Spark Plug Colour

[Ian Vincent]

I periodically pull the spark plugs on my car to check their colour and hence the mixture. I always do it after a run when the engine has well and truly warmed through and when it has not had a chance to idle before I turn it off.

 

Why is it that the plugs for numbers 1 and 4 are always showing slightly leaner than numbers 2 & 3? Not a lot but noticeable and if I swap the plugs (1 for 2 and 3 for 4), then next time I check the outer two will still be showing evidence of a leaner mixture

 

I have a TR4a manifold with HS6 carbs so I would have expected a near perfect mixture distribution but clearly it isn't

 

Rgds Ian

Edited September 19, 2016 by Ian Vincent

[Peter Cobbold]

Ian,

One possibility is the firing order: 1342. The manifold is symmetrical in shape but not in time.Fuel that wets the wall as a film will contribute more to the gaseous mixture the longer it has to evaporate off the wall. As folllows:

 

#3 inlet valve gets to see fuel vapour/spray/film that has had 360 degrees to stabilise while #1 and #2 were operating. Stabilising means a wall film of liquid fuel has more time to evaporate.But #4 inlet opens immediately after #3 so the fuelvapour/spray/film has to suddenly change direction in that manifold. And the wall film has less time to evaporate. Vapour and fine spray should respond to the sudden change in air flow but a wall film might not. When #4 opens it sees a manifold partly deficient in wall film so sucks less fuel in leaving it slightly lean. Same for # 1 which will also get less fuel form a wall film than # 2.

 

'Wall-wetting'' is well know problem with one carb feeding multiple cylinders and is cured with a hot-spot to increase evaporation. Rough surfaces inside the manifold improve film evaporation, while a polished surface makes wetting worse.

 

The mal-distribution should be more obvious at wider throttle openings as the air flows past a part-open butterly are sonic and have an important role in smashing liquid fuel droplets into a fog of small droplets, or vapour. I believe the standard TR3 SUs have the buttefly top swinging away from the jet, while a HS6 is more conventional with the bottom of the butterfly swinging away from the jet. This may ( or may not) change the wall film distribution...

 

An additional possibility is that the longer time the mixture has to dwell in the manifold ( # 3 and # 2) the warmer the air gets. This makes it less dense so the mixture will go richer. Also the longer dwell gives more time for liquid fuel to evaporate and the fuel vapour then displaces air, again making the mixture richer. Polishing the outside of the manifold and fitting a heat shield will reduce the temperature of the manfold wall and reduce air density loss by heating. But fuel film evaporation will be impaired

 

Wall wetting can be reduced by improving fuel spray atomisation at the SU jet, by increasing the velocity of air across the SU jet. This can be done by increasing the downforce on the SU piston with a stiffer spring. The needle will then need refettling to restore the correct mixture.

 

Peter

Edited September 21, 2016 by Peter Cobbold

[Peter Cobbold]

Some experimental data on liquid fuel films here, using an SU and transparent runner:

http://people.eng.unimelb.edu.au/imarusic/proceedings/9/Milton.pdf

The article doesn't cover branched manifolds but does show wall wetting with fuel films 0.1 to 0.2mm thick and

the factors that favour fuel puddling. The experimental manifold was very long, I reckon the TR's would be about 200mm between carb and inlet valve.

 

Peter

Edited September 22, 2016 by Peter Cobbold

[RogerH]

Hi Ian,

rather than simply coming to a stop and then turning the ignition off. Keep the revs up 2000+ for 30 seconds or more and then kill the ignition at that rev level.

It may be the idle mixture rather than the main running mixture that is at fault.

 

Roger