Also the mixture of the starting carb can be adjusted for summer and winter cold running. There is a little hex nut that the plunger needle passes through, limiting the stroke of the needle.
Interestingly, before itâs fully warmed up, it also adjusts while driving. As engine revs rise it sucks the needle further down, preventing the mixture from becoming over rich
Does it?
Vacuum sucks the needle down. At idle itâs the leanest, that mixture is determined by the nut. As the engine is loaded, there is a weaker vacuum, the mixture gets fatter as the needle rises. Thatâs determined by the needle profile and spring strength. So the ASC adds extra fuel under load.
At idle it doesnât (shouldnât?) suck the needle all the way down. As the revs rise it sucks the needle further down, weakening the ASC mixture as it begjns to draw more from the main carbs. Check it out and youâll see if you rev the engine at standstill and watch the needle getting sucked down.
When the valve has lifted, the needle disc chamber is in direct communication with the inlet manifold and the depression, dependent on throttle opening, varies the position of the needle by exerting a downward force upon the suction disc and needle assembly. Thus:
(a) At idling the relatively high depression will draw the needle into the jet until the needle head abuts against the adjustable stop.
(b) At larger throttle openings a reduced depression is communicated to the needle disc chamber and the spring will tend to overcome the downward movement of the needle, thus increasing mixture strength.
Idle sees the highest vacuum and needs the least amount of added fuel. The ASC is adjusted at idle while the engine is hot.
Maybe if you just increase the revs without load it will do something special but the needle is still all the way down at idle!
Itâs not only vaccum but total airflow that draws the needle down. Yes at a wide throttle opening the needle will rise giving needed enrichment, but at light throttle opening at higher revs it draws through the main carbs and doesnât need as much enrichment from the ASC.
I can see that at very light throttle openings at higher speeds the needle is further down. That is another low load, high vacuum scenario.
What is really important is that the needle is far up at wider throttle positions and all the way down at idle (aka very low at very low loads). Itâs a neat device.
1 Like
You seem to be in USA, so if you are utterly unfamiliar with working on vehicles, if I was you, I would offer to pay the expenses of a club or forum member if one is close enough and willing, that would cost you much less than having it towed to a mechanic
a member here mentioned he has looked at it
for example, If I was within 100 miles or so, I would tune it for you for less than $500, even if I needed to stay in a motel
It helps if the person has parts, such as a complete working distributor, ASC, carb parts
Thats not to say you cant do it yourself with advice from here, depends how much your time & money is worth
1 Like
Maddy has a property to drive around on, I got by in the fields with some YouTube videos, a manual and an approaching thunderstorm. I think sheâs more than competent to give it a try, and there is no pressure to get it right immediately. I would make sure everything works as supposed, take pictures and measurements of the current âbaselineâ at which Nick got it to run, then go through the parts that seem not to work (aka the ASC), and then set everything to the values quoted in the manual, balance, and then fine tune. I messed up twice in my tuning process and it wouldnât run anymore and always got it back to idling reasonably by setting it to the values in the manual (jet flush and down so many turns, etc.).
I just hope the mechanicals, like bushings, are okay. The distributor seems to do the job and Nick didnât do anything further than getting it to run so there is still potential to get it right.
The engine will tell you what it needs and eventually you understand what itâs trying to say. I like doing it
I actually use a vernier caliper tail to measure down from the bridge to jet, and the distance should be 1.5m for an HD6 or HS6 as far as I recall (according to Hamill)
Thats my start, I tune from there
Sometimes (always eventually) the Jet spring will rust away from water and scum in the fuel bowl, and you can see this with the bells and piston out.
No amount of fiddling with number of turns will help if the spring is collapsed
There is also a .040" drilling between the front carb and ASC.
If this gets blocked, the ASC wont work
There are a lot of minor things that could be wrong, I can think of at least 10
A non-Jag mechanic may have some trouble, or at least charge a fair whack
Well forum members you have certainly covered the topic very well. My response has been limited due to my involvement with home repairs and packing. I am not free to exercise the advice given here at the moment. There may be a Jaguar enthusiast who may help me. I think since the points do not appear correct I should purchase again the points then have some assistance. I will refer to this thread as I attempt to understand how this ASC is supposed to work.
Thank you to all who have shared your thoughts and experience.
Maddy
I will attempt the troubleshooting procedures shared by forum members here. I am curious to see if the ASC is functioning properly. I do not think this test will change the outcome regarding the engine starting.
Nick mentioned that the points were not right. This can affect the running of the engine and think it prudent to purchase another set whether I install them or the next owner does.
Maddy
The ASC IS what makes the engine start. If the engine starts cold WITHOUT the ASC it is running very very rich. That means if you set the carburetors right you will need the ASC to do its thing.
