Sooty plugs on 5&6

[matttnz]

Been trying to figure this out for awhile and with a couple of random questions before but I thought I'd crowd source some opinions... I'm barely competent mechanically (read interested and could figure it out but life skills lie outside of engineering & mechanics so unfortunately garage ends up doing a lot of the work on my beastie until I retire & can do a decent mechanics course)

 

I have a '74 CR PI car which has an irritating habit of misfiring under what appears to be 2 distinct situations:

 

Firstly if it hasn't been run for a week or 2 it'll run on 5 cylinders briefly and then after a block or 2 is ok. I assume that's a bit of leak back from one of the injector lines. I can live with this ok.

 

Secondly (and more irritatingly) if I have run the car for any appreciable time in even mild traffic (ie not running around open roads (some would say basically not running in its natural environment...) it starts misfiring and running like a bit of a pig. Pulling the plus shows 1-4 have a nice grey apprearance but 5 & 6 are sooty & cleaning them up brings the 6 back to a smooth state.

On the other hand, if I'm sailing along remote roads and revving freely the engine seems as happy as a pig in mud.

 

Setup:

Bosch fuel pump -draws a bit heavily on the amps but seems otherwise ok. Fatter wire & relay to boot.

K&N filter mounted directly onto the end of the plenum & the rad shroud removed. Assume that in v heavy traffic I would see warmer air to 5&6-hence richer but would it really make that much difference??. I could revert to stock if needed once I dig out the pipe from the original box to the plenum.

MU hasn't been played with for 15-20years I understand

Nylon pipes from MU to injectors (haven't pulled the injectors to check spray)

New pertronix & a slight improvement following it's installation but no cure. Points much the same/worse.

HT lead resistance ok.

Guys at (Triumph savvy) garage say spark is ok, fuel line pressure ok. IIRC the PRV was deemed ok.

I pulled the plenum off & the butterflies appear (to untrained eye) to open symmetrically.

Always run 98 octane which is the richest available locally downunder.

(I think that's everything...)

 

Any thoughts gentlemen (& assume the occasional lady despite what James May says)? Garage suggested injectors but if both 5&6 I'm wondering if its an issue in the common whatever-the-thing-that-looks-like-a-twin-choke-carb-but-isn't. Otherwise I'm wondering what else we could be looking at (short of a recon MU).

 

Or: is this just normal TR6 activity & I'm just expecting too much (although my old Hillmans & Austins of student years ran more reliably...)

 

Help! (and thanks in advance)

 

Matt the kiwi

(PS: please don't refer me to shops/people on the other side of my world!)

 

 

 

[RogerH]

Hi Matt,

haven't much clue on the TR6 but your running is not normal.

Have you tried swapping 5&6 injectors with 1&2 and see if the fault remains or moves with the injectors.

 

Roger

[Steves_TR6]

At idle cylinders 5&6 don't get much air, because the air bleed for idle is all the way down the other end of the manifolds.

 

I've read that some people run a slightly hotter plug in 5&6 to help, another approach is to install a second air bleed valve in the manifold for 5&6 to balance things out.

 

I will one day try the second option, when I can find a spare bleed valve!

 

Another suggestion might be to check the balance of you throttle butterflies, perhaps 5&6 are opening a bit late?

 

In summary it's probably a disproportionate lack of air to the rearmost manifold.

 

Hth

 

Steve

[matttnz]

Ok...good start & can try swapping the injectors as well.

How do poor injectors make it worse? Poor flow should = relatively lean or is the issue a poor mist = incomplete burn?

 

Have put hotter plugs in all around previously, do 1-4 need to be cooler?

[john.r.davies]

You only need hotter plugs if the OEs show signs of fouling.

And "Grey" plugs 1-4? Sounds a bit lean to me, pale brown shows correct mixture.

 

With engine warm and running, and a large tin or other container to catch fuel to hand, pull each injector in turn and observe the spray cone. It should be a complete cone and very fine. Then turn off and pull them in turn again. Any dribbling? Indicating a poor seal and possible over fueling.

The fineness isn't a factor in how well the fuel burns - the PI system isn't injecting straight into the chamber, nor accurately timed. The Bosch Jetronic system that came next injected ALL THE TIME, and not towards individual bores, just the main inlet, and that superceded Lucas Pi because it was much simpler, cheaper and worked! A fine cone indicates a properly working injector, no wear or blockages.

 

John

[matttnz]

Pale brown would be a more accurate description.

[astontr6]

Hi Matt !,

 

If your engine is not starting up on all cylinders it usually means a leaking injector(s) 'O' ring(s). When you switch off the engine some of the petrol in the injector line(s) drips pass the 'O' ring. On start up the fuel line has to re-pressurize.

This is sometimes very difficult to detect! It can be detected by feeling the injector line pluses 1 1/2 inches from the injector, on start up only. 2 people are required to do this check! No pulse in that line means this is where the fault is Or by pulling out each injector and check spray into glass jar looking for any very small air bubbles or droplets in the cone spray pattern.

