XK150 Stalling Under Acceleration

After setting timing on my 1958 XK150, I am getting blowback through my starting carburetor when manually revving the engine. Timing is set at 8 degrees before top dead center with the vacuum line disconnected and plugged at 700 RPM. The distributor has new cap along with new spark plugs.The car starts quickly and runs smoothly except when revved. When driving, the car drives fine as long as you don’t try to accelerate quickly under load. It stalls but recovers as you lessen the acceleration. I have just replaced the 10 year old pertronix ignitor with a new one and also the fuel pump. The carbs have been rebuilt. I did check the butterfly air openings and the needles, all good.This problem was the reason all the work above was done. Any advice would be greatly appreciated.

Is there oil in the Dampers of the carburetors?
Or try another thicker oil.

Check to see if the mechanical advance is not stuck. You should be able to move the rotor and have it snap back. How old is the gas? Is this happening when the engine is cold or also when warm? When cold is the starting carb engaged? Does it cut out when the engine warms up?
Pat H

Thanks for your replies. Su oil is in the carb dampers. Distributor advance is working, I checked it manually and with the car running with the timing light. Gas is good 93% non ethanol. I have had this car for years and it gets driven frequently and runs great. This happens when engine is cold or hot. It is kept in a climate controlled garage. I began to notice it about a month ago while accelerating on a drive and has gotten worse since.

Sorry. The starting carburetor is running correctly and cuts off in about 3 minutes dropping the RPMs from 1200 to 700.

Hi Edward,

Did the onset of the problem coincide with any of the work you did?

I would suspect mixture being well off or vacuum leak. The leak could be in brake system.

How do your plugs look?


Check to see if the linkage connecting the carburetor throttle shafts together may have come loose. Perhaps one throttle plate isn’t opening?

Advance is working and throttle plates are opening and closing together correctly, spark plugs are new and gapped correctly.

Just to clarify, are you measuring your ignition timing using the markings on the flywheel, or markings on the harmonic balancer?

I’m thinking that if you may be using the markings on the balancer to measure your timing, and the rubber in the balancer has degraded allowing the outer ring of the balancer to slip relative to the crank, those timing markings would now be inaccurate, and your ignition timing may be actually extremely retarded ?

Does your 150 have an anti-blowback valve?

The following is a picture of my timing marks, the long one is TDC and the other two are 5 degree marks before TDC. I apologize but I don’t know if my starting carb has a anti blowback, but I can include a picture if it helps. Thanks!

Hi there,
What Gary is saying, is the harmonic balancer that the timing mark is on is actually two pieces of steel; an outer ring and an inner hub, joined by a rubber ring. This rubber ring sometimes comes loose, allowing the outer ring to rotate in relation to the inner hub. So what is marked on the outer ring may not necessarily be actual TDC. this causes your timing measurements to be out. You want to determine if the mark accurately shows TDC.

Thanks for all your help, I will position piston #1 at firewall to the upper most point @TDC and check the timing mark on the harmonic balancer.

1 Like

I have checked the timing on #1 cylinder and the timing marks on the harmonic balancer are accurate. When increasing rpms I do get blowback coming from the starting carb even though it is off if it helps. This happens when I am driving or in my garage manually.

Have you tried a compression test?

I have an XK150 with a 3.8L; mine can backfire occasionally through the starting carb when cold with a quick throttle opening but not when warmed up. Yours must be backfiring through the inlet manifold which could be advanced ignition timing, incorrect valve timing (which doesn’t change much over time) or possibly a vacuum leak somewhere. Try retarding the ignition slightly first, look for any vacuum leaks second (including all round the inlet manifold to head joint, and lastly try to check the valve timing/valve clearances (maybe a tight inlet valve). Good luck and report back please.

If your 150 has the vacuum brake booster, you could try clamping off the hose and then revving the engine to see if it improves…

If your car has the Trico vacuum operated windscreen washer system, try clamping-off the rubber hose at the inlet manifold. A faulty Trico system can be the source of a vacuum leak.

Thanks for all your responses. I think the Lucas igniter coil my be my problem. It is date coded 4/58 so it probably came with the car when new. Pertronics said to also check the distributor leads as they are known to be finicky

the way the ends are screwed in to the stranded copper lead.
What could possibly go wrong. I am tied up until late next week but will let you know what I find.

Mine has copper center conductors and I soldered them to the brass discs

The screw in the end of the HT lead is definitely a bodge, they normally have what looks like a large brass carpet tack stuck in there, could be the source of the problem but I think it’s more likely a vacuum leak somewhere.