Odd spark plug colours

[Lebro]

Just taken the plugs out of a fellow members TR3a (but with low port engine)

Engine was re-built, and has been running since last August.

Runs very well, tightened head down at 100M, now up to 250ish.

 

It has been a bit spluttery when backing off the throttle, then re-applying, so we checked the plugs to see if the colours suggested that the car was running too weak.

 

 

 

What we found on all the plugs was black soot on the outer body, going part way up the side electrode, and covering half of the insulator. The other side of the insulator was white. majority of the side electrode was a light brown colour.

Front two plugs were a slightly lighter brown, so we richened the front carb 1 flat.

At the same time we changed the "3 in1" oil which was in the dashpots to some straight 30 oil.

 

Sputtering has gone, & car now running well, but puzzled by the apparently conflicting colours on the plugs

 

Thoughts ?

 

Bob. (& Mike)

Edited February 28, 2016 by Lebro

[Peter Cobbold]

Bob,

If the plugs were removed after a period of tickover or light-load running then the black sooting on the insulator may be normal. And plug reading will be unreliable. Thus:

http://brianesser.com/main/index.php/tech/9-tech-info/26-the-ultimate-guide-to-spark-plugs.

""The porcelain around the plug’s center electrode can be divided into three areas for reading. The area that is closest to the tip is affected by the idle and transition circuits carburetor circuits and is of no real concern to a racer. If this area is gray then you drove the car back to the pits and you cannot correctly read the plugs. The middle area is only colored when you drive down the road at around a steady 30-40 mph and is normally affected by the primary circuit jetting with the power valve closed and this is really of no concern to the racer. The area you are interested in is that third that is all the way up inside the plug where the sun don't shine. This area is colored when all is wide open under full power because the combustion chamber heat totally cleans off the other two areas. It will take a special plug reading flashlight with the magnifying glass to view it correctly.""

If you did a 'plug chop' ( eg 3000rpm full throttle, engine hot, kill ignition, coast to a halt) then the black insulator suggest its too rich, or the plug is too cold a grade.

Peter

Edited February 28, 2016 by Peter Cobbold

[Rob Salisbury]

Hi Bob, it was much easier to read and interpret plug colours in the old days when we had proper petrol!! but if I pulled those plugs from my engine then I would be looking to richen up the mixture somewhat (or run colder plugs), a light tan or biscuit brown is what I go for, also, hard to tell from the photo but the plug gap looks a bit small to me, 25thou works well for most applications, though with electronic ignition then 30thou.

Cheers Rob

[bob-menhennett]

Bob and Mike

I'd second Rob's thought that the plug gap looks small ( and electrode twisted slightly ? ).

Bob

[Lebro]

Hi Bob / Rob.

Yes agree about the gap in the photo.

All plugs had the same colourings (more or less)

One of the plugs was dropped on the floor, it may well have been that one, & the gap may have got closed up as a result.

They were all set at 25 thou previously. Will check that one out.

 

Thanks

Bob.

[BlueTR3A-5EKT]

Soot from oil blow by on a new engine where the rings are not fully bedded perhaps?

Otherwise the colouring fits with your actions.

Peter W

[Lebro]

Confirmed today No. 1 plug had got a very small gap as a result of being dropped ! now corrected.

 

Bob.