Interpreting vacuum gauge reading

XK-Engine
1

I am testing the replacement engine in my 420G, it drives well, but the idle was not perfect, so I placed a vacuum gauge and the needle flickers rapidly 2-3" at idle from ~16-19

I have even flow thru the dual HS8 carbs via a Gunson airflow, and a warm compression test showed 175psi on all cyl

I corrected timing to 8*BTDC, (I had set it for highest idle, and needle was flickering 5-6" 16-22)

Idle is ~750rpm in neutral auto.

My “vacuum gauge chart” shows such a fault may be caused by vacuum leak, incorrect timing, weak or broken valve spring/s, or various valve/head problems.

pulling plugs one by one did not seem to show any worse than others

I am not very familiar with vacuum gauge interpretation, but I am concerned that some valve spring fault is most likely indicated fault, I dont think the needle flickering rapidly can be a sign of perfect health on an air pump.

pulling plug leads one by one did not seem to indicate any cyl worse than others

2

When you say “rapidly” do you mean once per revolution or 3 times per
revolution?

Mike Eck
New Jersey, USA
www.jaguarclock.com http://www.jaguarclock.com/

I am testing the replacement engine in my 420G, it drives well, but the idle
was not perfect, so I placed a vacuum gauge and the needle flickers rapidly
2-3" at idle from ~16-19

3

I thought there is some way to post a video on this forum, i’m sure I have seen others do that?
(says I can only post picture formats)…maybe its done via a url “host” website?

anyway, I slowed my vid right down and the flicker seems to be quicker than the rpm
the needle flickers very rapidly indeed

a “clue” is that altho the airflow is balanced, the air idle screws are not,
the air idle screw on the front carb is screwed much further down to achieve that"balance"

driving, the engine does not feel quite right, like there is some imbalance, idle is somewhat unsteady

as i rule out anything simple, my next step would be to remove cam covers, check valve clearance,
and change springs with head in situ.

I would prefer to do as much diagnostic work as possible beforehand, as a next case scenario is head would need to come off to check valves, guides and seats

ok, found a way to insert vid (via photobucket)…there is a significant exhaust leak at mani/pipe junction

4

checked linkages, adjustments and hose connections, all ok

there is an audible whistle coming from rear carb at 1300-1600rpm

putting a hose against each carb at idle, I can hear a steady hiss from the front one, but the sound is more turbulent or fluttering from the rear

this to me suggest there is a malfunction in the valve train of at least one of three rear intake

looks like I will have to pull cam covers, check clearance, and likely change out intake valve springs

5

I cant believe its the valve springs. if you have to open the rear carb idle screw a lot more than the front I would start there. can you effect idle speed with the rear or is it effectively not working? you may also like to check that the front carb butterfly is actually fully closed so the idle air is actually being controlled by the idle screw and not leaking past via the slightly open butterfly. its always useful to separate the carb linkage so the two carbs are independent so you can ensure the butterflies are fully closed on each, then using only the idle screws check and balance the carbs on idle. The workshop manual has a proper description for this. Are you sure you have HS8 carbs?

6

I can hardly believe it either, as I have never heard of them failing,

but after listening closely to the hiss emanating from the rear carbs compared to front one, there is something not right to make the air distinctly turbulent, as opposed to loudness of hiss.

imo whatever this is, that is causing the vacuum gauge needle to flutter

I followed the correct procedure for Carb setting from the FSM (again), tried a few things, to no avail.

The idle is not smooth enough, and even though it drives a lot better than with the previous motor, i can feel there is some imbalance

they are HS8, fitted with correct needles for this engine, and manual choke.

the variation of adjustment is quite small, the air idle screw on the rear one is different, (much longer), that was misleading me a little…someone has been in there before me

I think difficulty balancing carbs could be due to different airflow back vs front possibly caused by unequal air draw from cylinders, one or more

the HS8 throttle plates each have spring loaded contraptions, which on other HS carbs I have seen brazed up, I suppose it is possible there is some issue there, so I will remove and check them.

  • edit I have 2 pair of HS8 spare, may change out the rear carb to eliminate any possible problem there

currently searching for my IR thermo gun to check ex manifold temps, which would be a good clue

(exhaust mixes in front resonator so dual tailpipes dont help much)

like it says in the FSM, valve clearance and other timing issues must be attended before tuning can be done, I can only work thru the issue systematically.

I showed the vehicle to an experienced mechanic and discussed it with an expert.

The first guy was clueless, but he agrees idle is not good enough, expert guy said just drive it till it manifests.

the valve springs I would only do as a last resort, prior to remove head, if all else fails.

the vacuum chart that most closely matches mine is “worn or loose valve guide/s”

7

Hmm. I am confused (could be my age) my response was in relation to HD8 carbs which have the air idle screw. I thought the HS8 didn’t have the separate air idle circuit and just use a screw to open the butterfly?

