So, I need to adjust the amount of advance that my capsule provides. Mine is an 86 on a Lucas system and it has an adjustment screw with a lockdown nut.
Does anyone have any information on just how many degrees total advancement the vac cap should produce at 15inches of vac?
The non-adjustable unit on my 88 is supposed to have 12 degrees total at 15 inches HG.
Does anyone have any information on just how many degrees total
advancement the vac cap should produce at 15inches of vac?
I think the spec is inches of travel rather than degrees of advancement, but I
dunno the answer anyway.
– Kirbert
Dave,
Thanks that helps!
Any idea if that 12 degrees the actual movement degrees of the rotor or is it that confusing half ( or double ? ) the actual degrees because the dizzy goes half ( or double? ) the degrees that the crank moves?
Hmmm. Nothing to do with the rotor. Crankshaft degrees- you should see the difference using a timing light.
Perfect! Thanks again!
Follow up.
I was able to adjust my capsule to its minimum advance setting. It was close to minimum already. I then added the vacuum regulator via tee to the line.
Now my vacuum advance only receives 10 in hg. These two combine to reduce the total advance contribution of the vacuum advance. Somewhere around 5-6 degrees.
Better performance, no detonation.
Thanks for the help everyone!
Does your vacuum capsule get 10 in Hg just at idle, or beyond that point? Just interested in how you connected everything up. Is there a dump valve involved. I’ve been going over my distributor with an annual inspection, as it is still snowing around here, so I’m curious.
I then added the vacuum regulator via tee to
the line.Now my vacuum advance only receives 10 in hg. These two combine to
reduce the total advance contribution of the vacuum advance. Somewhere
around 5-6 degrees.
That vacuum regulator is only supposed to be involved at idle. Once above
idle, you should be able to pull full manifold vacuum at the capsule. Without
it, your fuel economy will suffer.
– Kirbert
I’m pulling vacuum feed from the top of the A bank throttle body. So no vacuum at idle.
Vacuum comes on as you open the throttle. Builds to 10 in. Caps there. Max vac contribution is less than before. Static is higher than normal.
This combined with centrifugal adv gets me to the very edge of detonation. Tested repeatedly.
These changes from stock were made in order to overcome several issues I was having and may not be appropriate for any other xjs.
I was having trouble initially with a terrible miss at idle. This was because of a too advanced timing. Setting her at 18 BTDC at 3000 rpm with vac adv disconnected and plugged resulted in terrible, weak performance and I was getting detonation. Reducing the static timing to eliminate detonation resulted in undriveable performance.
Reducing the vacuum advance as much as it’s adjustment ( my vac adv unit has a bolt and nut ) would allow was insufficient to prevent detonation while set at 18.
Capping the vacuum input at 10in. Hg and slightly reducing my static timing resulted in excellent performance, better milage, and a beautifully smooth idle.
My advance curve is now different than before, but total advance is as much as she can handle on 93 premium ( USA ).
I will replace the centrifugal advance springs with new if I can find them, but until that time I’m happy with the results.
Ok, I see what you did now. No regulator, no dump valve, and just one vacuum hose to canister. I use the regulator and dump valve layout on mine, so I guess as long as you are happy with the performance, that’s good! Dave.
Or did I miss something here? Is the regulator used or not? If so, how did you connect it up?
Close. Here’s my current configuration…
That item very top left is a small engine fuel filter to filter the air bled into the Regulator (8).
Fascinating! I would not have thought of that approach. So the vacuum goes to 10inHG and no further… I also like the pictorial- very clear.FWIW, I’m with you on the overheated Ign. amp. I moved mine to the firewall on the starboard side, behind the washer bottle. Made a decent sized heat sink from aluminum, and ran a 2inch air supply from a scoop behind the grille.In traffic situations there is a 3 1/2 computer fan that comes on when the brake is used, but only when the A/C is running. Temp is monitored using a fishtank thermometer with a 36 inch cable to inside the car.Works great.
Vacuum comes on as you open the throttle. Builds to 10 in. Caps there.
Are you using a vacuum dump valve? The reason for its existence is that
the vacuum to the advance module cannot bleed fast enough through the
vacuum regulator. So, if you’re cruising along at 45 mph – max vacuum,
max advance – and suddenly tromp the throttle, the advance would remain
fully in for a second or so – a second or so of detonation. You’re not having
that problem?
