Further to previous posts. Now have idle speed correctly set on my XJC 4.2 (1976). About 850rpm in Park and 650/700 rpm in gear. This was a garage job as I can’t figure out the US spec Strombergs and after-market manual choke.
Anyway while the car runs nice and smooth I am still getting excessive fuel consumption (yesterday about 13mpg although this was a mix of 60/70mph, traffic jam and town … mainly highway though, I would have expected about 18mpg all in) and the exhaust still seems heavy on hydrocarbons and CO. Whiffy, you don’t want to be, for example, breathing nearby. I’m not using A/c.
Any tips on what can be causing this? Do I need to do a full carb and ignition service?
Sounds like it, although I don’t have stranglebergs on my 4.2 the SU gives me up to 22 Imp MPG on a run. And I have a high idle so I’m sure they are not set up correctly, I intend to refurb a spare set at some time and swap them over.
My experience is as Robin’s. Freeway and a little suburban work with two in the car gives 22.5mpg imperial. My SUs have been recently rekitted - including new throttle shafts. I bought all parts from SU in UK. Paul
with twin SU and auto gearbox I manage around 19 MPG (15 l/100 km) mostly motorway, highway, little suburban, no city, no cold running.
I was never able to get much better (is your indication rather typical, Robin, or more like “I did it once but never was able to repeat it …”?), but was clearly around your mileage with cold weather city driving.
If you smell raw fuel at the tail of the car it may either be overly rich tuning or a fuel leak around the tanks, pumps, plumbery in the tank area. What do your plugs look like? It might help to do an emissions test.
Some years ago I tried to really lean out the mixture and indeed it would backfire if I pushed back the manual choke too soon. Two years later we’re back to square one - even after long motorway trips the exhaust fumes still smell rich. So I guess the engine likes rich combustion and somehow the carbs adjust to it …
**
‘…traffic jam and town - mainly highway though’, together with different driving styles, makes comparisons difficult, Nick…
While I agree that your engine is likely running fat - only relevant emission measurement can confirm or refute. The xk likes running rich, and will react if lean - and of course backfire if too lean, as Jochen mention.
It’s possible that the garage set the carbs up more for ‘good’ running than actual best economy? Your hesitation in doing this yourself is fair enough - but in addition to by-the-book carb tuning; a personal touch based on driving doesn’t come amiss…?
Basically, I think you should run a gas analyser before ‘drastic’ action - and consider some hands on work yourself? Including checking that the carbs are synchronised - without necessary ‘doing’ anything with it.
Reducing idle will only slightly affect economy - Stromberg versions on automatics specifies 650 rpms. You are of course sure that the manual choke is fully ‘off’…?
How are you calculating your MPG? My 4,.2 on SU’s manages 17 overall (ie over a period of 12 months or more). If you have only just adjusted the carbs it may take time to settle down to an average. I run a book to record all fuel bought since purchase, dates, mileages and than do the simple arithmetic.
Thanks all. I’m pretty sure the choke is off, yes. Immediately after the mechanic did his tinkering the problem was better (per smell test), now the mix seems to have adjusted itself back and barely needs choke to start even on cold mornings. Hmmm.
My mpg test is admittedly imprecise. Fill tanks until the nozzle cuts out, drive, fill tanks until nozzle cuts out. But it was not a very long drive and included something town and traffic which I suppose gives rubbish mpg. Previously I was getting 17mpg over much longer drives so I guess it will revert to something like that. But on my previous XJC I got something closer to others, i.e. 20mpg or a bit better.
I might have to do an exhaust test anyway soon so I will see what that throws up. In the meantime, I have a 1000 mile drive lined up at year-end (weather permitting). Sounds as if the best plan is to leave it alone before then and take stock afterwards. I might burn 25 unnecessary gallons on the trip but then I can be surer of the issue.
Thanks all. So i took a look at the plugs. 1 to 4 are black and sooty. 5 and 6 are grey-brown. The gaps are about 0.7mm / 0.28". They have only seen 3000 miles or so. The O rings in the Strombergs were also renewed at this time.
