I have a 54 XK 120 SE and am experiencing a problem that I read an older thread on. The engine starts normally and the auxiliary carburetor shuts down properly and idles normally. As soon as I get her on the road, and go through the gears, the engine falters and dies. It won't restart. Then, Ilet her sit for about ten minutes and it will start but the same thing happens as I pout load on her. It sure seems like a fuel restriction problem to me, but I have changed everything. New NAPA fuel pump with regulator (tried settings from 2.5 to 3.5 lbs. and it doesn't matter). Took out the filter in the tank (and cleaned the tank while drained) and installed an inline filter that is clear and shows no debris or sediment, replaced the cone screens at the SU float bowls, put in new ball type float valves and messed with several adjustments. Tested between each step with the same result. I have the insulators at the carbs but have not tried anything electrical except for making sure that the rotor is free. I read a thread that the coils fail from heat, but can this be happening when she is not fully warmed up?

 

Did you put oil in the dashpots? 20W50 enige oil works ok for me.

 

Yes, Thank you. I use the oil from SU.
Could you explain the theory on why that may be a problem.
I am still trying to chase down a possible fuel restriction.
Fuel tank vent is clear.

 

I suggest disconnecting one of the fuel lines to a carby and ensuring that there is good fuel flow.
I just went through a similar situation with a MK2 and found just a trickle reaching the float bowl. Start and idle OK but died when power was required.
I found that some previous owner had inserted an extra in line filter in the supply line just as it entered the engine bay and wasn't it clogged.!!!
So check for adequate flow. You could also have a damaged fuel line or similar.

On the coil issue the engine does not have to get hot. A coil with internal shorting is quite capable of generating its own heat.
When it does it again check for spark immediately after it stops before it has time to cool down.

 

Check the air bleed hole to the tank is not blocked. Fuel tanks need to be able to draw in air as the fuel is used up or a vacuum is created and the fuel pump can't pump the fuel forward.

 

i have several british sports cars( xk120,austin100/4 tr3 tr4 and i have had a similar problem with each of them and all solved by installing a new condenser. my mechanic has an old fashioned condenser checker and each one was failing. this happens quite often on old outboard motors( the part break down voltage until they cool off) they are inexpensive0 just a few dollars from moss motors

 

Here's a long shot. Did you get any work done under the car before this started? I had a 1968 Thunderbird that would idle fine then die after running a bit. After sitting it would start and go through the same cycle. It turned out to be a fuel line that was bent when the exhaust system was repaired. It would pass enough fuel at idle but not under power. When it sat, the fuel line, now under vacuum, would refill then run until fuel in the line was exhausted.