Also the non-return valves in fuel lines / 'O' ring seals (2 & 5) can show a similar problem if filter gauzes are still in the injectors as they catch the bits of '0' ring seals as they break up. The injector will not spray properly. Re-con injectors do not have these filter gauzes in them.

 

N.B. A leaking injector can give a rich mixture on that cylinder and compound the problem. on 5 & 6.

 

Good Luck,

 

Bruce.

[Peter Cobbold]

Its a common problem 5 and 6 running rich.

Had it myself though not to the point of the engine stumbling.

""Secondly (and more irritatingly) if I have run the car for any appreciable time in even mild traffic (ie not running around open roads (some would say basically not running in its natural environment...) it starts misfiring and running like a bit of a pig. Pulling the plus shows 1-4 have a nice grey apprearance but 5 & 6 are sooty....."

When trickling in traffic the air in the plenum chamber enters the front cool and gets hotter. The longer the air spends in the plenum the hotter it gets - so 5 and 6 are fed the hottest air. Hot air is less dense so the mixture is richer.

When running on the open road the air flow through the plenum is faster ( more horsepower ) so there is much less time for the air to heat up

 

The plenum will be heated by the radiant heat from the exhaust so fit a reflector sheet of thin aluminium between the two. Or as a test wrap it loosely in a couple of layers of alumnium foil.

 

Peter

Edited August 15, 2015 by Peter Cobbold

[john.r.davies]

Pale brown!? OK!

 

The difference in plenum flow at idle and speed is remarkable.

Sparing the arithmetic:

At idle, say 800rpm the air entering the plenum travels at 5mph

At 6.5K it comes in at 40mph, ignoring the car's velocity.

 

At that speed, the relatively narrow, 3" plenum will exert some drag on the air as it travels to the rear.

I made myself the biggest plenum I could, 6" wide. Ignoring the front were the air comes in, which is an orifice with different aero effects, this will halve the air velocity.

But drag varies as the fourth power of the diameter of a tube, so that the drag on the air travelling through the wide plenum will be one sixteenth!

Those rear cylinders will be able to breath more easily!

 

John

Edited August 15, 2015 by john.r.davies

[Peter Cobbold]

 

The difference in plenum flow at idle and speed is remarkable.

Sparing the arithmetic:

At idle, say 800rpm the air entering the plenum travels at 5mph

At 6.5K it comes in at 40mph, ignoring the car's velocity.

 

 

John

John

The difference is even greater than that, as air flow follows horse power not rpm

Tickover will be around 2hp, while 6.5k at wot say 180hp

So if air flow is 40mph at full power it falls to around 0.4 mph at tickover.

 

A lot hangs upon when Matt made the plug inspection - after a period at tickover ? or a 'plug chop' ( kill the ignition at full power and coast to a halt. Dont do that with a steering lock!! )

Peter

Edited August 15, 2015 by Peter Cobbold

[Guest ntc]

Funny this as 5 and 6 run to hot if the block is full of crud

[Peter Cobbold]

Funny this as 5 and 6 run to hot if the block is full of crud

Neil

Hotter running cylinders will take in less gas volume so the standard squirt of petrol will make for richer running. Heating of the the intake charge, before the inlet valve closes, will be worse at lower rpm. I can't put numbers on it though... Peter

[Guest ntc]

Neil

Hotter running cylinders will take in less gas volume so the standard squirt of petrol will make for richer running. Heating of the the intake charge, before the inlet valve closes, will be worse at lower rpm. I can't put numbers on it though... Peter

You got it Peter been there worn the tee-shirt

[matttnz]

Ok, so you guys have been busy whilst I've been away sleeping (and watching the All Blacks demolish the Aussies...Go ABs!). Thanks v much.

 

So: I'm getting out of this something I suspected previously, namely that 5&6 are getting hotter air=rich mix=fouled plugs at lower revs. Next thought is: why this car more than other PIs? I'm assuming (as Roger confirmed) this is not normal behaviour. Is a leaky injector into the mix likely to be the culprit? I have the hesitancy on startup after a long time sitting-is that more likely a dodgy banjo or leaky injector? I'm assuming the latter could cause both problems? (apologies for any idiot questions-I'm medical, not mechanical)

 

Question is what to do about it:

I can try to source the pipework connecting the old airbox to the plenum and reinstate the stock breathing setup & radiator cowling. Will take a little time to arrange. The current setup (since PO in 2009) is a conical K&N straight onto the end of the plenum & no radiator shroud . Does anyone else have this setup? Any issues?

Swapping injectors: Put 5&6 onto 1&2 best option? (any advice on dos & don't here or is it a fairly idiot proof procedure?). According to the History file they were last reconditioned in 2009.

The heat shieldy thing over the manifold (stock). Advice & experience welcome here too but I'm assuming if it's not stock it shouldn't be needed to fix this issue?

An extra air bleed valve onto the back although that looks like it may lead to balancing issues?

 

Would running a slightly faster idle help (assuming increased airflow being subject to less heating-it idles happily at 600rpm, at least until the plugs foul)?

If this doesn't work am I next looking at reconditioning the MU? (looks like this was last done in 1998 by TRBitz before it emigrated to NZ)

 

Appreciate the assistance guys (although I may have lost the Aussies help here now...)