So seems definitely too rich… Question is how to address this…
it definitely looks like there’s a vacuum leak on cylinders 5 and 6. If you try to tune the car by the exhaust emissions you’ll get some cylinders running overly rich and some too lean - in your case too lean looks just about right, but the others are definitely too rich, if they’re looking like that right after some serious driving.
Once the leak is fixed the engine will be ready to be tuned correctly.
**
We agree that #1 is the rearmost cylinder, Nick…?
It’s the a typical sign that the rear carb is running fat, the front ‘normal’ - the two middle cylinders are sort of fed by both. Not entirely conclusive; both may run fat with an air leak associated with the front. If only one carb is running fat; carbs are not synchronized - and if it’s an air leak the they can’t be synchronized…
The plug gap is correct for the mechanical points ignition - though has no bearing on the fat running…
So I need to look for a potential vacuum leak… ok. I guess I start with the various hoses? One curiosity of the carb set-up is that the front carb has a vacuum hose while the rear one is capped in the same place (see previous photos).
By the way it has an aftermarket ignition system, see photo.
Further investigation reveals that there is no vacuum to the foremost carb. It is connected to the distributor vacuum capsule. Removing it makes no difference.
However if I disconnect the other vacuum lines situated towards the right hand rear part of the engine bay, the rpm rises appreciably.
I’m guessing from this that indeed the foremost carb is not set up properly due to lack of vacuum somewhere which is depriving the carb and / or distributor, but the rearmost one is correct.
However I am not entirely sure what the culprit is here. Could it be simply the vacuum hose? But if so wouldn’t I feel vacuum at the outlet at the carb? Next time I will of course check the hose itself. But in the meantime ideas welcome.
for the moment I can’t really make a picture of what you’re talking about as my car doesn’t use a distributor anymore …
Checking for vacuum leaks you might rather induce changes on the idling engine by spraying either water (safe, but less expressive) or brake cleaner (less safe, but more tell-tale; keep fire extinguisher at hands!) around the intake manifold. If RPM change, in particular: rise, at any point, you’ve identified the leak. Try to get as specific as possible to find out where the leak is located before pulling the intake (I’m writing from experience - in my case the intake manifold gasket was the culprit - yet it could be any other place behind the butterfly as well).
**
The carbs do not ‘get’ vacuum, Nick - they produce it…
The only exception is if you have the ‘anti run-on’ set-up; a vacuum hose is then connected to the carb bowls. A vacuum spigot elsewhere on the carb may deliver either ‘ported’ or manifold vacuum to the dist…
If there is no vacuum at the dist end with the throttle closed, idling, it’s ‘ported’ - and out of idle; vacuum will come, varying with throttle position. This is the standard US dist vacuum control…
Only one of the carbs is used for dist vacuum control. And disconnecting the vacuum hose, either end, just disables dist vacuum control - with no effect on idle with the US set-up. In fact; some run the engine without it, relying only on centrifugal advamce. Which works, after a fashion…
**
These vacuum lines delivers manifold vacuum - used for AC/heater and other devises using manifold vacuum. Disconnecting any vacuum lines connected to the manifold will let more air into the engine - and the rpms will rise…
**
The reactions described are normal, and in themselves not indicative of carb set-ups. However; spark plugs show that the carbs are indeed not synchronised; either because proper adjustments have not been carried out - or something is affecting the rear carb. My suspicion initially was the manual choke - which seems to operate only the rear carb?
Whether or not the difference in carbs relates to vacuum leaks is still uncertain. But there is every reason to suspect the rear carb for running fat - which is not caused by a vacuum leak.
A usual cause of fat running is incorrect float level - which cannot be countered by the normal carb adjustments. The prime suspects are float level adjustment and/or a faulty needle valve - which at some stage must be addressed…
As an aside; you have the ‘Crane’ electronic ignition system which means that the plug gap should be increased to 0,035". It will not eliminate the fuelling issues…
