My XJ12 will only run if I pull out the spark plugs and clean them, but then won't start again after the engine is turned off for a while. The plugs are getting blacked up very quickly. Does anyone have any idea what is causing this and how to resolve it? Also it runs very rough when I get it going and I get back firing out of the exhaust.

 

From log distance.


The engine is over fueling. Or normal fuel but restricted air intake. Result, mixture too rich in fuel. AKA "flooding".


It is seen in three ways.


1. Black plugs.
2. Removing them allows the over rich air/as mix to escape and
need air to enter.
3. Injectors are leaking in fuel while the engine is off and create the "flood" condition.


Carl

 

Just one man's theory...

74 xj12 should have carbs. The floats might be stuck open allowing the fuel from the float bowl to overflow into the intake stream.

You can test this by getting it running and shortly after killing the fuel pump. If it smooths out before It dies It would indicate over fueling. If so I see carb rebuilding and fuel tank replacement in your future.

 

My XJ6 was doing the same thing...was running too rich (see plug). After adjusting the mixture screw no more problems.

 

Hi mate, Thanks for that. Do you know where the mixture screw is on the V12?

 

This is the tool needed to adjust the mixture on a Stromberg CD carby, and you got 4 of them.

Attachment 203128

This shows the mixture adjusting needle, down the spout where the toll goes and adjust to requirements.

 

Make sure you don't have a leaky float on any of the carbs. Over time fuel can get inside the float, reducing or eliminating its buoyancy. My E-type (w/ Strombergs) had this issue on the front carb some years ago. If you have a sunken float no amount of adjustment will cure the over-rich issue.