Not running on all cylinders

(peder) #1

Recently rebuilt engine which has been good all along, suddenly runs on 4-5 cyls. The other day it picked upp the missing ones after a few mins of driving, but yesterday remained on 5 for the entire 15 min drive.
Cables appear tight and the inside of the distr cap looks very good.
What next to check?

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(- 1950 MkV, 1959 XK150,) #2

Hi Peder,
I would try putting my timing light clip on the leads whilst it’s running and the induction should be able to tell if the plug is getting the spark transmitted to it.
IF they all have power, then it is maybe the plug.
If one doesnt have the power, then try swapping leads to narrow down the cause…

Best of luck,

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(Nic Mauduit) #3

it is easy to find the non running cylinder(s) upon starting by touching the exhaust : the cold one(s) are not getting any combustion.

As for the cause, there are a number of steps to investigate… if all has been good for a while (say several hours) I would start by the plugs (in relation to combustion richness, but also trying a new set as it is not rare to have almost new plugs turning bad these days)

good luck !

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(Foggyoo) #4

Water in distributor?

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(- 1950 MkV, 1959 XK150,) #5

Another possibility is you’re having arcing from a lead to the block.
At night with no lights have the engine running and see if you can see any sparks jumping around.

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(David Jauch) #6

Can I suggest pulling the plugs now? They will either be wet or coked. If they are all plugged up it might explain why a cylinder picked up the day before yesterday. Burn them free or try another set, then put them on the head and check if they all see a good spark when you crank.
Then see if the spark can jump a reasonable distance, and is strong enough.
If they all spark nicely you can look at the plugs again. Ideally, make a picture and upload it if anything is amiss.
Also try the engine in the dark to check for arcing, but yours are relatively new right?

David

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(Pat Harmon) #7

I’ve had problems with plug wires in the past. Solved by soldering the little brass discs onto the wires.

Pat H

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(Phil.Dobson) #8

what leads a caps do you have?

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(David) #9

David’s suggestion is right on target, Since the copper end is recessed on my 120 there is an old simple trick. Take an old spark and screw off the end drive a nail into the threaded end. Place the spark plug end into the lead and start the engine. Using an insulated pliers hold the lead and follow David’s suggestions to see if the problem is the leads or distributor cap.

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(Nick 53 XK120 OTS) #10

start with easy…and most common first…a spark plug…you can remove one plug wire lead at a time…see if same or worse…when it is the same…then that is the bad one…whether plug or the lead. Other easy fix…since plus so easy to R and R…just replace…You could do one at a time…to find which is bad (if it is one)…or just do em all…and inspect if one maybe has a speck of carbon across the tip. If you replace all and no fix…then look elsewhere as…noted by others…a wire itself or the contact at wire ends…sometimes a gap. Nick

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(Paul Wigton) #11

Even easier test: run the engine for a couple of minutes: Shut it off, then wet the end of your finger, and the plugs’ insulators.

It will be obvious which one is not firing. THEN, work backwards.

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(Rob Reilly) #12

All the above suggestions concentrate on spark.
An old mechanic’s rule was “If it ain’t spark it’s fuel, and if it ain’t fuel it’s spark.”
So I have another thought, partial fuel starvation, dirt in the inlet screen or bottom of a float bowl.

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