Sparks plugs

[Blue cedar]

Took my plugs out for the first time since purchasing the car. Noted the front plug ( next to fan ) colour was a pale brown , but very progressively working towards the bulk head, each plug was darker and darker in colour. To the extent the plug nearest the bulk head was black, although not sooted up or oily. Can anyone offer some guidance. Is the metering unit tired. Thought they would all be a nice pale brown colour.

 

Many thanks Peter

[Peter Cobbold]

Peter,

I believe its normal - mine ran like that for years. Possible cause is the plenum limiting air flow to rear cylinders.

A search for plugs + plenum might throw up earlier posts on the topic.

Peter

[Rob Salisbury]

Hi Peter, that is a fairly normal distribution of plug colour for the Pi cars, but you can improve things if you are prepared to play a bit with the metering unit. First you need to make sure that the throttle bodies are fully synchronised with regard to the butterflies, and that you have equal air flow to each throttle port, then check the manifold vacuum at around 800 rpm, you are looking (depending on spec, CP or CR) for around 8 to 12 in.Hg.

With the engine set up like that and with a smooth tick over, you may find the improvement in plug colour you are looking for, if not the a simple tweak to the metering unit may work.

On top of the control unit where the vacuum pipe connects is a small black plastic cover held by two small screws, undo these and remove the cover to reveal three concentric lock rings, carefully undo the bottom ring and the whole body carrying the top two rings can move, unscrew the body by maybe half a turn, lock up in this new position put the cover back and try the car. If you are getting manifold vacuum above 8in.Hg you should not need to move the body more than one whole turn to get decent plug colour, if you do need more than a turn then it's time to get the metering unit re-calibrated or refurbished.

All of this assumes that your ignition system is correct, timing correct and you've got a clean air filter.

Another cheat is to run one grade hotter plugs in the rear three cylinders.

Cheers Rob

[Blue cedar]

Peter. Many thanks. I understand. Cheers Peter

[Blue cedar]

Rob. Many thanks. Will get my head round this before delving further. Cheers Peter

[GT6M]

There is another reason, one I found out on Mams car, moer so than Dads.

constant town work, not up to temp for v long, mostly short trips,

can result in the idle passages gett,n slowly blocked with a thick tarry gunge,

stuff ye see in the inlet log, or inside a rocker box no used to clean oil / short journeys

 

the last set, 5-6 were totally blocked up. found this on 2 other sets of manis I got of some scrappers years ago.

Not saying its yer prob, but maybe so,!!!

 

M

Edited November 27, 2014 by GT6M

[GT6M]

and you've got a clean air filter., Says Rob.

 

Ermmm, no quite true wid a PI, as they work back to front compared to other fuel delivery set ups.

 

A new filter will mek a PI run rich, a blocked,n will mek it run weak

another Lucas PI quirk that few realise.

M

[Mk1PI]

One fix for the oil in the plenum from the breather is to drill a small hole at the rearmost , lowest point - you will get a bit of oil on the chassis but that can't do any harm - keeps the plenum oil free rather than the constant small pool they seem to accumulate.

[Blue cedar]

Many thanks for all the feedback. Yes, new filter was fitted a few months back. Original style air box set up. The plenum not doubt has a pool of oil inside! The car will idle forever so I guess it's not running to rich, as it would become lumpy and then start to misfire and stall ? Cheers Peter

[Rob Salisbury]

and you've got a clean air filter., Says Rob.

 

Ermmm, no quite true wid a PI, as they work back to front compared to other fuel delivery set ups.

 

A new filter will mek a PI run rich, a blocked,n will mek it run weak

another Lucas PI quirk that few realise.

M

Hi M, I'm interested to hear the theory on this one? ....... Is a blocked air filter going to produce more or less manifold vacuum?

Cheers Rob

[GT6M]

Its no theory Rob, but a fact.

a clogged filter will mek the links in the MU rise up

as its pulling higher vac, cos filters blocked, so air for induction stroke is less,so vac is greater, follow !!!

 

mind you, its gotta be blocked solid for idle to be affected, but will deffo come in around 2-3K revs and wot throttle

 

Try a little experiment, put a rag owa the log intake, tape it up well,

and set off, it will bogg doon, but pull choke oot, and it will go.

 

M

[Rob Salisbury]

Hi M, interesting, I see the logic behind this, but I still think you need to take the datum point of a clean air filter and set the metering unit up to that, then change the filter regularly, (or fit a K&N and wash and dress it every couple of years), I have to add that I'm still a big fan of the Lucas Pi system and all it's little quirks, even though I no longer have a Triumph with Pi. I have sometimes wondered if I could adapt it to fit my 4A!!

Cheers Rob