Dear All,
I have followed the workshop manual a few times to tune my carbs and have the following queries.

Slow running jet/screw: the final positions (with about 500rpm on the car tacho and about 600rpm on an external instrument) are consistently very close to fully seating the screw (about 3/4 turn for front carb, 1 turn for rear carb). Is this typical? It seems very far from the starting position of 21/2 turns.

Mixture: lifting the pins does not give a clear rpm reaction as in the manual even with significant adjustments of the mixture screws. Is there a special tip for this method? I’ve also tried adjusting the screws to get 3.5% CO in each exhaust pipe. I found this value in the internet. Is this the correct value? Both methods consistently result in sooty plugs. I’ve tried adjusting the mixtures until the plugs are grey/brown but this quickly led to a badly running engine too weak)... Have I missed something?
Dave

 

If you need the slow running screws nearly all the way down, that suggests that air is getting in another way. Typically, that's from the throttle plates not being centered in the bore, or the throttle stop screws are preventing the throttle from fully closing. Another possibility is worn throttle shafts and air is leaking around those.

3.5% CO seems high. I use an air fuel ratio gauge to measure the actual exhaust emissions, that is a much more accurate way to tune the carbs. As a starting point, I'd have the jet about 2.2 - 2.3mm below the bridge of the carb and see what that drives like.

I use this gauge and it works well: https://www.aemelectronics.com/produ...uego-afr-gauge

You can either weld in a bung into the exhaust pipe near the manifold to screw in the O2 sensor, you can drill a hole and use a band clamp to seal the hole like this: https://www.aemelectronics.com/produ...-sensor-mounts
Or you can use a tailpipe clamp for temporary use:
I would be aiming for an air-fuel ratio in the region of 13.5-14 (14.7 is stoichemetric), as these older engines like to be a bit rich. I have found that idle tends to be a bit richer than under load; so measure the air-fuel ratio while driving. measure it at steady load driving. Acceleration enrichment is a different tuning technique, using the damper oil in the carbs.

 

I run 13.5 AFR and she's smooth as a baby's butt and just the smallest almost imperceptible wisp of brown smoke from both exhausts as I take off hard & then totally clear. (newly rebuilt engine that's being run in the oil man way. The Jaguar method is completely obsolete and could give you cylinder bore glazing or polishing ~ two different things, and an oil burner for life.)

 

Thanks gents!
I did check the throttle valves visually but I‘ll now repeat by loosening the linkage clamp bolts. Perhaps curing any leaks will mean the pin drop test will work better...

Does anyone know how they did the tuning on the production line?

Dave

 

I have used the Gunson colortune kit to adjust the A/F mixture . You can Youtube it and determine if its something that might be useful to you. Pretty simple to use.

 

I use the Gunson kit as well and its always been pretty good.

Regarding the lifting pin, i've never had any success with them but one thing that you do need to be aware of is that you only lift it slightly not all the way to carry out the test. As I say though never worked for me.

If you want to check for any induction leaks spray some carb cleaner around the manifold with it running, if the engine note/speed changes then you have a leak and until its fixed tuning won't work. The other things to check are that the pipes into the inlet manifold from the starter carb are not cracked (again spraying carb cleaner around underneath the manifold will show this) and check the float level in each float chamber is correct.

I assume the electrical system is all in good health and the timing is correct? If the timing is out then it will make it hard to get the idle speed down the same as if its running too lean. These engines tend to run quite rich, the old guy who I learnt from used to reckon it needed to be between around 7% CO on his emmission tester to run properly.

With problems like this its always good to stop and take stock and then start from first principals, set the timing, points, plug gaps etc. then move onto the carbs and set them up by the book (albeit I would use the colourtune to get the mixture right). Don't skip any steps just beause you have already done them, check them again and hopefully in the end it will be right.

 

I have the colour tune kit in fact I have two and I put one in cylinder 1 and the other in cylinder 6 that way I can adjust the two carbs independently to get the optimum colour across the cylinders. I found in the past using one was a bit of a guess with twin carbs. I know the inlet manifold is linked but using two in the outer cylinders shows the spread of mixture in the inlet manifold. You can achieve the same I would think by swapping one colour tune from 1 to 6 and back but this takes up a lot of time and you are juggling with hot spark plugs.

 

I'm lucky to have access to an exhaust gas analyser. If both exhausts give me the same reading ~ I'm happy. It also allows testing at multiple RPM. I'm not really interested in idle as long as it's smooth.

 

Exhaust gas analyser after basic set up.

 

The gauge I linked to isn't too bad for price, $245 CAD/ £160 and I have found it indispensable for tuning while on the road. As i mentioned on the other thread, I found that my basic setup was correct and I needed to change the weight of the damper oil for the acceleration enrichment. I couldn't have determined that accurately any other way

 

Thanks for all the ideas! I like the idea of using 2 Colourtunes simultaneously. I’ve in fact had one since 1973 which I used on my first car (Mini) but never needed thereafter.
Dave

 

Just don't take the revs up too high with a Color-tune, or you will have a hole in your bonnet or worse. They were made for idle. I've witnessed what they can do.

