XK 120 engine very poor running when warm

XK
1

Have seen similar problems many times and unfortunately now it’s my turn asking for help. The situation is as follows:

Completely rebuild 1954 engine with SE spec, engine did meanwhile 50 mls (running in between 1500 and 2500 rpm).

Engine starts immediately with original Otter switch, starts at 800 rpm and climbs up to 1100/1200 rpm within 2 minutes (car standing, not running). Engine runs very smooth with AED on, without becoming “lumpy” towards the end of the “choke” period. Otter switch then cuts AED off and engine runs at 600 rpm without any problem. Temp gauge still “cold”.

Start driving the car and engine runs beautiful, picks up fantastic at any rev in all gears and lots of power. Temp gauge climbs up to 75 C after 20 minutes driving. So far, so good.

Once the engine is warm (so after 20 to 30 minutes), it’s very difficult (or better almost impossible) to accelerate from 1500 rpm upwards (in all gears). Lots of hesitation, not wanting to rev above 1500 rpm. The only way to get higher revs is to very slowly and gradually push the accelerator pedal carefully down, in which case the engine will slowly increase revs to about 2000 rpm, no more. When pushing the pedal down quickly the engine will protest and much more hesitation. Don’t know whether I should call this “misfiring” but fortunately there are no “explosions”.

After the engine has cooled down (several hours), it runs fine again until it becomes warm and the above story repeats itself.

I replaced the (hot) coil and quickly switched it for another (cold) coil: no changes, same engine behaviour.

Compression is fine (all around 140 psi). The dash pots are filled with SAE 30 oil. Spark plugs have a nice colour and no “soot”. I’ve already set the mixture a quart of a turn of the screw downwards (so richer) but nothing has changed.

I know it’s very difficult to diagnose from paper, but where should I now look first: ignition or fuel? Is the engine hesitation caused by “misfiring” of the ignition system or still a mixture problem? Or something else?

Your experience is welcome.

Bob K.

2

A wise old mechanic once said to me, “If it ain’t fuel it’s spark, and if it ain’t spark it’s fuel.”
I’m thinking your centifugal advance is stuck, or vacuum advance is broke.

3

Wow, until the plugs were mentioned I was very set on far too rich…
Was this a gradual change, did it come and go, or out of nowhere?
Same in neutral?

4

Bob

Had a very similar problem on our Healey.

Don’t start by rebuilding the carbs, retiming and adjusting the distributor, changing points and condenser, changing over to Petronix, lubing the carb linkage, adjusting the choke, adjusting fuel line to get smoother elbows, and spending time staring the engine into submission.

Check the fuel filter.

Just sayin’.

Bob England

5

David,

I was initially thinking more the opposite: far too lean. But plugs look fine.

My first drives were just a few miles and all went well ( and I was so proud…). But when I started making longer trips I noticed this problem immediately and it’s always the same.

Once this phenomena starts it’s also noticable in neutral although starting perhaps at a somewhat higher rpm like 1600 or 1700 rpm possibly because the engine isn’t under load.

Bob K.

6

If it was lean it wouldn’t run best during the warmup phase, accelerating well only when pushed slowly, bog down when pushed fast. That’s a sign of over rich. So is dropping rpm to almost stalling after coming down from rpm in neutral.

When my fuel line was plugging up (fuel starvation, lean, rough under load, idle alright ) it would only run smooth when I stabbed the throttle and the damper enriched the mixture briefly.

30 minutes screams coil or condenser doesn’t it? Vacuum in the tank is another possibility, or if it has a filter sock, this might plug up. Most issues with carbs are ignition. Condenser?

7

Thanks Bob,

I will do that, but I’ve fitted the AC filter as used on the XK 150 with the glass bowl.

In line with your thinking, I also heard that an older SU fuel pump sometimes “misbehaves” and gets hot, leading to fuel starvation.

Bob K.

8

Rob,

I will check both but I saw the vacuum advance doing its job when the pedal is pushed down.

Bob K.

9

Let’s think, where else can crud build up and block fuel flow?
screen on the tank drain plug
cup shaped cavity down in the drain plug
screen inside the SU fuel pump
screens in the float bowl banjo tops
small drilled holes in the bottom of the float bowls leading to the carb jets
To all of the above, I say Guess How I Know!! :grimacing: :face_exhaling:

10

Hi Bob,
Sorry you’re having problems.
Might be worth checking the advance through the rev range with a timing light. That shouldn’t change under load, so would help to eliminate spark. I wonder if something on the fuel side isn’t seating properly, maybe something in the starter carb? If you get really stuck, perhaps a diagnostic session on a rolling road would help. Might point you in the right direction, there are so many things this could be.

11

Bob

Mine was a in-line non-Classic type filter.

Your note on the fact you were seeing issues at idle was a bit different than I was seeing. Fuel starvation at highway speeds was my issue.

Bob England

12

Thanks everybody for your ideas. I made a long list with items to check (see below).
As additional information: the (aluminium) fuel tank (made by NAR) is new, as are all the fuel lines.
There’s no fuel screen/filter at the bottom of this aluminium tank, just a (smaller) drain plug. But the risk of particles at the bottom of the new tank is minimal and I have the additional AC fuel filter as a back-up.

