Do i have a vacuum leak?

Saloons
1

I’ve been chasing mixture and timing trying to perfect my idle (4.2 S1 motor), fully warm it’s about 8-900 rpm in neutral, 6-700 rpm in drive. Steady but with an intermittent miss and the engine sort of vibrates, it’s not smooth. Plugs looks tan, about right, maybe slightly lean. I’ve tried richening with the screws but it doesn’t make that much of a difference and the engine never pings.

I’ve gone thru and tightened everything I can get a screwdriver or wrench on, but I’m not sure if I still have a vacuum leak. My 123Tune has a vacuum gauge on it that looks like the image below. At idle it reads 0. At speed it goes to -0.3 and holds there.

123tune4rvb21b23
123tune4rvb21b23.jpg800×800 82 KB

Is that normal or should I be seeing something else?

Thanks.

2

Check the brake servo. Plug it off and see what happens to the vacuum. There is a seal around the large piston that can leak

3

Where are you picking up the vacuum (where on the engine and where in the world)? Does S1 refer to series 1 XJ6? Your readings are about right for ported vacuum to the dizzy. Assuming XJ6 that would be possible outside the USA but not in it, as vacuum retard dizzies were fitted.

4

Theo, you’re living my life with my XJ6. Drives well and pulls like a steam train - but idles roughly in drive. Tightened an isolated as much as I can. Vacuum gauge reads about where it should - about to kit the carbs. Cylinder head rebuild two years ago. Paul

5

I should’ve been more descriptive, it’s a 4.2 from a S1 E type, 9:1 with 2x HD8s. Carbs rebuilt last year when the new motor went in. I’ve tightened and retightened everything, though there are a few fittings and hoses under the intake that I can’t get to easily or at all.

The pick up is on the top of the front carb, I believe this is the usual place for distributor vacuum advance?

6

Theo – you’re using the HD8’s ported vacuum, which means the throttle plate closes off the vacuum when the plates are closed (at idle). This would explain no vacuum at idle.

I’m running an EDIS setup on my engine (3.8 with HD8s) and have been learning about ported and non-ported vacuum. Suggestion is to used non-ported for the reasons they mention below. I have my engine running on my test stand and am taking the vacuum connection used for the power brakes and plugged the ported vacuum on the front carb.

Doesn’t 123 Tune also suggest using non-ported vacuum?

Tom

Below snip came from: https://wiki.autosportlabs.com/MJLJ_V4_vehicle_installation_guide

image
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And a good writeup on why you want to use non-ported vacuum:

7

To check gauge vacuum, I take the hose off the brake booster and measure there. Paul

8

Yes gents, I am chasing a vacuum issue…fit a manual vacuum gauge to a non-ported off take.

needle vibrates rapidly between 1-2 degrees at ~17Hg

there is an audible whistle between 1000-1500rpm

many things have been done to chase this issue, likely next step, change out rearmost 3 cam springs, head still on car

9

Tony, the note on my vacuum gauge says “late ignition timing” immediately under 15 to 18 in-Hg. I did advance our XJ6 a little and it did show an improvement - just as a check. Will be interested to see how you go. Failing to manage a good steady idle is annoying, especially when you expect its one hour’s work! How have you determined it’s likely the three rearmost springs? Paul

10

Their is an audible whistle emanating from the rear carb throat, its rythmical, sounds like air rushing over a disturbed surface when a tube is held close.

have checked for all leaks, replaced rear carb, tried all variations of timing & tuning.

its hard to balance rear carb airflow, the sound is present, its possible the motor has been over-revved, most possible causes ruled out, vacuum gauge chart indicates broken or weak spring, or lose or worn valve guide to give this symptom.

This is a used high performance motor, but I do not know its full history

One thing yet to do is a leakdown test.

additionally, if the distributor advance is set via correct dwell angle, the points gap is very small around .010…not sure why this is

to change out the rearmost 3 springs will rule them out, cost $60, I can do it in one day

a poster confirmed this was his issue.

If it does not fix it, I will need to consider whether removing the head should be considered

it performs well enough, but does idle a bit rough, and I feel like it is not pulling equally hard on all 6 cyls to deliver expected additional power to motor it replaced

compression is equal on all cyls ~165psi 9:1

11

Ok learned something, about ported vs. manifold. Perhaps the rough idle is related to timing at idle? I will try to get a vacuum gauge and see what kind of readings I’m getting.

Also planning in installing 02 sensors so I can dial in the mixture in the next few weeks…

12

I’m confused about this and would be interested in hearing some opinion. The article states that ported vacuum came about with emissions requirements. Proof is that idle speed rises if vacuum advance is provided by substituting non-ported vacuum. In practice, you would slow the idle speed down after you made the switch, benefiting from less fuel consumption under the low-load condition of idle, just as you do via vacuum advance when cruising.

I tend to agree with all of this, but I am uncertain about two things.

First is the issue of obtaining a smooth idle. By adding variable spark advance to the mix of things that determine idle speed moment-to-moment, it would seem that you would add instability. If the idle surged spontaneously, the vacuum would change, changing the spark advance, changing the idle speed, etc. That’s potentially an unstable positive feedback loop.

Second, in designing a dizzy, does the correct range of mechanical advance (programmed in by the manufacturer by selecting springs, stops etc) depend on whether vacuum advance is present at idle? I can’t figure that out. If it does, then even if the principle of switching to manifold vacuum has merit, would it require a new centrifugal advance timing curve?

There’s a lot on the web about this issue, with most “experts” agreeing that ported vacuum is an emissions add-on. But it came relatively early on, ca 1960 or earlier, before emissions tests in the US.

13

Intuitively, what you say makes some sense - introducing another point of potential variability using non-ported vacuum might compound problems vs ported vacuum equals zero. From my understanding at modest RPM to redline the two become equal: there is no net difference to impact other settings, weights, springs etc. Shouldn’t require a new curve? Good discussion and happy to be corrected. Would like to get my 4.2 to idle like my 3.4 - some say it is the smoothest of all! Paul

14

Well, steady 20 inches of vacuum at idle. I put the gauge at the end of the line to the booster, then put in a T to see if the booster was leaking - same deal, 20 inches at idle. So it seems vacuum leak is not the problem.

15

  • have you measured identical airflow thru each carb ?

  • could try a new set of plugs

( with an infra-red tacho running off harmonic balancer and pulled each lead one at a time to see if the VERY accurate tacho could detect any variation in the respective drop, no luck)

initially set ~9*BTDC static, eventually just set the dizzy by hand for the best idle with engine running, wasnt far off that

mine idles as you describe, a bit lumpy with the odd miss

had some other jobs to do, so just let it as is, been driving it 6mths

16

Thanks, Paul. I think you’re right. You would just need to disconnect the vacuum when setting the static timing, which isn’t necessary with ported vacuum.

17

Does anyone have the full size version of this image? http://www.jag-lovers.org/snaps/snap_view.php3?id=1297375037&n4=

I have the following timing curve in my 123 which I found in the archives…

2%20curve
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10* at idle, I might try a few more see if it makes a difference.