My 1953 XK 120 seems to dislike modern fuels. When the engine is cold, it runs fine. When the engine warms up, the idle and slow speed performance is very poor. High speed running when the car is warm seems unaffected. It stalls easily and then is difficult to restart. I installed a Pertronix unit, new ignition wires, replaced the coil and plugs, set the timing, just re-built the carbs; no change. What I find odd, is that the engine will freely run up to the red line and runs fine at speed, but once warmed up has very poor slow speed drive-ability. Some times just stepping on the gas pedal will cause the engine to stall like it's not getting fuel.

I'm wondering if the "summer blend" of fuel we buy has something to do with it? I bought the Jag last December and it ran very well until summer.

Any thoughts????

Mike.

Submitted by NC19-27130J on Fri, 01/11/2013 - 13:02

Although it's been a few months, I wanted to pass on what I have learned............

I installed the insulation blocks between the carbs and manifold; the studs needed to be replaced as well; too short (you were correct William!). Did some driving tests and, while better, I could get the same poor idle, hard starting effects after the engine was driven fast (expressway speeds). I have several other British cars that are all driven and all with SU carbs. Some, the MG's, have their intake manifolds on top of the exhaust manifold, and they are never a problem. Why???
What was I missing?
It was in the gas tank. All my other British cars I run on premium. The 120 has 8:1 comp so I thought I could get away with cheap gas. Not so!
I drained the tank and re-fueled with premium, and sure enough, the idle/ hard starting problems went away.
I also acquired a set of WO2 needles (thanks, Dick) and now have to set them up once it gets warmer. I was told it needed RF needles (it had those in place), but after some research, I see these are too rich for this application. The FHC has several feet of intake tubing between the air filter and carbs, so I can see why the needles are a little weaker.
I have to assume that premium fuel has a higher boiling temperature, which would make sense, if it's to be less sensitive to heat from compression.
Always something new!
Mike.

Submitted by NC19-27130J on Wed, 09/19/2012 - 12:19

May have found the answer!
My 120 does not have the fiber insulators between the manifold and the carb bodies; that was introduced about 400 engines past mine (at W.6919). They were added because the carbs suffered from heat soak (duh!).
I have ordered a pair. I wonder if the studs were changed????
Mike.

Submitted by NC19-27130J on Tue, 09/18/2012 - 15:50

Dick,
There are no insulator blocks. I don't believe the 120's had them. I was thinking it may be something to add.
Went through the float bowls and tried different settings with no improvement.
Using standard needles (new).
My next task was to do a vacuum check cold and hot to see if something is changing with temp.
Changing the mixture has not helped; seems to run a little rich already.

Thanks for your suggestions.

Mike.

Submitted by SW03-09811 on Tue, 09/18/2012 - 13:56

Mike it sounds like a "lean mixture" problem. You will do well to eliminate the "usual suspects":
- Do you have carburetor insulating blocks in place?
- Are the float levels correct and the bowls properly vented?
- Do you have proper needles? (WO2?)
- Have you checked for vacuum leaks?
- Have you tried enriching the mixture?

Good luck,
Dick