Spark plugs - sooty

[NYK]

I’ve been setting up the tick over balance and mixture on a recently fully rebuilt engine, I’m pretty sure it’s not running rich (using colourtune) but during the time it takes to do the adjustments the plugs NGK BP6 get a bit sooty. I can’t think this is right, could I go to a NGK BP5?

[ericbushby]

Hi Nyk
I use NGK BP5 in mine and they stay clean for a year or so. I replaced them last week for occasional misfire.
I could check how long they have been in but I am quite content with them.
Eric in Burnley
1967 S3SE DHC

[NYK]

Hi Nyk
I use NGK BP5 in mine and they stay clean for a year or so. I replaced them last week for occasional misfire.
I could check how long they have been in but I am quite content with them.
Eric in Burnley
1967 S3SE DHC

Thanks Eric and are you just running a standard tune engine?
Nic

[ericbushby]

Hi Nic,
yes it is a SE engine on Webers supposed to be 115HP which I think just means the next step up of the cams ( the two ring groove marks) but without any other mods.
Until I got the mixture settings better, I was running on NGK BP4 in order for them not to soot up very quickly.
best of luck
Eric

[NYK]

Thanks Eric, mine is just a standard big valve so based on your, plus others comments I’m going to try the BP5 which I’ve ordered from fleabay. Under £10 for four spark plugs can’t be bad in this day and age. Interesting to see that you ran BP4 for a while.
Nic

[rgh0]

Thanks Eric, mine is just a standard big valve so based on your, plus others comments I’m going to try the BP5 which I’ve ordered from fleabay. Under £10 for four spark plugs can’t be bad in this day and age. Interesting to see that you ran BP4 for a while.
Nic

They are probably fake on Ebay. By from a known reputable supplier. It costs a little more but they will work

cheers
Rohan

[rgh0]

They also should be BP5ES. The ...ES extended nose style is what is required for the twin cam. Generally a BP6ES is hot enough but if only drving gently then a BP5ES may help

cheers
Rohan

[promotor]

I’ve been setting up the tick over balance and mixture on a recently fully rebuilt engine, I’m pretty sure it’s not running rich (using colourtune) but during the time it takes to do the adjustments the plugs NGK BP6 get a bit sooty. I can’t think this is right, could I go to a NGK BP5?

Are they getting sooty to the point that the engine won't run? Adjustments regarding plugs are likely to give different results once under proper driving loads and conditions.

I agree with the former comment about plugs - buy from a reputable motor factor / dealer. I've even had supposed NGK plugs from a famous parts re-seller where the sealing gasket just fell off!

[Andy8421]

I’ve been setting up the tick over balance and mixture on a recently fully rebuilt engine, I’m pretty sure it’s not running rich (using colourtune) but during the time it takes to do the adjustments the plugs NGK BP6 get a bit sooty. I can’t think this is right, could I go to a NGK BP5?

Plug temperature is optimised for running conditions, not tickover. I have a tuning manual from the '60s that recommends cutting the engine after a run and coasting into the garage to check plug colour as any time spent at tickover will mask the true state of the plug. Any deposit build up from tickover will get burnt off when the plug comes up to temperature at running speeds.

Having said that, with modern electronic management systems, I am quite prepared to believe the plug is just fine all the time, but with a carb it is a miracle that they work at all, so a bit of sooting at tickover isn't a surprise.

[NYK]

I’ve been setting up the tick over balance and mixture on a recently fully rebuilt engine, I’m pretty sure it’s not running rich (using colourtune) but during the time it takes to do the adjustments the plugs NGK BP6 get a bit sooty. I can’t think this is right, could I go to a NGK BP5?

Plug temperature is optimised for running conditions, not tickover. I have a tuning manual from the '60s that recommends cutting the engine after a run and coasting into the garage to check plug colour as any time spent at tickover will mask the true state of the plug. Any deposit build up from tickover will get burnt off when the plug comes up to temperature at running speeds.

Having said that, with modern electronic management systems, I am quite prepared to believe the plug is just fine all the time, but with a carb it is a miracle that they work at all, so a bit of sooting at tickover isn't a surprise.

So perhaps the question I should ask is “does everyone get sooty plugs if they leave their engine idling”?

[rgh0]

The answer is "it depends" on the tuning of your idle jets and driving usage but generally twincams need to idle rich and the plugs will get sooty if idled a lot or driven mainly gently on the idle jets.

BP6ES seem to be a good compromise for everyday use aound town. BP7ES may be best if you mainly high speed cruise or track days or climb mountain passes. Also BP7ES if you have a race engine using normal premium unleaded and normal compression ratio under 11:1. BP8ES if using high compression to 13:1 and racing fuel on the track.

If you dont have a problem with excess fouling and misfires with BP6ES and they just get a bit sooty when idling a lot then you probably need to do nothing. Whether you want to go to a hotter BP5ES plug due to usage rasies the question of idle jet tuning for your usage also.

cheers
Rohan

[NYK]

The engine has done around 50 miles since rebuild so engine revs are being kept low and obviously very gentle driving. Plugs after an 8 mile run were darker than expected biscuit brown, more like burnt biscuits! Tick over is set to 900 rpm with colourtune plug just turning blue, so if anything a little on the lean side. Plugs tend to soot whilst at tick over as I set up engine.

[loueelotus]

Hi I read the is in one of mine élan books I got many, many years ago .. Which states run the car fast then coast to a stop and close the ignition right away and vola you will have the true plug colour.

[2cams70]

I can’t believe how long this post has gone on for! Worried about soot on the plugs at idle? That’s automotive hyperchondria!

[NYK]

I can’t believe how long this post has gone on for! Worried about soot on the plugs at idle? That’s automotive hyperchondria!

I see it as more of a technical conundrum. If one believes that the colourtune plug is a valid means of determining the richness of the mixture at tick over, then why does a correctly tuned engine result in sooty plugs at idling speeds. However I would agree, there is probably little more to be said on the subject.