How are my plugs?

At 1500 rpm I seem to have a bit of a surge or stumble of the engine, it accelerates, there is no hesitation but this slight stumble while at a steady rpm is very annoying, the photo shows my plug color after a 45 minute run at all speeds.
Am I running a bit on the lean side?
I have 150 with a fully rebuilt 4.2 with 9-1 comp, and a 9 degree btdc, sporting two HD8’s. with UO needles.
I have a 150 distributor with electronic points,with a new vacuum.
Any ideas from the gurus’s out there.

As far as can be told, using modern fuels, the plugs read pretty good.

Pretty simple test, for too-lean steady state, is crank the jets down two full turns. See if it continues.

Make sure the carbs are balanced, and the linkage has them opening the same amount, and that the float levels are equal.

If you are using points and condenser, I have seen this behavior emanate as a result of a breaking-down condenser, as recently as thisweek, on my Onan generator.

I see you are using electronic ignition.

New condenser = glass smooth steady state.

1 Like

I had the exact same symptoms on my MKII.
I replaced the points, condenser, and rotor cap.
I also cleaned each carbs throat and needle.
Not sure which of those particular items was responsible for the stumbling, but it now runs good as new.

Using old-fashioned ignition systems? About 70% of carburettor “problems” are igntion-related!

If it’s a slight stumble ,with a light throttle at a steady speed, then back off the ignition a tad, the ign is to advanced.

I shall richen both carbs a tad and retard ignition a tad.
Watch this space.
Thanks for the tips guys.

Do just one thing: doing both, and if it stops, you wont know which fixed it.

Yes I was planning that , i think the mixture first before I leave the house, and then the ignition, on the road if I have to.

Those plugs look pretty good. The UO needles is on the rich side, so with your plugs looking NOT rich, I would give the mixture screw one quarter of a clockwise turn to marginally richen it.

I have almost the same set up on my XK120 3.4 with two HD8’s.

One thought, normally a 4.2 in E Type would have 3 HD8’s so maybe you are a bit lean.

What is you depth with a digital micrometer of the jet below the bridge.
Is it about 0.06 to 0.07” ?

Further to my previous post, this morning I took the bells off the HD8 carbs. I measured the jet heights down for your information:-

On the small mixture screw on down on the right of each carb.

2 00 turns down is 0.057”
2 1/2 turns down is 0.071”
3 00 turns down is 0.085”

So you can see that half a turn make quite a lot of difference to the jet height and thus the IDLE mixture and will affect the rest of the mixture when the needle is sucked up under acceleration.

Later this morning I will take a look and measure what I have, with a jet sitting down at .085 the idle mixture will be quite rich.
Thanks for that info, i will get back to you later , however its a lousy rainy day today so no road test till tomorrow.

My Jet heights are .100 when I took the picture of the plugs
1/2 turn up = .086 1/2 turn down = .115
1/2 turn more up = .074 1/2 turn more down = .130
it seems every 1/2 turn raises or lowers jet by .014, same measurement as you have .
Are my jet height too low to start with? are my float levels too high?

Ok, 0.100 seems to be pretty low down. I am assuming you are measuring from the bridge across the carb ? Not from the outer brass shank around the jet. According to the books you should start with
2 1/2 turns down which as I know and I assume you have discovered is about 0.069 to 0.071.

But what is strange is that you jets seem to be very low (rich) but you plugs looks good.

Your needle of UB is richer than UM.

So, unless your float level is wrong, it sounds more like ignition?

Take the float bowls tops off and turn them upside down, check that a 7/16 bar will just slide through the gap between the underneath of the top base and the float lever which should be just resting on the little float closing pin. Also take out your brass floats to see if they are sinking because they are leaking internally. Shake them in your hand and listen to them. This float symptom would cause flooding so not your problem, but worth checking while float bowl top off.

Sorry meant to say the UO needle is richer than UM.

Yes measuring from bridge.
A bit of history.
I had these same carbs on my 3.4 and the car perfect, no stumbling,
I have now a 4.2 with a series 2 inlet man. Which use to have twin Stromberg, so l am trying to sort these few hic cups after the main install. The 4.2 is such a massive improvement in performance and l strive for a perfect running machine, and with help from you chaps l will get there.
I will remove float chamber tops and start with float levels. And possibly set jets to .070 and see what happens, but first l will take a drive after lowering jets as Wiggles mentioned to see if that has any effect.
Cheers and thanks.

Not sure if Wiggles means another two full turns from where you are or two turns from top. If it is another two turns from where you are it will be like running with a choke fully out !

I think he meant from where l am now, just to see if eliminated the problem, if it did then you could rule out ignition.
Anyhow l did one full turn taking it to .130 and take a short run, but l feel l will have to start again with float level and set jets at .070 , l have also another HT set with distributor from my original motor so l will give that a trial.

I am making an assumption, but assuming your HD8’s are such that the butterfly fully closes on idle and takes air through the volume screws that are out about 3-6 turns. Unlike the original H6’s on a 150 that had idle stop screws to hold the butterfly open a slight amount?

Thanks: I should have clarified!

I use “1/2” turns," at a time, so when I state “turns,” 2 “turns” of adjustment is one FULL turn of the screw.

That’s what I did , 1 full turn, now waiting for weather.


1 Like