 

Agreed ~ I'm lucky I have access to a Schenck rolling road dyno & EGA very close to where I live. I can set IAT etc. I will never have to tune on the road. I've been ill for 2 years & only now we are starting to win. I've had every orifice checked e.g. colonoscopy etc. ~ nothing found. It's inherited from my Mom. Now that I have my mojo back with the right medication (I'm Diabetic) I need to put some miles on the car. I've only done 200 miles from rebuild. I'm running in on an SF 40 & it's time for bursts at full throttle while watching the temp gauge & using much higher revs. You will never run in an XK engine with modern lubricants using Jags obsolete methods. That could lead to cylinder bore glazing or polishing, 2 different mechanisms & an oil burner for life. My engine is never going to be babied or it will never run in. I want it properly run in so that I can move to synthetics. I'm going to use a 10W-30 or 5W-30. I have substantial oil pressure. I Plasti-gauged every bearing in the engine.

 

If you have the inclination, would you mind starting a thread on the right way to break in a new engine? I'll be doing that soon with the friends E type engine that I talked about assembling earlier in the month and want to do it the right way.

I plastigauged the bearings as well, and some of the crank bearing clearances were about half a thou (0.0005") smaller than the published range in the service manual, so I was wondering if an SAE 40 would be too heavy for a break in oil? I can get the Motul break in oil locally, but was wondering if that was too heavy for the bearing clearances? Once broken in, any advice on a grade to use? This will be a summer car with mild ambients, so probably will only operate in the 10-25°C range.

 

An SAE 40 is fine for break in. Shoves up the oil pressure while you are wearing off asperities. This is Petroleum based & not synthetic so lower VI. Thins more at high temperature SF 40 is about the lowest grade of oil we make. I would use an SE if I could lay my hands on some.

 

Just for the record...
I checked for leaks by spraying carb cleaner and found no leaks. I set up the carbs linkages again and checked the throttle valves are closing fully. I repeated the reference set up of the slow running jet and the mixture and balanced the carbs with correct idle speed. The screws for the slow running jets were again near to fully closed. Perhaps this is normal.
Then I used my Colortune at sparkplug positions 1 and 6 to adjust the mixtures for both carbs. That was successful and the first time with my Mk2 that I had a positive result on adjusting the mixtures. I already reported of having no success with the pin lift method or using a CO tester.
Dave

 

How's the condition of the seal around the slow running jets, if it's leaking there, that would account for you having to turn them out so very little to achieve the correct idle speed and balance, it may be sucking air around the threads ?
If too small of an O-ring is used it stretches and does not come in contact with the female part of the idle screw area.
If you're satisfied with the over-all running of the car though, I would just leave it.
On my car the butterfly valves do not centre themselves very well as that whole area is quite worn, it takes an extra a stronger spring to force them shut.

As for the lifting pin, it can take some practice to work it and reading the reaction of the engine, it's very subtle.
I find it only has to be lifted very little, so little that it's almost imperceptible, and only for a split second.
The engine will do what it says in the manual, the idle will slow _ too lean; no change in the idle _ still a little lean, but OK; I slight rise in engine speed then normal idle _ just right.

I don't use it that much, I like to listen to the engine when I turn the mixture screws and look at the plugs after a good long drive.
After I think the engine is adjusted correctly, I may play around with the pin and get a reference feel for it.

Ironically I never had any success with Colour Tune _ maybe if I had 2 of them.
To each his own.

 

Good Job ! glad the colortune worked for you. I'd leave it alone at this point and check the plugs after a good run.

 

You seem to have her idling OK. For high speed/power readings you need a dyno. Perfect monitoring of IAT etc. Most proper Dyno rooms have a cooling tower on the roof so you can control IAT (inlet air temperature) as it makes such a huge difference. Otherwise you must monitor IAT & do all the necessary calculations.

Just make sure your throttle butterfly shafts are not severely worn or carb bodies.

SU's don't like what they term "false air" (Sorry I can't find an overhead cam engine example but this does the job)






Buy this book. Not expensive.

 

I reckon you still have a problem because of those miniscule air bleed opening for idle
Apart from the possibility of internal leaks in the carbies there are a few other areas where "false air "as in Glyn's picture can get in.
Vacuum line to the brake booster reservoir. Try clamping this hose off and see if there is any difference. Could be a crook hose or leaky reservoir.
There are rubber hose joints joining the starter carby to the distribution pipes on the bottom of the inlet manifold. I have found these leaking a couple of times.
The rubber coupling on the vacuum line to the distributor. I found one split.
I use a piece of flexible PVC hose with an internal diameter of about 3/16 inch or 5 mm with one end in my ear and the other end moved around suspect points to hunt down vacuum leaks. It is very effective