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Triumph TR6 - TR6 PI running very very rich.!

I have a 1973 TR6 and it is running very very rich. It has just had a new Unleaded Cylinder head and it seems to be since this was put on. The plugs are very black and the car has now got to the point where it is choking itself up, misfiring and loosing power when I try to go up hills (i.e when I use a little more throttle). It is also sending black smoke out the back when I accelerate hard.

I want to sort the problem myself (The garage labour prices in Switzerland are phenominal.!!)

Does anyone know the things I should check?

It had a new metering unit 2 years ago. Could there be a problem with something stuck in the metering unit?

Any suggestions would be well recieved.


Adrian Gould
Adrian Gould

The PI metering unit will run rich if it is not sensing the depression in the inlet manifold correctly. It should work thus: when the engine is running on a small throttle opening, the depression in the inlet manifold is high ( low pressure with respect to atmospheric)and this lifts the neoprene diaphragm in the metering unit,which pulls the cam upwards. This pushes in the end-stop and so reduces the movement of the shuttle, and reduces fuel delivery. I would have suspected a punctured diaphragm -if the metering unit was old( you can test this easily by sucking on the end of the hose and blocking with tongue- the suction should hold for a few seconds). I guess you might have a problem with transmitting the manifold pressure- is the hose connected properly to the inlet manifold? ( it would be disturbed when you took head off). Also, are you sure the choke lever is returning to the off position - towards rear?
Peter Cobbold

Unleaded cylinder head, eh? Does that mean that you have just started to put unleaded in the tank?

Many specialists offer 'unleaded' conversions, or conversion kits for the METERING UNIT - apparently the seals originally used, or fitted until recently, are not compatible with unleaded fuel. If yours have suffered as a result of the change to unleaded fuel, leaks might make the unit, as explained above, malfunction.

I can't give an opinion on how necessary are these new seals, only relay the many reports, eg in TRaction, the TR Register magazine. The tests Peter Cobbold suggests might reveal if there is a seal problem.

Please report back!
John D

I've just bought a fully restored 1972 TR6. I'm keen to run the car on unleaded fuel if possible, but I don't know what (if any) work has been done to the cylinder head or metering unit in the past. The metering unit was reconditioned in the mid-90's but I don't know if the seals used will be compatible with u/leaded fuel. How big an issue is this? I am surprised to learn that the seals originally fitted are sensitive to fuel composition. Have people really had problems, or is this a myth that has gained popularity. I'm sure that the commercial concerns selling overhauled metering units are only too pleased to reinforce this! Have owners genuinely had problems?

Paul walker
Paul Walker

i have been advised by a well known fuel injection supplier
that all that needs replacing is the seals in the metering
unit(i have had main changed all the other rubber in the
fuel chain is compatable the fact that your unit was
reconditioned in the mid 90s suggests that it may be ok
simon connolly

I have a '68 TR5 with some of the same problems. I've had a gradual deterioration in performance - the car begins to misfire very badly almost every time I pass 3k rpm. It's not a intermittent misfire; It's as if I have an electrocial component cutting out and limiting rpm. The engine operates without problems at lower rpm (but it is a dog to start).

fuel pump is good - replaced couple a years ago. Fitted with cooling coil.
Changed fuel filter.
Changes points and condensor.
Replaced HT leads & plugs.
Checked butterflies.
Choke lever is properly located (with the right gap between it and the actual fuel distributor arm).

The fuel distributor was replaced 24 months ago, (Moss exchange unit). And the fuel injector leads have strong, sharp, pulses, (indicating the injectors are not leaking badly).

My next steps are:
Substitute coil.
Check fuel pump pressure (any idea where to get the equipment to check fuel pressure?).
Check injectors.

Advice appreciated!

Jim (by the way - I live in Texas with a UK spec TR5. I run on unleaded fuel with superblend zero lead 2000 susbstitute. I use 100 octane fuel which I get from a local fuel distributor). As you can see - I lve a challenge!
Jim Fitzpatrick

Jim's misfire above 3000 rpm sounds to me like retarded ignition. Try advancing static ignition by say 5 degrees(crank) and see if running improves- and keep going gradually until its optimal. My blown TR6 has a US-spec low compression head ( 8.5:1)and static timing of 18 degress BTDC seems about right.
Peter Cobbold

Dear Adrian

My friend had a problem with his injection system after it had been serviced in a garage. It mis-fired and fuel was actually coming out of hte exhaust. It turned out to be that one of the injectors needed bleeding. To bleed the injector,
have the engine running, unscrew the Keep Plate, and one by one
pull out the injectors. There should be a fine pulsating triangle type mist.
If not, tap the injector with a wooden object, untill you get a fine pulsating mist, if not the injector might be faulty (be careful not to damage the
injector pipes, and let the fuel spray into a container and not on the
exhaust manifold). My friends car was in the garage for 2 weeks, I cured it in
10 minutes! Please let the website know how you get on.


simon connolly

Hi All,

Thanks for the comments regarding my TR6 running rich. The problem still exists and after a number of attempts I am giving up (not in my nature so I am a little annoyed.!!) and taking the car in to a garage. Hopefully they will solve the problems. In the meantime I will check again the injectors (as Simon Connolly said)

My main problem with the car is still that it starts OK (Not much choke needed, suprpise surprise) and then runs Ok when cold. When it has warmed up it starts playing up. The main problem is that it chokes up when you apply a little more throttle to go up hills (or if you pull away by putting the throttle down a bit quickly) If you are going up a hill, and you don't remove the throttle within a few seconds of it starting the choke, it pours black smoke out the back. If you then remove the throttle and dip the clutch, the engine will stall (Choked to the point of stalling). You then start the engine agian, the smoke pours out and clears and you may be Ok for a mile or so and it will do it again.

I have heard somewhere of delayed fuel regulation but would not know how to prove (or solve) it.

I have checked the Vacuum in the inlet as well as I can, put a new fuel pressure regulator on, timed the engine, lubricated the parts that I can in the metering unit and still no difference.

Lets hope the garage can sort it.! I will update everyone on Thursday 21st Sept

Could it be the metering unit itself?

Thanks for your advice and if you have any more it will be received very well.


Adrian Gould

Are you sure the choke mechanism in the metering unit is not seized "on"? Try driving and seeing if pulling out choke control worsens the rich running. I think you are right to suspect the metering unit- but it not a subtle problem judging from black smoke. There is no temperature-sensitive mechanism in the metering unit, but a cold engine needs richer mixture than when hot, hence problem seems to get worse as engine heats up. Perhaps the shuttle has seized in the metering unit, or is unable to follow the cam fully. It is easy to dismantle the unit and refit it yourself, providing you keep it clean and refit it as it came off(but note: my copy of Haynes manual has the rotor-timing diagram printed upside down) Sometimes gum from evaporated petrol reduces the fine tolerance between shuttle and housing, a common reason why laid-up cars hane more PI problems than daily drivers.
Peter Cobbold


This is the originator of this problem just updating on what has happened. I finally gave up, put the car in a garage and they told me that it was the fuel pump causing all my problems (Not something I had thought of). Basically it was pushing out 1/4 of the pressure it should do and this was totally messing up the injection system and causing the car to run rich through poor fuel mixing, fuel vaporisation and general lack of pressure in the metering unit. I was a bit dubious but purchased a Bosch replacement pump (Great bit of kit.!!), fitted it the the other day and fingers crossed it has solved all the problems. I've only done 100 miles since the change but it has had no problems at all.

Anyway, thanks to all who gave their ideas, I appreciate it.


Adrian Gould

This thread was discussed between 14/07/2000 and 13/10/2000

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