Other than that, it’s interesting what you’ve been doing. Jag used the
vacuum regulator at idle, you’re using it everywhere BUT idle! Jaguar
wouldn’t have used the vacuum regulator in this manner; rather, they would
have simply mechanically limited the vacuum advance range – which, in fact,
is exactly what they did.
On my '83 – which was plumbed largely stock, other than the cold start
scheme – I never had detonation at WOT but I did get detonation at light
throttle – indicating the vacuum advance was giving me a bit more than it
should. So I can see where you’re coming from.
– Kirbert
In traffic situations
there is a 3 1/2 computer fan that comes on when the brake is used,
but only when the A/C is running.
Interesting! So the brake lights and the A/C must be on for this fan to run?
Temp is monitored using a fishtank
thermometer with a 36 inch cable to inside the car.Works great, but K.
would probably disapprove.
It’s a waste of time, but at least it’s creative. Someone else, years ago,
simply relocated the amp to INSIDE the LH air filter housing, so it was
continuously air-cooled. I thought that was cute, too. But all this is for
naught, since overheating of the AB14 amp is only a problem when there’s
something wrong with it. The entire notion of relocating the ign amp on a
Jaguar V12 came from the Opus system, which truly needed to be relocated.
– Kirbert
I’m not using a dump valve in this scenario. Because the vacuum comes direct from the throttle body it vents immediately. I don’t have an in-cabin vac gauge plumbed so I can only test while stationary but, in those tests, the drop is fast as it was when I did have the dump valve plumbed. And no detonation on the road even when I try to force it out.
Right. I’m working with the components I have to try to correct for some issue that’s generating too much advance. I’m assuming my centrifugal is giving me too much because of weak / stretched springs. I could be wrong about that but it seems the likely suspect.
[quote=“Kirbert, post:14, topic:351017, full:true”]
On my '83 – which was plumbed largely stock, other than the cold start
scheme – I never had detonation at WOT but I did get detonation at light
throttle – indicating the vacuum advance was giving me a bit more than it
should. So I can see where you’re coming from.
– Kirbert[/quote]
My vac adv appears to be behaving as expected, but as a whole the system generates way too much when plummed as stock. This is just a working solution. Anyone know a part number for cent. adv springs that are a suitable replacement or where the OEM springs are available from?
Based on everything I’ve read in your “Book”, the ignition amp is more likely to heat up in traffic. Living here here in snow country I have six months a year to play with the car without actually driving it, so to keep my mind active I try to think of ways to “improve” the reliability. I understand that the intake manifold is an air-cooled heat-sink, but after taking temperatures, and then relocating the Amp, and watching the temps. fall a great deal, I’m happy with the result.
I wired the fan so that it only runs when ambient temps. are at their highest, when I’m using the A/C. And stopped.
Now that I’m retired, I have LOTS of time to waste!
I’m working with the components I have to try to correct for
some issue that’s generating too much advance. I’m assuming my
centrifugal is giving me too much because of weak / stretched springs.
I could be wrong about that but it seems the likely suspect.
Weak springs could be causing you to get centrifugal advance too soon, but
there is a hard limit on the max advance that is independent of springs.
My vac adv appears to be behaving as expected, but as a whole the
system generates way too much when plummed as stock.
I’m suspicious that there is something else afoot here. Like, perhaps the
timing plate under the crank damper is mislocated or something.
– Kirbert
I’ve heard that is a possibility, but not on my 86. The timing plate is not adjustable in any way. Thoroughly investigated that avenue. No slotted holes and the plate has a curved cutout that would impinge on the damper if moved 2mm. It wraps around it.
Also, I’m judging the excessive advance by detonation under load, not just relying on a light on the timing plate.
Good point about the max centrifugal advance though. It can’t go farther than the cam will allow. Hmmmmm… No idea now.
I’ve heard that is a possibility, but not on my 86. The timing plate
is not adjustable in any way.
Interesting! I wonder when they changed that!
Also, I’m judging the excessive advance by detonation under load, not
just relying on a light on the timing plate.
There are LOTS of things that can cause detonation under load. In fact, I
woulda been happier to hear what the timing marks were saying.
BTW, my '83, with 11.5:1 compression, ran essentially dead stock timing (18
deg setting, OEM centrifugal advance, OEM vacuum advance) and would
SOMETIMES ping lightly under light throttle when running on regular 87
octane fuel. Seemed to knock worse on brand name gas, less on no-name
brands. Whatever, I just switched up to mid-grade 89 octane gas and never
heard a ping again. It’s that experience that makes me wonder what’s up
with your car.
– Kirbert