choke setting richer or leaner.

Unlike the choke housings in the XKE and other cars, the TR6’s have a weep hole for some reason. There have been complaints of dripping fuel from this hole. I assume this is residual fuel draining out of the choke housing. I have used the non-drilled housings to solve the issue.

Expert: Carl Heideman

Eclectic Motorworks
(616) 355-2850

Joe Curto has some great tips for the carbs. Here’s something I can add. If you’re using ethanol- blended fuel in your TR6, you’ll likely experience hard starting when the engine is already hot. Many people misdiagnose this as vapor lock. The actual problem is fuel boiling out of the carbs and flooding the engine after it has been shut down. The best fixes are to avoid ethanol-blended fuel (if possible) and to install a heat shield kit (available from several suppliers). The other fix is pretty low-buck: While it seems paradoxical, holding the accelerator pedal to the floor while cranking a flooded engine will usually get it started again.

There’s an old saying that most carb problems are the ignition system. You cannot properly tune the carbs on a TR6 unless the ignition is right. With modern fuels, I find that about 15 degrees BTDC of timing at idle and about 35 degrees at 3000 rpm (vacuum disconnected) offers the best performance. If you can’t get those numbers when timing your TR6, it’s time to send out the distributor to be recurved at a company like Advanced Distributors. Also, most TR6 distributors have a vacuum retard rather than a vacuum advance. The retard was used to pass 1970s emissions regulations and degraded the idle quality. I disconnect the vacuum retard on TR6s.

Have a backup plan: TR6s (as well as TR4s, TR5s, and TR250s for that matter) have a front-hinged bonnet with a cable-operated latch on the firewall. If the cable ever breaks or gets damaged, it’s seemingly impossible to get the bonnet open without damaging the bodywork. We always put a backup cable on TR6s. There are kits on the market to do this, or it can be done with some mechanic’s wire. Either way, it’s cheap insurance.

Bonnet busted? If you’ve ignored our advice about the backup bonnet release cable on your TR6 and the bonnet gets stuck, we do have methods to release it that usually work. The first procedure tends to work if the cable has just stretched or slipped rather than broken, or if the bonnet latch adjustment is part of the problem. Start by pulling on the remainder of the cable and holding it out as far as possible–use a pair of locking pliers so the cable can’t slip back into its hole. Then drive the car on the bumpiest roads around, rattling everything as much as you can. The bonnet will often pop free. Make sure you do this at low speeds. Although the bonnet won’t fly up, it can still react in some unfortunate ways, so we want to be able to stop the car as soon as possible for safety’s sake. Once it’s free, we’ll make the appropriate repair. The second method works best if the cable is completely damaged. Take a screwdriver that’s about 2 feet long and extend it through one of the grommet holes found under the dash–you’ll obviously want to use a hole located near the latch. Feel around for the latch and pop it out. This method is tricky unless you’ve practiced it first. The best way to prepare is to have access to another TR with the bonnet open so you can see what you’re aiming for.

Expert: Paul Dierschow

Sports Car Craftsmen
(303) 422-9272

When considering a TR6 for purchase, scrutinizing the items on the following checklist will quickly sort a good candidate from a bad one. This 10-minute examination is not intended to spotlight the relatively minor faults that might be expected of any 40-year-old used car. Instead, it should reveal most of the basic structural issues that can turn an ill-advised purchase into a looming disaster with huge repair costs. Here are the top 10 danger signs to look for:

1. With the engine off, push and release the clutch pedal. Then, with a pry bar placed between the crankshaft damper and the nearby crossmember, push the crankshaft backward into the block. The amount of backward movement ideally should be 0.004 to 0.008 inch–barely noticeable. Upward of 0.030 inch is common and salvageable with immediate attention. A movement of 0.125 inch indicates that the engine is a goner, as it’s most likely fatally damaging to both crank and block.

2. Look under the car, checking sideways from the front edge of the driver’s door, and notice the angle of the clutch release arm–it sticks downward from the left side of the bell housing. If the arm points straight down, all is well. If it points back from vertical, the bolt that holds the clutch release bearing fork to the cross-shaft has sheared, requiring a transmission removal for repair.

3. With the engine running and your foot very firmly on the brake, slowly release the clutch while using the brake to hold the car in place. Then shift into reverse and repeat. You are listening for a sharp clunk from the differential. If you hear one, then the right- front differential mount stud has broken loose from its woefully underdesigned frame attachment. All cars will need this repair at one time or another, and properly making the repair requires removal of the differential.

4. Open the hood and look down at the front corners of the engine. You’re looking for two steel boxes that are welded to the frame, right where the lower control arms mount. The boxes should be perfectly square and solid to the frame. Frequently they are injured, particularly at the right-rear mounting point. Also, look at the number of shims found behind each control arm bracket: more than five or less than one on any of the four bears further examination.

5. Check the chassis member where each rear control arm mounts. These points are very frequently rusted out and may be indicators of more serious chassis rust.

6. Look carefully at the seams between the body panels. The four fenders should join their related scuttle, tonneau, and front and rear panels with a clean, seam-sealed groove. The fenders should join the rocker panels below the chrome strip in a perfectly flush fashion, with the rear fender attached to the rocker with a small dot of brazing to enforce that flushness. The joint lines frequently disappear with shoddy rust repair methods.

