Running Rich

[Martin Nash]

Having replaced cylinder head gasket on my 1931 Chummy, stripped down and cleaned the Zenith carb, checked ignition etc the engine  seems to be running a little roughly (small adjustments to the ignition make no difference) and the plugs are very sooty indicating that the engine is running too rich. Looking into the cylinders there appears to be small amount of oil/unburnt fuel (not certain what) Understand that it is not possible to adjust the main jet mixture  (apart from changing the jet size). Can any one suggest anything?

Regards

Martin Nash

[Stuart Giles]

Unless someone has either drilled or changed the main jet, over rich mixture is usually caused by the fuel level in the float chamber being too high.

If this is the case there is usually a bit of a leak from updraft carbs when you turn off the motor and close the fuel tap; this indicates that the fuel level is above the main jet level rather than just below it.. You adjust the fuel level by moving the needle up or down on the collar which is connected to the float levers.

[Martin Nash]

Stuart

Thanks very much for your advice. I will get into this later today and report back

MN

[Reckless Rat]

When you say you have replaced the cylinder head, what have you replaced it with? If it's the later Ruby type with 14mm spark plugs and a two bolt fixing for the top water branch then your problem may well be plug oiling and not a rich mixture. The design of this head places the plugs right over the edge of the bore and if there is any wear it can allow oil to foul the plugs. A hotter range of plug, such as a B5HS can help.

[Martin Nash]

Thanks for this which is very interesting. Actually what I wrote was that I had changed the head gasket, however having just checked it must be a Ruby head as it has two bolts/nut holding down the water inlet pipe. I can see that the plugs are located exactly as you describe. Wii get some hotter plugs and see if that helps.

Also Stuart Giles suggested that the richness could be caused by excess fuel in the chamber. I have lowered the fuel 'thingy' by 40 thousandths (by adding another washer). Will see if this makes a difference. Engine is sounding sweet at tickover but seems to get lumpy at and above  2500 revs. Am thinking this might be a another issue.

Thanks for your help

MN

[Malcolm Parker]

My experience of fitting Ruby cylinder heads to standard 'unknown' engines is that they become rough and suffer from plug oiling.   Reverting to the earlier cylinder head restores the status quo.   Rather than fit a Ruby head, I would consider a Ricardo aluminium head with the 18mm plugs, this would give you the increased power the Ruby head should provide without the problem of oiling plugs.

[Ivor Hawkins]

You beat me to it Malcolm.

I had a Ruby high compression head on my otherwise standard motor in the Chummystein and it was quite harsh compared to the old low comp head...I returned the low comp head to its rightful place and the engine ran so much sweeter and if anything, the performance improved!

[Malcolm Parker]

That's because four low compression bangs are better than three high compression ones!

[Nick Turley]

As is mentioned by RR earlier in this thread, you need a B5HS plug definitely not a B6HS, this is because of changes in modern petrol.
In fact if your engine is worn you might need a B4HS plug.
Do avoid the plugs with the resister built in.
These days once you have a plug misfire you may as well throw it away unless you have a plug media blaster or you are prepared to mess about with oven cleaner..

[Martin Nash]

Thanks to everyone for your comments and advice. I discovered that the fuel valve is leaking and hence probably more petrol getting into carb than should.

Also I have got a 'spare' cylinder head of the older type with 18mm plugs which I will fit.

Much appreciate the help

Martin Nash