If the ASC works you can move on to the main carburetors and set them correctly.
High idle rpm mask a lot of problems. If itâs not running smooth at lower rpm try the new points, if you can get it to tick over nicely put them in the glovebox.
David,
Although what you say is correct, I am meaning that I am going to walk through the steps without changing anything to make sure I understand what needs to be done. I will solicit help in correcting the ASC should I have the time. In case you did not know, my husband and I are readying the house for sale and packing for our move.
Maddy
Hi MaddyâŚas I have saidâŚit starts, it runsâŚyou have video of that as well, no bad sounds from engine, good oil pressure. Our quick check of the ASC was that it was not workingâŚwe sent power to itâŚno click. In my opinion this is info for the next owner. Actually a minor easy fixâŚwith possible some new partsâŚnew ASC solenoid etc. Same with pointsâŚit starts, it runsâŚthe points in it are newâŚbut with the arm holding one contact bentâŚso not flat to flat contact when closedâŚand not correct dwell-duration due to the offsetâŚInfo for next ownerâŚeasy fix. If I were next owner Iâd be sending the dizzy out for a rebuild anyway with new vac unit.
It is a good dogâŚbut Let sleeping dogs lie. Waking oneâŚturnis it suddenly into a snapping dogâŚat least temporally. Keep packing up.
Nick
Nick
I was interested to read this, Tony. Iâve just done exactly the same procedure. Both measurements were over 3mm and now they are right on the 1.5mm mark. I suspected the engine was running quite a bit too rich, and I was having endless problems with the plugs sooting up and eventually misfiring if run too long at low revs. Engine and carbs recently rebuilt - new throttle spindles and bushes.
Iâm on my 3rd set of NGK BP5ES plugs in a few hundred miles (if that!). Yesterday I threw in a new set, but of course, the old ones will clean up fine.
After adjusting the jet heights to 1.5mm down, and running the engine for a bit, I took out the front plug and it still looked like new - no colouration at all.
In your experience, do you tend to fine tune a bit richer than the mixture produced by adjusting the jet to 1.5mm, Tony? I have a feeling my carbs are now a tad lean⌠Idle isnât as smooth as before, now sounding even a little bit âsplashyâ. I balanced the airflow on the carbs with my airflow meter.
I donât want to run the risk of burning pistons. Adjusting the mixture is such a faff on RHD 120s with that bloody steering column in the way of the rear jet assemblyâŚ
Yes, a bit rich is better than lean, spark plugs are cheaper than pistons
I cant recommend Hammill strongly enough, he details the best way to tune an XK engine, anyone with a nice XK should own this book, its very inexpensive
I just set up the carbs and move the dizzy by hand for best idle when fully warm
check with a strobe light that its good at higher RPM
Hammill recommends a road test with stop watch ( and looking at the plugs), and a few other things to make sure its not too lean
That is why the ASC must be tuned up correctly, our cars cannot be tuned or run for the first few minutes without that being right
You canât go by a certain depth, that is your starting point. Do the piston lift test - lift the piston a little bit. If the rpm drops, itâs lean, and if it rises, itâs rich. Slight rise and fall back to previous rpm is what you want. And if nothing happens you are way out (lean).
Maddy I didnât know you are moving and in that case it might be better to just sell it of as running but needing adjustment, were I a prospective buyer I would understand. Clearly the engine is running and it just needs some fine tuning.
David,
Yes we are moving from Washington State to Oklahoma to help with my husbandâs elderly parents. We have our 2 drivers and I have 2 collector cars that need transported. Selling the Jag means one less to transport.
Maddy
Dear David,
While it is clear that the ASC is the starting carb for the engines for a reason, their importance will depend on the weather: both on our XJ6 SI and in the XK, at anything around 26-30°C ambient (as now here), the engine will start without ASC, at yet get a healthy 1-2% CO at hot idle. So, the ASC may very well be needed in <20°C conditions (or whatever) but otherwise my experience is that the engine will start and run fine, except for perhaps a maximum of a minute.
I see the piston lift test as an indicator, but rather go for the (inexpensive) CO measurement at any decent shop.
The 3.4 manual, states that after a 6mm lift, the engine should run first increase lightly rpm and the âstutterâ -le moteur tourne en boitant- in my original French version, for the MKVII.
I personally like running all my classics, SU, Weber or after-market injection at higher CO levels (2-3%), but admittedly my spark plus are slightly blackened in all my cars, and yes, I already had a holed piston in a Cooper Mini with a HIF44 carburettor adjusted too lean <0.5 CO idiotically set by myself.
Best,
Ll.