7. From the front of the car, sight down the gap between the hood and fender–and continue sighting that line down the top of the door and along the rear fender joint. This line should reveal a gentle, consistent bulge in the middle, with each side being a perfect mirror image of the other. A lack of uniformity reveals some sort of serious incident in the car’s past, which demands further investigation.

8. Inspect the upper fender swage line. The width should be uniform down the entire length of the car, tapering to zero at the end of each fender. Lack of uniformity not only indicates bad bodywork, but it may also be an hint at what lies underneath.

9. Inspect the corners of the fenders. TR6s have numerous built-in rust traps that, if revealed on the outer panels, most likely indicate similar damage to the related inner panels. Look in the pockets above and around the headlamps as well as below the chrome line at the rear of the front fenders. The rear fenders are particularly vulnerable above the tail lamps and along the edges at the top and front of those panels.

10. With the car on the ground, note the gaps at the front and back of the doors relative to the adjacent fenders. The front gap should have a consistent width; the rear almost never does with an original frame. If that rear gap is twice as wide at the top as at the bottom, that’s normal and only a visual annoyance. If the top gap is triple the lower width, however, the car’s mileage is most likely well into six digits, regardless of what the odometer says.

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View comments on the CMS forums
LanEvo HalfDork
10/30/17 11:40 p.m.

Very useful to me, as I recently got started on a TR6 project. I didn't have much of this information at the time I bought the car, but it looks like a got lucky!

JoeTR6 HalfDork
11/2/17 6:45 a.m.

One thing I'll add.  The rear hubs are prone to fatigue and catastrophic failure.  The stub axle breaks at the threaded end resulting in loss of the wheel.  Given that these parts are now over 40 years old, I would replace them with a newer, stronger alternative as a matter of course.  New hubs aren't much more expensive than rebuilt originals anyway.

wspohn Dork
11/3/17 12:25 p.m.

They also have trouble with the sliding splines in the rear axle shafts - they can bind right in the middle of hard cornering.  Guys racing TR-6s almost always swapped them out for different axles.

1/27/21 2:13 p.m.

I saw where the TR6 metering needle (Stromberg 175 CD) can be fixed or adjustable.  Which is generally preferred?  I currently have what I believe is a fixed needle, but there are not any markings on it to tell for sure.


Purple Frog (Forum Supporter)
Purple Frog (Forum Supporter) HalfDork
1/27/21 3:39 p.m.

I could probably get agreement from Tim.  wink

If only we could travel to Tallahassee and do a brain dump of J.K. Jackson's brain and somehow put all that knowledge in a easily assessable format.  He is like a walking TR6 encyclopedia.   yes

JoeTR6 Dork
1/30/21 9:10 p.m.

The Stromburgs on my '73 TR6 had adjustible needles.  I liked being able to tweak them, but it wasn't a lot of range.  The only downside I can think of with the adjustable needle was that the o-ring sealing the threaded adjuster would leak some oil into the carb as it aged.  AFAIK, the fixed needles could do the same thing.  I would think fixed needles with the adjustable jets from a TR4 might be a better setup.  There are kits to convert TR6 carbs over to adjustable jets, but I never found the need.  That car didn't stray too far from stock, just a slightly hotter cam.

wspohn SuperDork
1/31/21 10:53 a.m.

I never  liked the Strombergs despite the fact that they probably have better mixture control than the SUs (perhaps why they used them to meet smog regs). They were smogged carbs and the later adjustable needle models didn't help all that much.  The rubber diaphragm also introduced another wear/age service issue that the SU didn't have.  But the older SUs are a bolt on so not a big deal to backdate.

murphmi New Reader
6/7/21 12:34 p.m.

When I bought my TR6 30+ years ago it was somewhat of a "basket case", and came with the transmission out and with several other parts in boxes. I think I got it cheap because the PO couldn't get the new clutch to fully disengage because the fork pin was sheared.

That was a fairly cheap and straightforward fix, but the more subtle problem was that he had misplaced the two tranny-to engine mounting bolts that act as alignment dowels, and replaced them with regular bolts. Neither of the two shop manuals I had at the time mentioned that you had to use those special bolts, so when I assembled the whole thing the clutch disengaged fine but continuously "chirped" with the pedal down, until I came across the info about the dowel bolts and properly aligned the throwout bearing to the pressure plate. 

it also had too much crankshaft end play, along with steadily declining oil pressure as the engine got hot, but I was able to drop the pan, change the end play bearing, swap out the main and rod bearings, check the journals with plastigage (all good),  all with the engine in the car. Oil pressure is great now. Also had a body shop replace rotted rear chassis trailing arm extensions, and install the differential mount reinforcing kit. My mounts were just starting to crack, so it was very fortunate timing. 

A tip when replacing chassis rear trailing arm extensions: make sure chassis loading is on the four wheels while the work is done. If just the chassis is supported, when you put the car back on its wheels you'll find the rear door gaps will be all screwed up. That can be fixed (I know!) by loosening the rear body-to-frame mounts and inserting shims, if you can back the nuts off without breaking the bolts/studs, but what a pain-in-the-ass job!

wspohn SuperDork
6/7/21 12:58 p.m.

PS - when you service the rear axles etc., may as well swap out for some new rear suspension springs to eliminate excessive squat on acceleration - the factory ones weren't well specced.

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