8

not confused, you are correct, the HS8 air idle screw acts directly on the butterfly, which kind of simplifies things

I do not think carbs (even if they were slightly out of sync) can cause the air turbulence and audible squeel from the rear carb only in the particular rythmic way that exists

in any case, I have checked carefully several times, and the butterflies are closing completely and acting in sync

found IR gun, and nothing really obvious in terms of inconsistency in exhaust manifold temps

next step will be to check & swap out rear carb body, bell & piston

also got to fix steering column bushes, the lower one has disintegrated, so adequate test driving is not completely safe, this job entails removing entire steering column for O/H,the parts are awaiting me…oh what fun

9

an update, I changed out the rear carb with no effect.

the vacuum gauge needle and description are most alike to “loose valve guides” or “weak or broken valve springs from the chart below…it exactly matches “loose valve guides” description…if held at ~2000rpm the needle oscillation is only about 2”…the needle moves very fast

(it seems my video is no longer viewable)

vacuum gauge-crop.png1110×676 132 KB

rear carb emits a whistle in the rpm range ~1300-1500.

googling “frequency of human whistle” indicates ~1000Hz

this indicates whatever is causing the whistle is at crank speed, not cam speed…its disturbed airflow, not mechanical noise

next test i just performed was to use a laser tacho to see if pulling each spark plug would show any cylinder not pulling its weight (I could not detect any difference by ear)…but the digital tacho did not show anything amiss with one cyl in particular.

compression is ~170psi on all cyls

I do not think there is any further tests I can perform that need a running engine, so now I will remove the camcovers, check clearance, air will be injected into cyl to see if its leaking out any valve, next swap out valve springs, using the “rope in hole” method

cant ignore it, or drive vehicle, as if the problem is valve/spring/guide related, dont want to drop a valve

prefer to conduct all diagnostic tests prior to removing head

10

havent had a lot of time

tried a few things and had a couple of good guys look

showed Jag mechanic a phone vid, and he said he had not seen the like, but thought valve train likely

I changed out rear carb HS8 throttle plate from later unit with spring to earlier flat plate HD8.

One strange thing I found; with correct rebuilt dizzy, with points set .016, dwell was ~20degrees…setting the points gap low.012 made dwell ~28"…this dizzy had points originally set to `.008"

I replaced it with the earlier 420G, 22D6 dizzy with points at .016 and dwell was ~35degrees
another 22D6 gave 40degree dwell

Its my understanding dwell should be between 34-40 degrees on XJ motor?

I also used alternative gauge, and tried mine on another vehicle…mine has a larger hole, which seems to make it less damped…1/16" vs 1/32"…my gauge now shows fluttering 17-20", the other gauge shows ~50% less flutter. On a 320k Jeep motor, my gauge showed 1" flutter, the other was steady

my fellow Jag examiner reckons it is not unusual for a carb to whistle, and that I should leave it alone.
(from major work)

I will however pull camcovers and check clearances, and also have ordered a leakdown tester, and a new set of leads.

I do think there may be more than one issue at play

11

yeah i am very happy.

12

Thank you so much for this.

13

I had a similar problem with the 4.2L XK engine in my 1969 E-Type FHC several years ago and it was due to failed springs on one of the exhaust valves. This problem first showed up as an occasional lumpy idle. I was surprised when the compression tests were OK. But the spark plug color on one cylinder was different than the others and the exhaust on the left side was occasionally uneven indicating a misfire. So I knew something was not right in one of the forward cylinders (4, 5 or 6). In trying to verify which one for certain, I hooked up a Gunson Colortune and watched nice blue combustion in 5 of the cylinders but weaker yellow or sometimes no combustion in the suspect cylinder, #4. Convinced now that something was odd in that one cylinder I removed and disassembled the head. Sure enough the valve springs to one of the exhaust valves had failed. When compared to the other valves, I could easily compress the springs for the bad cylinder with my two fingers. The valve springs were considerably weaker than the other springs to the other valves. I was in a hurry to get the car running again so I installed one of the spare 4.2L heads that I had sitting on a shelf, along with a new head gasket that I had on hand, and the car was running nicely again right away.
I did not do a vacuum test at the time, but I wish that I had. If I had, I suspect that the needle would have been jumping around like what you experienced because the valve was not always sealing properly due to the weakened springs. I seem to recall there might have been some valve guide wear as well that made the spare cylinder head the my obvious solution for a quick fix.

Paul

14

Interesting…I still havent attended the issue, but I feel compelled to do something soon, and the choice is head off, or replace rear 3 springs with head in situ, by removing cams, and using compressed air, (or making a positive tool out of an old spark plug and threaded rod,I think it will work). .would be nice to just nail it down to a single cylinder, will check the spark plugs asap

I have not done a leak down test yet, not sure that will show anything, but will do one before dismantle

15

you can always see best robotic vacuum for pet hair here.