The XK engine idling speed is supposed to be 500 rpm at working temperature but my car did not. After tuning it two times at different specialists the idling speed is 1100 rpm after the first and 1500 rpm after the second (I am using premium 98 unleaded gasoline). When I try to low down the idling speed unscrewing the two throttle adjusting screws (two carburetors) it goes down without problems but, when I accelerate and the I leave the gas pedal, the engine stops…
Could this be a problem with the strength of the mixture? I suppose that the tuning specialists think, most of all, on the amount of Monoxide when they tune it.
Sounds like there is an air leak have you tried spraying engine start around the inlet manifold to see if the rpm’s change?
Are you running 9:1 comp? My XK engine (S11 XJ6) runs quite well on 95 and will tolerate 91 (NZ) this is a 8:1 engine.
your symptoms are indicative of some issue causing throttle plates not to return to the same place due to such issues as loose couplings, binding, improper return springs, or incorrect initial settings
follow the FSM exactly
jet depth setting, idle air (and timing) are all interactive
alternatively the throttle shaft/carb bush may be worn and admitting air.
to find out, if the carb has the throttle shaft disconnected from its neighbour, and you try to wriggle it in the body, there should be no detectable freeplay
and yes, make sure there is no vacuum leaks first
Your symptoms do sound like a vacuum leak(s) at the throttle shafts so that needs to be checked and corrected. Having said that, in my experience with a 3.8S with manual gearbox car, the engine seems to be happier with an idle speed of about 650-700 RPM. Five hundred is a bit low unless an automatic car in drive.
agree on Idle RPM,
spray some ether based product around the throttle shafts while idling
try other suspect areas, quicker than dismantle
in addition, I have always found idle speeds creeps up as engine warms
it takes a surprisingly long time to do that, 10-20mins
no xk-xj engine I had would hold 500rpm idle and not want to stumble or stall
1 Like
Let’s be sure whether you have the original SU H6 carbs, or has it been converted to something else?
The H6 carbs have the mixture adjustment underneath, hidden by a covering nut.
The specialist tuning shops may or may not be working with the right information.