All in all, I still think my problem must have a “heat induced” cause (as it runs so beautiful the first 20 to 30 minutes).

  • Ignition related:

    • Distributor malfunctioning
      • Centrifugal advance
      • Vacuum advance/retardation
      • Condenser
      • Points
    • Coil overheating

  • Fuel related:

    • Vacuum in fuel tank
    • SU fuel pump
      • Coil overheating
      • Screen blocked inside pump
    • Fuel lines blocked
    • AC fuel filter blocked
    • Carb blocked
      • screen in the float bowl banjo tops
      • small drilled holes in bottom of the float bowls to the carb jets

Bob K.

13

I would examine the points very close, I have twice had weird points problems

(appeared to be poor quality new parts)

Once a condenser, it went short, luckily in my driveway, and luckily I diagnosed it almost immediately no spark at all), dont know what happens if they go open or dodgy, but I would buy another and substitute it anyway, very cheap

same with coil, carry a spare and substitute when it plays up. I have even seen Jags with a spare coil fitted in the engine bay (never had coil trouble on a Jag)

When it gets iffy, pull over, pull a lead and see what the spark looks like compared to usual

Check the dizzy cap spring loaded main lead terminal

This should allow you to eliminate ignition as the problem

I am gueesing its much hotter here, than there, never had vapor lock on a Jag

14

Thanks Tony,

At present it’s just between 60 and 70 F overhere, which isn’t really hot, not even for our moderate climat. So what might happen at temperatures over 100 F…

I will check each of the potential causes that I listed and hope one of them is the culprit.

Bob K.

15

I listed all the cheap, easy, clean jobs for ya 1st, almost rule out ignition

If it is not power related…ie scum blocking fuel tank tubes, you would think that would show up on v hard acceleration, cold or hot

I would not do it for long, but if its easy, I would remove air filter when it plays up and see if that makes a difference

My old MK7 could be finicky about things like that

If you have not already done so, I would completely isolate the ASC, and make sure it isnt possibly leaking somehow, these things are so old, tiny nuggets of metal can do funny things, you need a magnifying glass and strong light

16

Hey Bob, probably a longshot, and I might have missed this item but is the new fuel tank vented?..and if so, have you verified that the vent is open?

If the tank is NOT vented, is the fuel cap vented? - easy to tell, there will be a small hole in the top of the cap…blow into it to verify the air comes out the hole on the other side of the cap.

In either case, try driving it again, and as soon as it acts up, pull over and slowly remove the fuel cap, and if you hear/feel a whooshing sound as the air enters the tank, that’s probably the cause of your problem.

1 Like
17

Tony,

I will try that as I have to admit that I tuned the carburetors without the original AC pancake air filters fitted. But on the other hand, why would the engine run fine until it’s warmed up to about 75 C?

The ASC should be fine, as when it switches off, the revs drop from 1200 to 600 instantly.

Bob K.

18

Chris,

The new aluminium tank is a (reasonable) copy of the original steel tank, including the vent pipe running over the top of the tank. That pipe ends under the “flap” that acts as a fuel cap. See pics.

Bob K.

New alu fuel tank rear
New alu fuel tank rear1920×1440 524 KB

Vent Pipe Extension in Filler Box
Vent Pipe Extension in Filler Box1920×1449 134 KB

.

19

Ahh, I should’ve paid more attention…sorry!

Well, nevermind then! :wink:

…oh, just a thought - are the washers for the float bowl overflow pipes orientated correctly? - the red fiber “serrated” washers go under the banjo (between it and the top of the float chamber cover), and the metal one goes above it, under the bolt. Also, the serrated washer needs to be in new condition. A mix-up here’s not uncommon, and can cause similar issues.

Hope you get it sorted soon, this stuff can drive ya crazy!

20

You have said–runs great at start up and for a while until engine reaches full warm–coolant 75C or so…is when it loses power. Cools down,then will run well again until fully warmed up.
Sure seems related to HEAT. So heating of what…loses willingness to power…
We know coils are notorius for heat problems…: I’d try yet another coil.
Fuel–does not like to get hot–if vaporizing causes loss of power. Try shielding fuel lines and carb from heat.
distributor condenser…not usually heat releated…but could be–To eliminate some things…that are relatively easy–try a new (another) coil. then a new distributor cap, run thru the folloiwng possibilities:
carb chokes–change running mix–from cold to warm—what will run well cold, will not when warm. Choke means blockng some air flow–and a rich mixture…can there be a blow back vave in the intake that is stuck.
What spark plugs are you using–what heat range, heat range becomes more noticeable when–ta daaaa…there is heat.: and what gaps, what dizzy gap or is it electronic. What spark plug wires–heat can cause electrical resistance in wires to increase…and modern plug wires have waaaaayyyyy too much resistance. Combine that with a R spark plug which adds another 5,000…to the 15000 to 40,000 per foot of modern wiring…
the fact that it runs well cold…and not when fully warmed…is the factor…What things in the air-spark-fuel recipe will do well cold…and not well when warm.
–timing and timing advance should not change–cold or warm- but still an easy check to eliminate. And could if a little off, combine with whatever the main issue is.

Nick