Hi all
New to this forum. Located in Denmark Europe and new owner of a low milage 1984 XJ12 series 3. Been around classic car for 20 years and educated marine- and electrical engineer.
Car has only 50.000 miles and not driven for 10 years before previous owner bought and drove 200 miles in two years. Car drives but felt "dead" when I got it and didn't seem like a 300 hp motor.
Car started as it should, idled at 1000 until warm and 7-800 rpm at hot.
Part from changing fluds, greasing, cleaning grounding points, tires etc. etc, I have replaced the spark plugs and in that process discovered that the vacuum advance unit on the distrubutor didn't function.
Removed distributor after marking right position and gently having removed the rotor without pulling too hard on it. Replaced the vaccum unit with a new from SNG Barrett, installed the distributor and timed it to 18 deg. BTDC at 3000 rpm.
Everything works as it should, moved when applying vaccum and rotor can spin the 11-13 deg. freely as it should and returns immediaetly to correction position when let go.
Car idled at 7-800 rpm hot as it should.
Started the car the other day, cold idle went to 16-1800 rpm, and settling to 1000-1100 at warm. Checked the timing and it was way off the scale at BTDC. Readjusted the timing to 18 deg. BTDC at 3000 rpm without observing anything loose.
Wouldn't go lower than 1000 rpm hot and now 13-1400 rpm at cold again this morning.
Checked the AAV, had it off, and piston moved as it should. Have checked all other vaccum hoses and no issues. When removing the hose to the vaccum tank you can hear it has stored vaccum (drawing in air), so system is "tight". Idle doesn't change even if
adjustning it by means of the bolt in the aav all the way in.
Any hints on what can be wrong?
My suspection is the counter weights in the distributor but before taking it apart I'm looking for some advise/experience.
Thomas

 

And a few hours later. Started the engine, ran at 1000 rpm for a few seconds and then raising to 1200. Let it warm up and experienced following, if advancing the timing by the excentric to 6-8 degree ATDC the idle drops and settles at 800 rpm. If returning it to 2-4 degree BTDC it raises again.
Makes sense that it will raise the speed if igniting at the highest compression ratio i.e. just before TDC but still trying to understand what`s going on.
Pulled 8 of the spark plugs to ease turning the engine to TD cyl 1A and check alignment marks and then simply start from there. Not too keen on taking the distrubutor apart as it really seems to be in good condition.
Advise appreciated.

 

had a similar problem with the idle, hot and cold, your problem sounds like central weights sticking, worth the while cleaning the weight system, in my earlier days of mechanics we'd use a bit of HT grease on the weights plates etc but i find if you clean up - lube shaft and use silicon spray instead of grease - it holds on the surfaces better, no need to remove cam if it's not stuck.

 

Have you set the throttle butterflies to 2 thou gaps and are the throttle return springs good?

 

Yes butterflies set at exactly 0,05 mm and checked that they will come to rest at the stops when in idle.

 

I have many thoughts.

Most covered in the attachments below.

The fact the AAV is not responding indicates a Vac leak somewhere. Emissions nonsense is #1 in my book, always. It does not take much, and some markets have a nightmare of hoses etc to search through, we had zero.

 

Thanks Grant.
Hoped you'd respond as I've already dug into a lot of your experience.
I think it' something else, cause while replacing the vacuum advance I also cleaned up the cold start system acc. to this recommendation
What'll happen if the vacuum dump valve is defect?

 

OK, I ditched those dump valves waaaaay back on all of them.

Theoretically, vac on one port holds it closed so the required item does its thing. When a vac drop, manifold rear spigot (memory here), the valve opens and dumps the line vac, allowing, in this case, the capsule to spring back to zero, as apposed to "drifting back, as vac drops out of that line.

I ran a hose direct from the capsule to the INNER port under the A Bank throttle body, then looked at all the by-passed bits and removed them.

There would be???? a small vac leak if it failed, but I doubt enough to upset idle.

Unplug the vac hose to the capsule, plug the line/s, and see what difference it makes to idle integrity.

If you have a mirror on a stick, warm it up, pop the Inlet hose to the AAV and have a look, ensuring that piston is really closed, they are a fickle thing on a good day. Dont burn yourself, stuff will be hot back there.

Look carefully at the inner curve of the 90deg hose on top of the AAV, they crack for no reason, and that vac leak WILL cause what you have.

I will keep thinking haha.

 

Told you I would keep thinking.

Thomas,

Read all this again, and thought some more.

You have had the dizzy out and apart for service, good.

Here goes what I think may be happening, as I had it with the 85 Red Beast.

The 2 shafts of the dizzy have a Lucas Plastic Thingy as a bearing/spacer/anti rattle device, where the top shaft meets the lower shaft. This Lucas Thing dies after about 5 years, mainly the Anti Rattle wings, but the washer section stays there mostly. Until disturbed, when it crumbles.

That washer section does 3 things;
1) Acts as a bearing for the mechanical rotation of the Rotor Shaft,
2) Sets the Rotor Tip to Cap Segment "Arc Height".
3) Prevents the nubs and their plate from binding on the weights, thus "jamming" the Rotor Shaft somewhat.

Its explained a bit in the attached PDF, which I forgot to include earlier.

If its as clear as mud, let me know, I will sort it.