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Triumph TR6 - PI mixture (gremlins)

After getting frustrated with a progressively increasing misfire over several months, I have come to the conclusion that there is a 90% chance that there is something amiss in the metering unit.

The strange thing is that there appears to be a different mixture setting for each cylinder!! This conclusion is entirely due to the spark plug visual clues - Cyl #1 is slightly lean, where as #6 is rich. The most puzzling thing is that the lean-to-rich increase is progressive through the cylinders!!!!! (2 is about right with 3 being slightly richer than 2, 4 is richer than 3 and so on.

My basic knowledge of the way the metering unit works tells me that this cannot(!) happen - in theory anyway.

Two things come to mind - restriction in the manifold air box flow that somehow gets less towards the rear of the engine thereby increasing fuel/air ratio, but I'm confident it is not that. Nothing has physically changed in that area and I use a good K&N air filter.

The other thing is the bung seals - is it possible that these may be the problem? - can unleaded fuel (or anything else) affect them and if so has anybody come across this little horror before?

The metering unit was recond a few years before unleaded was introduced here so it may be that the seals are not suitable for unleaded use. I use a lead replacement additive but I have no idea how this affects them. I don't want to have to send it in for an expensive make-over if it is not the problem.

The misfire is certainly caused by the plugs fouling (carbon buildup - no oil) but I have thoroughly checked the ignition system and apart from a dodgy lead, it all checks out.

Timing/advance is OK, injectors overhauled, manifold air balance acceptable, vacuum OK and peak performance about 85% with brand new (hot) plugs. This deteriorates rapidly as the carbon build up starts.

Any experiences or advice would be very much appreciated.


Roger H

Hi Roger

This is a clip from an older article regarding unleaded and the seals.

"THERE IS some evidence to suggest that seals in older fuel injection systems, such as Lucas PI systems on Triumph TR6s, are affected by unleaded petrol. If you get any problems, suitable exchange metering units are now available from specialists. In Sweden, damage to turbocharger vanes has been blamed on sodium-based LRP."

In north america there are vey few PI trs so don't have direct experience. But from other cars and injection I can't see why some cylinders would be affected more than others if seals are the problem?

The only change I can think of front to back is block temperature and air temp in the air intake?

The fuel injection on my Buick has always needed a can of injector cleaner once a month with the unleaded otherwise it tended to clog up fast. Cheap fix if you haven't tried already.

Hope Peter or John U.K. see the post they have a lot of PI experience.

Bill Brayford

Roger - Most on this BBS for the TR6 are from USA or Canada and we don't have more than a few PI TR6's here. I suggest you ask your question on the TR Register Bulletin Board. Most who go to that Forum are UK members and there all always lots of TR6 PI owners who respond there.

Go to :-;CODE=00

Then click on "Register" near the top LH corner and you will get a screen on how to register. I go there quite often. I signed up. It's free and also very professional.

If you don't get the reply you are hoping for, ask Phil Vella, the Tech. Dir. for the TR Register (UK). He has a TR6 PI.

E-mail Phil at :-

Don Elliott, Original Owner, 1958 TR3A
TR Register(UK)Member since 1987
Don Elliott

Thanks Bill and Don for your replies.

I'll try the TR-register link first and see how we go.

Just to update, I religiously use 98 RON unleaded fuel but in addition, tried an octane booster and hotter plugs (Denso equivalent of about RN16yc). They are looking (and feeling) decidedly better - the misfire ie more subdued, but there is still variation and room for improvement.

Who knows - the fuel companies put octane ratings on the pumps but what you get could be anybody's guess. It may turn out to be a combination of factors coming together.

Also a note to Don (I think) who asked in a recent thread about photos of the manifold vacuum setup standard in the early PI saloons - I haven't forgotten!!
I had hope to find one in reasonable (original) condition, but they are a bit thin on the ground. I should be able to get something in the next couple of weeks.

I have also ordered roller rockers from Goodparts but before I instal them and complicate the tuning further, I need to sort out this little hitch.

Thanks again

Roger H


I have TR5 PI with a very similar problem. The missing is usually only up to about 2200 RPM then things sort themselves out. Itís particularly bad when pulling away from stop and at lower revs in second gear. On the weekend I replaced coil, plug leads(I was told they useless), plugs (now iridium) and rotor button (which was breaking down). The problem is much improved but not 100%. In my case plugs 4 and 6 carbon up. I suspect the new ignition bits and particularly the plugs are masking a mixture problem.

Like you I use high-octane fuel with a lead replacement (valve master). Maybe itís the valve master? Ė but that wouldn't explain the problem with only two cylinders.

Iíve only had the car a couple of months Ė the previous owner had it completely re-built (including engine) and bare metal spray etc.

Iíd be very keen to hear if youíve found a solution.

Gary Crutchley

There was a discussion on this very problem on the TR Register TR6 BBS but it was more than two months back and the threads do not seem to be archived. You could try posting again.
IIRC the consensus was that:
1 Its a common problem,the front running weak and the rears rich, even when properly set up
2 Its not caused by unequal lenghts of PI pipes
3 Its likely to be due to the inlet plenum chamber either restricting air availability for the rear cylibders or because those see hotter air. Improving the cold air supply seemed to be the favoured action, but the means was disputed.
P H Cobbold

G'day Gary ;)

I've been using valvemaster as well and over the last week I've had more than one opinion that it's 'not good' - words similar to that anyway!!

Don Elliot kindly referred me to another board - check it out as I got some interesting feedback to my query.;act=ST;f=7;t=1619

I'll use the Redline product at the end of this tank of gas and see how it goes - it was strongly recommended by two people here (I think they were covering up the Redline logo on their T-shirts at the time!!)

Thanks Peter - it has been very informative and may generate some further interest when I repost in a few weeks time with some basic results.

Thanks again Don for the referral - I checked out the pic - fantastic - I must say that IMHO, the TR3 is the most beautiful of all Triumphs ever made!! (I may get some flak for this).

I think my love of the '6 developed around the same time as the hairs on my chest as they had the performance to match the look. There was one in the neighbourhood I was lucky enough to drive one when I first got my licence - sounds a bit like 'the-girl-next-door' syndrome.

Just as a matter of interest - I visited Racetorations in UK some years ago now and Darry Uprichard had/was entering a 3A in the Monte Carlo rally. He told me that he could get 200bhp out of the (slightly overbored) baby - it was a beautiful thing.

Roger H

Roger - Glad to have helped. Darryl gets the 4 cylinder Triumph tractor engine in the TR3 and TR3A up to 2.5 litres - as you say - by increasing the bore.

Ken Gillanders sells these kits in USA and he has modified his "Red Rocket" this way. I went for a ride with Ken about 10 years ago in Ventura, California and he spun his rear tires at 1700 rpm in top gear going uphill.

I bought the last set of original 1991 cc pistons, rings and liner kits from Peter Hepworth in Yorkshire in 1987 and they have over 78,000 miles on them since. It's fast enough for me.

Glad you like the photos.

Don Elliott, 1958 TR3A.
Don Elliott

This thread was discussed between 09/11/2003 and 13/11/2003

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