Jaguar Repair: Aux Carb, head gasket leak, vacuum leak

Question
I have an XK150S 3.8. Starts from cold and shuts down when temp' reached. alarming 'spit back' through Aux carb when revving engine - oily looking fuel. Carburetors recently serviced. Has been running OK since service.

Answer
Hi Bill,

The Aux carb takes the place of a choke and supplies a rich mixture for cold starting. Usually a spit back on a quick throttle opening is a lean condition either from a lack of oil in the top pots of the 3 SU carburetors or it can be the thermo switch that controls the solenoid on the Aux carburetor has cut it off too early. Another possible is that the jet setting of the carbs are set too lean. An intake manifold or any vacuum leak can cause this too.

If all that is correct you need to run a "leak down" test to see if you could have a leaking intake valve. A better test I like to use is to remove the spark plugs and use compressed air to charge each combustion chamber by setting each piston at TDC of the compression stroke and in 4th gear with the hand brake on and listen in the carburetors for any hissing sound due to a leaking intake valve.

While at it, listen for any hissing at the tail pipe. (a great test for any valve leakage, even the slightest)While the pressure is on it, open the radiator cap and look for any signs of the coolant raising. (A perfect test of the slightest head gasket leak) These tests don't have anything to do with your "Spit back" but are good tests while the chambers are pressurized.

Howard