Plug fouling

[Wilf2019]

Hi all,

Since rebuilding my A65 Thunderbolt engine and going through the running in process the spark plugs are seen to be sooty when removed.

The engine has been rebuilt with new pistons, rings, honed bores, valves and guides etc.

Since rebuild the engine has covered approximately 275 miles.

I have a Pazon ignition and have strobe timed it to 32 degrees BTDC at 3000 rpm as per the instruction manual for when running in.

The carburettor is a new Amal 928 set to the manual specification.

I have set the mixture screw initially to 1.5 turns out and found it needed winding in very slightly to achieve the highest tick over RPM.

The bike does seem to wet sump as the oil level in the tank drops slightly if left for a few days.

However, the bike seems to run very well and pulls well through all the gears.

What is the likely cause of the fouled plugs? Could it be oil passing the rings, or running too rich?

Thank you in advance for any input.

[EDDIE SIMPSON]

hi wilf,
assuming the plugs or ignition system, or carb wear, flooding is not to blame.
then concentric carbs have a fixed pilot jet and so to weaken the mixture you need a larger throttle slide.
fit a choke  and a slide that needs the choke to start and will restart easily when the engine is hot. normally a 31/2 or a 4.

[cdsdorset]

Is the whole plug sooty? With modern fuel you only really need to check the colour of the centre core and earth electrode.

[chaz]

bear in mind, a rebuilt engine will need time to bed in. oil control rings need to seal, as will moving valve gear. Then there is the re torquing of head etc and sealing of gaskets, so I wouldnt put too much time into worrying about it at the moment unless the plugs were white.
as for fuel, I will only use the highest grade I can get from now on.

[Wilf2019]

The carb does have a choke and a 3.5 throttle valve. The choke is required when the engine is cold.

The whole end of the plugs appeared sooty at first but have now gone a slightly browner colour in the centre.

I have been using super unleaded fuel.

I topped the oil up this morning with 600ml and drained 300ml out of the sump. So the bike has used 300ml in 100 miles.

Not entirely sure that the ignition timing is correct. I statically timed it to 34 degrees BTDC, and then strobe timed it. The strobe was showing that the ignition needed to be retarded approximately 6 degrees on the ignition plate.

At this setting the bike seemed harder to start but run ok. This is where I have been running the bike so far.

Just as an experiment today I put the ignition plate back to the zero position, and the bike starts so much easier and also seems to go just as well.

Would it be best to leave in this position and would that have contributed to the sooty plugs if it was running retarded.

Although I do agree that the engine needs more miles and hopefully the oil rings will bed in more.

I have checked the fuel consumption and this is 55 mpg.

Thank you all for the advice so far.

[EDDIE SIMPSON]

if it was my bike, i would strobe it to 5,000 rpm - 6,000rpm  and try a no4 slide. it sounds as jf your very close to optimum settings.
what ignition system?

[Wilf2019]

I am running a Pazon electronic ignition.

I’m a bit wary of revving it too much to strobe it, with it just running in. 3000 rpm felt bad enough. 😄.
But even if I did that - wouldn’t it need retarding even more that I had it?

I’ve just checked the plugs again tonight after advancing the ignition back to where the static timing was set, and they are now not sooted up on the ends. More of a grey colour as you can see. That’s after about 25 miles.

[Wilf2019]

Attached picture of plugs now.

[idie]

You will not be able to get the same plug colours as before ethanol. The ethanol will stop that. So long as you don't get white tips, very weak or sooty black very rich ride the bike and enjoy it.