[Peter Cobbold]

Mine ran 5 and 6 richer than the others for 100k miles, standard, and swapping injectors front to back made no difference. But it was not to the point of fouling plugs.

Try a couple of hotter grade plugs in 5 and 6.

 

Peter

 

and check butterflies are opening in sync with front ones.

Edited August 15, 2015 by Peter Cobbold

[matttnz]

Have the hotter plugs in already.

(plug inspection was on return from a run across town running lumpy-not after idling at rest but not free flowing traffic either-a bit stop start)

[astontr6]

HI Matt!

 

My suspicions are still that you have injector problem(s) to start off with! But if your M/U is over 15 years old was it fitted with unleaded seals? I assume that you have unleaded petrol on NZ? The other 2 points are, have you done a manifold vacuum check off the servo take off and compression check on all cylinders, all these factors effect mixture. In Roger Clark's TR6 Tuning Book, you don't not remove the rad cowling and if you put a K & N Filter on the end plenum, you keep the ducting and inner air box to flow the air to the K & N. As already suggest by others this may compound the richness.

 

Bruce.

[matttnz]

Thanks Bruce.

 

No idea about the seals, I don't have records of what they actually did-was UK fuel unleaded at that stage? NZ fuel is unleaded-I use additive, but...

Compression ok, Don't recall a manifold vacuum check.

 

This plenum K&N mod looks increasingly likely to at least play a part... and I'm trying to sort an injector loan from the local TR club

[TriumphV8]

 

So: I'm getting out of this something I suspected previously, namely that 5&6 are getting hotter air=rich mix=fouled plugs at lower revs. Next thought is: why this car more than other PIs? I'm assuming (as Roger confirmed) this is not normal behaviour.

 

The MU can not deliver more to some cylinders than normal,

only less and that is always a problem of two directly in

ignition circle followed cylinders.

 

It is essential to understand that dripping of injectors

can only let drip out what the MU devided to that that line.

So all in all these cylinders will run leaner in theory because the dropplets

might not have the time to evaporate fully and leave the cylinders partly unburned.

 

I never noticed the rear cylinders going richer from the principle of air connection.

The connection pipe and especially your CR who has two of them provides sufficient

connection between cylinders to get same manifold pressure and they are constructed

even to do that at part throttle where the difference between throttle opening is much bigger.

 

And there is the base of your problem in my opinion:

The two rear throttles are opened via the middle spindle and all the play

is added to open the plates later/less. You simply have to open them more and thats it!

 

My estimation is that the sync has been done at too less opening or the time of the last

sync is a long time away and the spindles and connectors are worn out in that time.

Edited August 16, 2015 by TriumphV8

[Peter Cobbold]

+1 for Andreas' suggestion of making sure the rear butterfly pair open the same as the front pairs.

[matttnz]

They do to the untrained eye. Any more accurate way needed

[matttnz]

(fat thumbs)...to check?

[Graze]

Matt

 

You are lucky I have no interest in rugby

 

I spent yesterday addressing balancing the airflow to the manifolds on my 6. It's pretty simple. Get your self a weber carb balancing airflow meter from pentastar parts(cost $90)

http://www.pentastarparts.com.au/store/index.php?l=product_detail&p=4654

and follow the manual (around page 174), take off the plenum, measure the airflow at idle (should be around 800-900rpm), no 1 should be around 3, use the adjusting screws to bring them all as close as possible to each other

 

I suspect as Andreas says the linkages are worn, the butterflies not synchronised or closing completely

 

Good luck

 

Graze

Edited August 16, 2015 by Graze

[matttnz]

Lucky indeed!

I'll try to source one of those locally-if not will have to get one sent over from the Western Isle.

[TriumphV8]

.............and follow the manual (around page 174), take off the plenum, measure the airflow at idle (should be around 800-900rpm), no 1 should be around 3, use the adjusting screws to bring them all as close as possible to each other

 

 

In my opinion the manual is wrong at that point.

 

1.) If the butterflies are not fully closed at idle that does no harm as long as the air bleed screw

can be closed that much to establish a suitable idle. Sometimes after refurbishing the manifolds

this can become a problem because the air pops around everywhere till there is some coal

dropped in the butterfly area to seal.

 

2.) The balancing tubes transport the air from a partly opened butterfly to the other 5 manifolds

and the above mentioned is a CR with two tubes.

 

3.) The problem occurs when butterflies open a bit more because there is totally more air passing the

butterflies. This amount is too much that it can be balanced by the two little tubes and from that

in that state the butterflies must be balanced.

 

So the best way is to open the butterflies by shortening the cable and best get something about

idle of 2000-2500 rpm. As the engine makes a terrible noise and gets hot I switch the engine off

between measuring and working on the setup. To let the engine run only shortly I gether

information only with two manifolds and do the setup isolated for those.

 

At CR I take number 3 as the master and compare him with the others.

So numer 4 is the first to look if the spindle is twisted.

Than number 5 is compared to 3 and set and than 5 to 6 again for twisting.

At last 3 is compared and set with 2 and 1 and 2 checked for twisting

....... and thats it!