Hesitation when cold

Well, more like ‘not fully hot’ rather than ‘cold’.
It’s a 140SE, virtually standard with twin H6s. Pertronix module in the original distributor, everything fully rebuilt. I have set the engine up using a Crypton synchroniser, stethoscope backup, O2 lambda meter and a pair of Gunson Colourtunes, so everything bar a rolling road session. I have isolated the hisser to a dashboard switch.
Cold start procedure is ignition, hisser switch, start button. Once the engine has started (very easy, one or two spins), I give it about 30sec. then kill the hisser as it starts to run very lumpy with it still in. The car sits for a few minutes while I check for leaks etc. (yes, I’m still at that stage with a new restoration), and the temp gauge starts to move up to around 45-60˚.
I then drive off (hisser still switched off), and for the first 2-3 miles the engine misses under reasonable, not heavy load at higher engine revs. It’s showing 70˚ or so on the temp gauge by this time. The miss eventually goes away and it pulls cleanly, with normal oil pressure and water temp.
I’m using NGK BP6ES plugs which always look sooty black when removed, but at idle the Colourtune is definitely blue, not orange. Lambda meter shows around 12-12.5 at idle.
Is this miss for the first few minutes normal for an XK engine? Should I try a hotter plug, like a BP5ES?
Opinions greatly welcome.

Roger, my AED is controlled by the Otter switch which is set to switch off at 40 C, I can start the engine and let it warm up until the AED shuts off without any excessive richness, just an increase in idle speed from initial start @~850rpm to ~1000rpm and then @ 750 fully warm. From my experience it seems like your mixture setting on the AED is too rich and you are switching it off too soon. This sounds like more of a lean hesitation than a misfire. I know many XK owners prefer to have control over the automatic enrichment via a manual switch for fear of cylinder wash down and justifiably so but when properly set up even though the AED is fairly rudimentary it does work. Do your spark plugs all look the same, black and sooty even after an extended drive before a restart? Your Lambda numbers indicate 3-4% CO where mine is set but my plugs are consistently light brown. As I recall you do have a pair of mild performance camshafts installed which could have some effect during the warm up period. Lastly, and forgive me because I know you know your way around Jags and many other cars but do your carburetor dampers have sufficient oil in them?

Thanks for the reply, J.E. - The cams I have in now are the mildest ones Newman make for the XK engine, which are very close to the original factory profile. They’re sold as the factory replacement, not even fast road.
From what you say, I think it may well be the lack of the AED’s effect more than anything else. I really don’t want to re-engineer that again just now - I’m assuming the AED is either on or off - I don’t think it has a progressive action? I may be completely wrong on that though. Maybe it’s the mixture on the AED that’s out - I haven’t touched that. Certainly at cold start up with the AED switched on, within 20-30 seconds the engine starts to run really quite roughly to the point where I suspect it would cut out, Hence my switching it off. It may be that the thermostat has only just opened when it hesitates so it could be that. I know that after a mile or so the hesitation goes.
Yes, SU damper oil from Burlen carefully topped up regularly. But as it’s fine when warm, I’m not sure that could be the culprit. Don’t worry about what I know and don’t know, I’m now at an age where I forget stupid things!

Yes, both fuel and air are fixed in operation although fuel is adjustable. Because the combustion chambers heat up very quickly as compared to the rest of the engine too rich an AED setting will over fuel the engine very quickly ie the lumpy idle you experience shortly after startup. It sounds like your carburetors are well dialed in so when they are then is the time to make the fuel adjustment on the AED. I set it to as lean a setting as the (just started) cold engine will allow which combined with the carburetors will be slightly richer than the carburetor base setting. When the AED is on and you open the throttles the drop in manifold vacuum temporally closes the spring loaded disc in the AED essentially shutting it off regardless if the solenoid is activated. Because of the on/off nature of the AED different climates and of course state of tune and overall engine condition may require some fettling to get it just right for your particular conditions. As I mentioned I currently have a 40’ C Otter switch and I also installed an orifice of 8mm (.315”) in the vacuum supply tube to the AED to reduce the excessively high idle prior to the AED switching off.

In my experience what you are experiencing is normal if you have switched the hisses off. The hisser will give a progressive enrichment (as the butterflies open there is less vacuum on the hissed circuit) to prove the point when you first start up with hisses connected go for a run. If all is well you might like to tune the hissed (raise the needle a bit) to a point where it idles better. The procedure is detailed in the manual.


I went “back” to the old fashioned vacuum gauge and discovered quite a difference between setting the mixture according the manual and trying to obtain the highest vacuum value. I have to admit that I haven’t checked it against more modern equipment that you have used.

I have a very good pick-up now, at any speed and even just after the Otter switch reached 35 Celsius. Would be interesting to know what the difference is in your case. Have a look whether the vacuum further increases if you adjust the mixture screw.

