TR6 1976 spark plugs turn black

[martinTR6]

Dear Members,
I have an issue with my TR6. After a few months of refurbishing, I want to go to the car inspection so that I can use the car on the road.
The issue I have is that even after a short drive around the house, all spark plugs turn black (see attached pic) and stop firing.
Initially I thought it had to do with using the choke too much but even without choke the plugs turn black after less than a mile.
Original US 175CD-2 carbs installed.

This is  what I already tested and tried untill now:
- messured compression, all cylinders are about 130psi warm
- timing set to 4° advance
- checked that choke is really turned off when engine is warm
- iddle screws (front side) are closed
- disconnect the rocker cover outlet (avoid oil sucking in)
- checked Temp compensator (close at ambient, opens above 50°C)
- set mixture to min (tried also 1-2 and 3 turns clockwise)
- used champion N9Y and N12Y plugs: no difference
- when I do the mixture test described in manual (lift air valve +/- 6mm) engine stalls => too lean? 

Suggestions and thoughts are very much appreciated!

thank you
Martin

[Peter Cobbold]

Martin, I think we can rule out carb faults as all 6 pluga are sooted. The timing may be far too retarded. Many USA 6s have low compression heads where a static timing nearer 18 BTDC is optimal. if that works we might need to invstigate inoperative vac capsules on the disy. Peter

[Mike C]

If you want plugs that will fire reliably regardless of the mixture condition try the 3 electrode type- I use NGK BUR 6ET's. Of course you could be curing the symptom of the problem and not the cause.

[martinTR6]

I have disconnect the retard unit and adjusted the timing to 12BTDC. Result is the same: plugs turn black

I'm gonna test the NGK BUR 6ET plugs tomorrow.

However, I'm afraid that they will not take away the root cause of the problem.

 

[Peter Cobbold]

Martin,  original never refurbished carbs?  check for split diaphragms,maybe both are kaput. commom fault. Peter

[martinTR6]

these are original cabs, refurbished by myself. O rings and diaphragms renewed. Since all plugs are turning black, it has to be something common or an issue with both carbs...

[Peter Cobbold]

"plugs stop firing after less than a mile"

they dont look sooted enough to do that. do they start working again after a cooling down period?  I,m thinking maybe coil. Is it correct for the 6, later 6s have ballast resistor wire in loom and need a coil that runs on ca 9volts. If you fit such a coil to an non-ballasted loom it will overheat. Is coil hot when firing fails? also lots of duff coils around.

[john.r.davies]

If the photo in your OP shows a typical plug, Martin, it is not sooted up!  Nothing like!   Pursue a different diagnosis, along Peter's line.

John

[DRD]

2 hours ago, john.r.davies said:

If the photo in your OP shows a typical plug, Martin, it is not sooted up!  Nothing like!   Pursue a different diagnosis, along Peter's line.

John

They look pretty clean to me!

[Peter Cobbold]

Check, by replacing, the disy condenser.  Also check the short braided wire that  earths the moving plate to the disy body is intact.

[martinTR6]

Concerning the coil: I do use a balast wire, but I think it's a 12V coil. Could this combination result in too weak sparks?

[North London Mike]

Just a quick confirmation (as an oily 4 potter) plugs look pretty clean to ma also. LNK has continued to fire/ run with much worse which, in my mind, points to ignition breaking down elsewhere ? 

[RobH]

43 minutes ago, martinTR6 said:

Could this combination result in too weak sparks?

A 12v coil requires 12v supply to work properly. The primary coil energy will be 25% low if you really are running it on 9V from the ballasted supply.