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Triumph TR6 - hesitation

I'd appreciate any thoughts on a problem that's driving me nuts..

Car is an early 1974, 42K miles, bone stock w/ all emission controls in place and appearing to function (incl. egr, thermo switch, vac retard, evap cannister, etc.). Carbs are newly rebuilt and the car has new points, cap, rotor & condenser, TRF 'repro' green plug wires, NGK iridium plugs and a Bosch 'red' coil.

The car hesitates under moderate to full acceleration at low RPM's, <1,800+/-, in the higher gears. The problem worsens as the car warms up.

I noticed the problem developing a while ago then it worsened. I checked & adjusted the mixture using the lift the air valve method and it seemed to be lean even with the needles at the rich stops.

I also flushed coolant and noticed that the Fail-Safe thermostat was locked open...I though the car ran cool ever since I put it in. Replacing that brought the car to a proper temperature but exacerbated the hesitation problem. I've never had an overheating problem.

I had the carbs rebuilt by Apple Hydraulics (it was time aside from this problem, nice job by the way) and the mixture still seems a bit lean, I'm close to the rich stops, but the plugs are OK, light grey/tan w/ light deposits. I checked and didn't see any obvious intake leaks (brake booster, vac retard system, line to carbon cannister). The hesitation is still there, but much less and, except for that, driveability w/ the rebuilt carbs is excellent.

I am not sure that the vacuum connections to the thermo switch are correct, does anyone have a diagram? My switch has 4 ports. Not a lot of good reference material for that.

I can't think of what else to check and I am leaning back towards the ignition system, but aside from the diustributor itself, which seems fine, and the thermo switch connections I can't think of anything else.

Thank you.

You're right . . . it's probably ignition-related. So:

1. Ditch the green 'boutique' cables; they were a source of a lot of my ignition problems a few years back. Quite honestly, I'm disappointed TRF still sells them with a straight face. I run magnecor KV85's and recommend them for the life of your TR.

2. Ditch the iridiums (sorry!) and put the venerable Champion double coppers back in.

3. Do a visual inspection of your cap/rotor for carbon tracking or cracks.

4. Hook a timing light up and verify that your distributor is advancing smoothly and your idle timing is somewhere near 10 BTDC.

Rick O.
72 TR6
Rick Orthen

Hey, Wayne. A couple things come to mind...
1) The damper oil could be too thin which allows the carb piston to open too soon on acceleration. This would result in too lean a mixture and possibly the hesitation. You might want to try 10W-40, 15W-50, or a mix of the 2 in the carbs.
2) The float level in the carbs may have been set on the low side during the rebuild. That could give lean conditions even with the needles close to the "full rich" position. A solution is to pull the carbs and close the float gap 2mm at a time to increase the level.
3) Do you have an external fuel filter in the system and when did you change it last? If it's beginning to plug the carbs can run "starved". It's definitely time to replace it if the engine begins to miss at high rev's in 1st & second gear.

Brent B


I recently had a similar situation and in my case it turned out to be a combination of timing and a worn distrbutor shaft causing the rotor to hit the contact points inside the cap just a little late. Over time it wore a couple of the the corners down at the contact points and when it got hot or I demanded power it would end up missing and leaving me with only 5 or 4 cylinders firing.

You can only really see the indications inside the cap if you take it off and look at the inside with a good light.

Replacement of the cap and adjusting the timing helped a lot and switching to a Petronix igniton to compensate for the age induced wobble brought the system back to 100 percent.

Shall be interested to know what you find out.

S.E. Paling

Solved! The distributor cap itself was the culprit.

After going through the fuel, intake and ignition systems I went back again and followed the spark path.

The distributor cap contacts were partially fouled w/ plastic from the cap itself. When I first installed the cap, new Lucas, I noticed that the molding around the inner contacts was pretty sloppy w/ bits of cap material standing proud of the contact faces, some on the face, etc. I removed some w/ my findernail but didn't think much of it (figured it was a new part). When I took a real close look yesterday, what I first took for normal contact wear/oxidation was actually a smear of cap material across the contact face...essentially a dielectric layer across parts of three contacts... Swapped back to an old Lucas cap I had around and its a new car!

After confirming it was the cap, I threw out the TRF repro wires, some had weak grips on the plugs, didn't hold the dizzy cap too well, generally weak terminals. Certainly not TRF's usual standard. Thanks Rick, I'll check out the Magnecores.

I also have to say I am a not quite happy w/ the "new" Lucas quality as of late....

Keeping the iridiums though, when I was diagnosing this, they certainly reduced the misfire which I'd expect from a fine wire plug...lower voltage requirement for firing. They don't add performance, no plug will, but could help in marginal situations; weak spark, less than perfect mixture, etc. and are very resistant to fouling. Certainly helped on the Piper (different conditions though, leaded fuel, etc.)

Thanks all!


what iridium plugs are you using?
matt persanis

1 Replace the points ignition with a Pertronix Ignitor module, about $100, fits inside the distributor cap, & is easily fitted in 30 minutes.

2 Replace the distributor cap.

The first fix cured my 74 TR6 hesitation when fitted with twin Strombergs. The second fix cured a recurrent hesitation after I fitted twin 2 inch SU carbs.
David A Webb

Matt, I'm using the NGK Iridiums, ordered them through NAPA. They came pre-gapped (spot on) to .025. A little pricey, but worth it.

500 miles since I swapped distributor caps and no problems.

wayne, do you know the number of the plugs? I would like to try them, MAtt

I believe they are BPR6EIX. W

This thread was discussed between 29/08/2005 and 14/10/2005

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