It might well be that the SU carbs behave different in this respect, compared to more modern fuel systems.

Bob K.

According to the manual, if the engine is fully warm and the AED is actuated the engine should not run with “noticeable irregularities”. Since the OP’s engine runs “very lumpy” with the AED on even when it is not hot, that tells me that it is adjusted too rich. The AED can be made leaner by screwing the stop screw DOWN. The proper mixture is achieved when the engine is fully warm and still runs well with the AED on.

Thanks all, some great tips there. Never come across an AED like this before - triple 45DCOEs or BG Mighty Demons don’t use them!

There should be a little spring on there that prevents the mixture adjustment nut from rotating due to engine vibrations.

In my case it is, the remedy would be to tune it richer or go to a different needle - but then it is way fat once it is warm. Live with it.

As the hisser seems to be too rich, it should run reasonably (!) smooth, but faster, and then as Phil wrote there’s a procedure on how to adjust it when the engine is hot. Keep in mind that the hisser needs to cover a lot of scenarios, so in fair weather it will be too rich if it is expected to start the engine on a cold winter morning. You decide, and adjust accordingly.
The AED is the later version, the solenoid actuated auxiliary starting carb is usually called ASC.

The ASC can not be disabled by throttle operation, that is not how it works. If the ignition (or power) is disrupted it is unable to open against manifold vacuum until the throttle is blipped, at which point the vacuum is weakened and the solenoid can overcome the suction. This is a quick workaround to shut off (automatic) enrichment if not needed.

Just an update - resetting the ASC didn’t really make any difference, but changing the plugs did.
I had originally fitted NGK BP6ES, but these were consistently very sooty all over but especially around the edges, so I switched them to a set of BP5ES which are slightly hotter. The faltering has gone away.

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Great, so basically fouled plugs, it will be interesting to see how the 5’s perform and their coloration after some miles.

Hmm, it did it again yesterday - proper misfire somewhere until the gauge was up at 70C. So I ran a compression test, which gave me 150, 152, 150, 148, 148, 150. Not much wrong there, I’d say. Photo of plugs below - these are BP5ES, which have now done around 150 miles. The BP6ES I had in before were a similar colour.

I’m wondering if these are showing a bit lean, or if the BP5ES might be a bit too hot - perhaps better to go back to the BP6ES, richen up a bit?

Opinions welcome. I may just need to let the engine get properly warm!

I would call that lean, the darker portion of your #3 spark plug is what would be a correct mixture color if all of the porcelain of all of the spark plugs presented that way. The hemispherical combustion chamber like on the XK engine is great for breathing but that design is not ideal for mixture ignition and thus requires a richer mixture to propagate the flame front effectively. Richen the mixture overall to the point where the idle setting will just manage it and see if there is any improvement if the problem still persists the different needles may be necessary to increase fuel in the low to mid range.

Thanks, that’s helpful. I’ve looked at the set of BP6ES I had in the engine last week, and they are pretty much as No.3 in this photo, so it might be worth going back to those and investigating the mixture further as you suggest.

its been a while…how is it running…seems a fuel/air mix was more of the issue…than the spark plugs…but that said–it all matters in the recipe for smooth optimal running, including ignition timing. As to plugs…depends on each engine…the NGK have a wider range within a numbered plug than do Champion…I find the 5 is suited to my varied driving…(not much in traffic…but also not sustained high rpm…which would be the 6. Note also the NEW NGK are a R-resistor plug…some NOS BP5ES or BP6ES without the R can be found. The R adds 5,000 ohms resistance. IF…a car accidentally also has modern high resistance plug wires…(can be 15,000 to 40,000 per foot) for todays very hi energy sytstems) .that can also be an issue.
Smooth running is a recipe of many small ingredients.
DId you get it resolved? My engine takes a while to warm up.

Hi Nick, I haven’t run it since the last run with the BP5ES plugs, as the oil filter has sprung another leak (top seal this time, hence the other thread) and it’s losing a fair bit, but only whilst it’s running. The top seal is under pressure. Luckily I don’t seem to have difficulty buying both 5 and 6 NGKs here.
I’m also about to ditch the Evans coolant, as on a long section of motorway the temp gradually rises to at least 90, then drops back in traffic - which is weird, completely the wrong way around in my experience. I suspect the water pump is not working well at higher rpm with the more viscous evans.
It’s booked for a rolling road setup in a couple of weeks so we’ll see what’s what then with regard to further down the needles in the carbs. I suppose a hotter plug combined with a slightly lean mixture could contribute to engine temps, but I wouldn’t have thought by that much. It doesn’t pull very strongly at higher revs so needles are a possibility, but never having driven a Jag of any variety before I don’t really know what to expect. It’s done around 500 miles in total now so should be loosening up a bit. I was planning on going over to the K&N pancake filters from the wire basket pancakes, but not sure I need another variable at this stage. I’m guessing the K&Ns would let less air through.