Loss of power in Triumph 6 cylinder

[TriumphDriver]

Hi guys - I thought I'd post this here rather than my usual Triumph forums to get an outside(!) perspective.
My Triumph GT6 Mk2 engine is running like a bag of sh*te. Fresh petrol, new carb diaphragms, new sparkplugs and the timing reset. The engine starts well enough, almost first turn of the starter, but vibrates / clatters / rattles on any kind of acceleration. Cruising is no problem but top speed is well down, and on steep hills it struggles in any gear. Inside the car it almost sounds like an air-cooled Beetle engine on any kind of engine labouring, but yet the plugs are brown coloured and it doesn't seem to be seeking fuel. Ignition is electronic inside the original Lucas distributor and the cap and rotor are both good, only fitted last year so less than 200 miles so far. I changed the plugs on Friday, changed the ignition leads around, reset the timing and changed both carb diaphragms before setting the mixture up from scratch, but it's still playing about.
Anyone recognise the symptoms?

[darrencambs]

Hi there,

First thoughts are that you need to do a compression test. Possibly a broken ring or two could be the problem. It may also account for the clattering sound.

I'd also take the plugs out and rotate the engine by hand, shining a torch down the plug holes looking for any obvious problems. When I rebuilt my TR6 engine, a couple of the exhaust valves had some serious fractures across them that would have no doubt resulted in part of the valve splitting off and dropping inton the bore.

Also an idea to take the rocker cover off and rotate the engine to check the valves / rockers are operating correctly. Unleaded fuel could have resulted in valve seat recession, closing up the valve clearances, and resulting inthe valves not seating fully - the 'aircooled engine' sound could be due to combustion gasses blowing past the valves.

A couple of other thoughts...

Possibly a stretched timing chain? The TR6 has a duplex chain, but GT6 maybe a single chain?

The crank pulley is a bonded 'harmonic damper' on the TR6, so probably the same on the GT6. I've heard the rubber bonding can sometimes let go. This would explain the vibration you speak of, but I would expect this would be pretty obvious as the pulley would ricochet around the engine bay if it had come adrift!

Also, just because the ignition system is new doesn't mean it's OK. Once I'd checked the mechanicals I'd be tempted to swap back to the old points and maybe an old cap and rotor to see if there's any difference. There have been a number of problems with repro rotor arms for these dizzys.

I hope that helps. Please let us know what your findings are.

[TriumphDriver]

It's not the compression - all cylinders are around the 139 / 140 mark, all very close to each other. The valve clearances all appear ok, it's an unleaded head replaced in 2005. Ignition is a lucas distributor running an electronic system inside the cap with a new red rotor arm, the best fit I've seen in any.
Plugs are very sooty, but any weakening of the mixture results in backfiring through the carbs. Having said that there's been a lot of idling in the garage and no real driving recently so this may account for the soot.
I've changed all the plugs and reset the gaps; swapped the ignition leads and am checking for fuel supply, but there's no sounds of a weak mixture and no visible effects - the engine just clatters on load and top speed is a measly 70 mph.
I'm going to try again in a few minutes with more fresh petrol (Superplus 98 octane) and see how it performs now.

[Morrisand944S2man]

I think you still may have the leads the wrong way round! Sounds like you have a bad misfire- i.e. one or more than one cylinder is not working.

Get out the manual and go back to first principles of setting the static timing and MAKE SURE you have the firing order right.

I would also remove the electronic ignition unit- these are notoriously unreliable. Re-fit the points and condensor and a good known rotor arm and she will be purring like a kitten! Many new ignition parts are very poor quality- see if you can get some new old stock ignition parts or simply replace an old set that was working well.

[Morrisand944S2man]

Nice car, by the way!

[TriumphDriver]

It's not the leads - the wall of my garage has it displayed in large letters: 1-5-3-6-2-4 and I've confirmed they're all on the right way.
Ignition is one of the SimonBBC units from ebay, he's always good, but i put on the old 123 electronic version I had previously fitted and got the same symptoms.
However: I"m starting to think it's dodgy fuel, given the link below:
http://www.club.triumph.org.uk/cgi-bin/ ... -new/#num2

Perhaps this Libyan affair is starting to hit supplies? Drained the tank last night with a siphon - how I hate the taste of unleaded!! - and will refill with new 98 ron from a different supplier.
It's a nice car when it goes.....

[tractorman]

I could be well away with the faries, but have you checked if the auto advance weights in the distributor - also vacuum advance if it's fitted? I can't really equate this to the noise - but what you call noisy may be a rattle to someone else!

I've held back from suggesting this until now - others have suggested my first and more logical choices!

[JPB]

Had it not been for your trying the 123 distributor too, I'd have suggested that "SimonBBC"s repackaged and price-hiked Britpart device would be the likely source of the trouble since these things don't like hot engine bays and are very prone to leakage.
Not bad for a tenner, and as stable as the much more expensive Optronic by Lumenition when we tested several systems for another car forum a few years ago, but these Britpart devices - even if fitted with the correct thermal putty on their base and not the sanitary silicon mistakenly advised in the original repackaging by Simon's outfit - really do need a big, airy engine bay or their leaky nature will see to it that they effectively short the coil once the engine's running.

[TriumphDriver]

I find his red rotor arms and the magnetic collar of the unit a far better fit than Aldon's, although as a precaution I swapped the SimonBBC electronic unit for an old Aldon unit I had removed as the collar was too big so the rotor arm fell off... so I'm using an Aldon electronic unit with a SimonBBC collar and rotor arm, and the problem STILL persists. The engine idles perfectly, but on revving it vibrates quite harshly until the revs are allowed to drop off, whereupon it reverts back to a steady idle.
I'll solve it eventually!

[Wicksy]

Oh boy you have really eliminated most of the stuff already but I notice you have not mentioned the fuel pump - is it mechanical or electric?
I had a very similar problem to yours with a modern Mistsubishi V6 which turned out to be a duff emersed tank pump.
Also have you an inline fuel filter in the system? This could be blocked causing starvation at higher demand.
Good luck with it.