Fuel consumption, CO

(nickhill) #21

Thank you all. My ignorance is a little less than it was…

Will have another look in a couple of weeks and report back…

(nickhill) #22

Found this handy vacuum schematic


(Frank Andersen) #23

Note that this is the European set-up, Nick - specific to that model year and equipment level. Specifically that the dist vacuum source is to the manifold; US versions the source would be ‘ported’ - on one of the carbs…

Also, that the one-way valve between the vacuum reservoir is not shown - it’s there on all models/market/years…

xj6 85 Sov Europe (UK/NZ)

(nickhill) #24

Thanks. Frank i note you advise a spark plug gap of 0.035 Inches based on the Cranes ignition system. But does that mean I should have N11YC plugs or N12YC / 2404 plugs? Or something else?

Haynes says 0.025 for the former and 0.032 for the latter…



(Frank Andersen) #25

Haynes probably does not know that you have the Crane, Nick…

The point here is that electronic ignition can stand, and should have, larger plug gaps. The electronic CE system used on SIII specifies N12Ys and 0,035" plug gap - and should be suitable for the Crane…?

By and large; plug selection is not really vital, except for mechanically fitting. However, the heat range of the plugs used should be selected based on the kind of driving expected - cold plugs for racing…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(nickhill) #26

Thanks. Racing seems an unlikely pursuit.

I did away with a suspect connector on the vacuum line from the forward carb to the distributor vacuum capsule so that line should be leak-free now. I may be starting to imagine things, but my impressions are that (1) idle speed has crept up to 750 from 650 in gear; (2) the exhaust smell is different (cleaner, fewer unburnt hydrocarbons to my nose) and there seems to be less CO; but (3) mpg is about the same at 16/17 on the highway at 50-70 mph.

Does this make any sense?

We’re into colder weather now, just above freezing, not sure if that would change anything, I guess it shouldn’t.

Incidentally while driving my fuel and temp gauges suddenly went to zero and then back up. Gave me a fright for a moment to see all my fuel apparently disappear.



(nickhill) #27


When I suck on vacuum line to the distributor vacuum capsule, shouldn’t something move under the distributor cap? Nothing seems to happen.

(Carl Hutchins, Jr. ) #28

You may not be able to “pull enough Hg’s” in that manner. A Mitty Vac does that test nicely.


(David Jauch) #29

I can pull enough, but barely. Try moving the assembly with your fingers, can you do that?

(Carl Hutchins, Jr. ) #30

Fingers will prove mechanical freedom, but not diaphram health.


(David Jauch) #31

Would I be overlooking a failure mode if it held my ‚vacuum‘ and moved mechanically?
Maybe the lever detaching from the diaphragm.
Idea: connect the distributor to any car and have high vacuum at idle to see whether it moves. No mityvac (sp) required?

(Paul Wigton) #32

Move it to full advance, then cover the tube and let go of the advance plate: if there’s any leakage, the plate will not stay in the advanced position.

(David Jauch) #33

…and put your finger on the port, advance and remove the finger: if it hisses the connection of the diaphragm is sound…
All cleared up now so, to the post before the distributor: gauges hiccuping can be a bad dash ground or a common voltage regulator, I think fuel and temp have something in common there?
Idle mixture would make sense because a leak would have most impact at idle, but the idle went up whilch happens with more advance, more vacuum. Also means longer combustion time, fits right in. That the mileage did not improve, no clue. Could be you imagine that, could be that the vacuum leak downstream somehow has the most impact at idle somehow-?

(tony) #34

thats a good idea

our 35 yr local Jag mechanic advises they are all stuffed, dont bother replacing them, just seal off the vacuum ports!..not a view i necessarily subscribe to

(Frank Andersen) #35

While mixture changes will change emissions; the changes in fuel consumption may not be all that noticeable, Nick…

‘Idle in gear’ is not really a valid measure of changes - all idle matters should refer to idle in ‘neutral’ or ‘park’.

While sucking on the vacuum hose should elicit some dist reaction, but you might have to remove the dist lid for visual observations?

Is there a direct line between the carb and the dist? Have you checked vacuum at the dist end; you likely have ‘ported’ - which should then read about ‘0’ vacuum with throttle closed and varying vacuum with increasing throttle. Ie, with throttle closed, any work done on the vacuum line connections should make no difference to emission or rpms in idle. However, if the carb vacuum connection delivers manifold vacuum in idle; repairing the vacuum connection will substantially change idle timing. Which may affect emission and idle…

While driving; if the vacuum regulations was ‘inactive’, and still is - there will of course be no change to petrol consumption.

xj6 85 Sov Europe (UK/NZ)

(nickhill) #36

Thanks all. I checked a few more things:

(1) Carb gives vacuum at higher revs
(2) distributor advance moves when I suck hard!
(3) rpm seems normal at about 850 in Park

I think I will change the plugs and set the right gap and then see how it goes over a longer run.


XJC4.2 1975 (Canadian spec)

(Frank Andersen) #37

Try lowering idle rpms to 650, Nick; it is the general specs for carbs - and will slightly improve overall mileage. If this makes the engine uncomfortable to drive - you can then increase idle to suit…

However, when talking fuel economy; while your mileage is not way out - one cannot neglect carb set-up. With plugs changed and after some driving - recheck plug deposits for signs of excessive fuelling. Either caused by lack of proper carb synchronisation - or something deeper. Like unmatched float levels, too high fuel pressure - or that manual choke.

Plugs should be uniformly ‘brownish’, indicating well adjusted carbs - but checking plugs is not a good substitute, for a gas analyser…

And what actually is your ign advance - ign timing is not irrelevant to fuel economy…

Mind you; you are OK - you can pursue matters at some leisure to get the mileage up to 19 or so…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(sawyer10702) #38

Nick i bet a fiver your car will start witout you pulling on the choke cable? Check your needle on that left carb. Stick a screw driver under the needle body and just lift it 1/8 of an inch when car is warm and idle. Maybe the choke is not really off. Or that carb needs to rebuilt sometimes those floats in the zs need to be replaced and adjusted. Also relace the fuel shut off valve
Had same problem.

(nickhill) #39

Thanks, yes doesn’t really need choke even on cold mornings…

Will take a look although likely to recruit a mechanic for all the timing and carb tinkering.

(nickhill) #40


Brought the idle speed down to 650rpm or near as can be. For the moment it seems comfortable enough. Also replaced the plugs which came pre-set at 0.9mm which is almost exactly 0.35”. Champion rather than NGK though.

Long drive tomorrow so we’ll see how that works out.


XJC 4.2C (Canadian spec)