XJ6 & XJ12 Series I, II & III 1968-1992

Spark

 
  #1  
Old 07-11-2016, 07:10 AM
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Ok, have fuel and engine revs fitfully and excitedly. but no fire.
I get a powerful spark from the short lead from the coil to the distributor when I hold it to the engine block.
the cables from the distributor to the plugs do not appear to be sparking at all when held to the engine block.
voltage seems to be getting where it should in and around the coil, but something ain't firing.
 
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Old 07-11-2016, 07:41 AM
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Remove the cap, turn it over and observe the condition of the carbon brush in the centre.

They fall out, wear away.

This is the small item that passes the spark from the coil, to the rotor, and then the rotor to the plug leads and then the plugs.

Without that carbon brush, some engines will run, albeit rough and odd. Some do not run at all.

This is a common thing on the V12, but can happen to ANY car.
 
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  #3  
Old 07-11-2016, 05:27 PM
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thanks
rechecked the dist. cap and while doing so discovered the broken rotor arm. so the cap looks find but the dist. rotor has cracked, stayed in place on the first check so I didn't notice it.
 
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  #4  
Old 07-11-2016, 08:45 PM
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Goodo.

Another simple repair on a simple car.

Ya gotta luv the Jag.
 
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Old 07-12-2016, 01:09 AM
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Most of us have been there. At least two causes:


1. Us. Not getting the rotor on square and fully seated.


2. Maker. Some after market rotors are not worthy of the name junk.


Carl
 
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  #6  
Old 07-12-2016, 10:59 PM
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well got a new Rotor from Repco a fuelmiser Euro BD250 and it fits okay but no change in status. the original one certainly needed changing as broken in two. But same fitful bright revs but no fire. I can see spark at the end of the leads from the dist. now tho I am fairly sure. might try reseating the rotor a few times to see if is a seating issue as JagCad suggests. Before I more on to wondering about the cap etc again.
 
  #7  
Old 07-13-2016, 12:02 AM
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Remove the spark plugs and clean or replace them.

They may be sooted up from "trying" to do their thing.

Since the rotor was busted, I would be suspecting the cap some sort of damge.
 
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  #8  
Old 07-19-2016, 06:28 PM
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dusted plugs, checked leads, replaced dist. cap to be sure.
can see spark from coil to dist. and now see sparks on end of cables from dist. to plugs too. fuel is reaching the carbs. have cleaned the air intake and checked oil in carbs.
spark plugs only 5 months old.
so the fuel is likely not getting from the carbs to the cylinders, right?
that begins to sound seriously complex. next move?
 
  #9  
Old 07-19-2016, 08:11 PM
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SU carbies I will assume.

2 different models used, HD8, or HS8.

The HS have a small hose underneath that transfers fuel from the bowl to the main jet. Extremely simple carb to clean and set up.

The HD has no such hose, it has a "cast alloy arm" that transfers the fuel, and has a diaphragm on the base of the main jet, and is a general PITA carb, outside of cleaning as mentioned below.

Either way here, remove the bellchamber (that large bell shaped thing in your face as you look at the carby), and CAREFULLY clean ALL the soot and rubbish from inside that bell and the outer circumference of the piston. Do ONE carb at a time.

This will 99% get any SU back up and working. Those pistons stick inside the bell, and that causes fuel starvation issues big time.

It may have Stromberg CD carbs. If so, good luck, I know nothing about them other than trashing them for SU.
 

Last edited by Grant Francis; 07-19-2016 at 08:14 PM.
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Old 07-19-2016, 09:15 PM
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AUC 8102 is marked on the top of bell-housings.



 
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Old 07-19-2016, 10:49 PM
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Maybe HS judging by images on Google that look same?
 
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Old 07-19-2016, 11:15 PM
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Aden I had a no start issue a little while back and Grant kindly walked me through it....I was getting fuel to the carbie but after removing the bell housing I found that the little black (don't know what you call it) like grub screw had come loose and fallen off into the bottom.....worth checking.
 
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Old 07-19-2016, 11:20 PM
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Also mine were filthy and cleaned up well (thanks again to Grant and Clarke)


 

Last edited by Daf11e; 07-20-2016 at 05:55 AM.
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  #14  
Old 07-20-2016, 03:31 AM
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Yes, mine look very similar. Two further observations, down inside the lip under the bellhousing back toward the engine is a lot of watery-oil substance, cleaned the otehr day but has returned. Ideals where this is from?
Took a sample of the petrol being pumped to the carbs and it looks cloudy like two fluids failing to mix, that does not seem good. Would water in the fuel (no idea how if so) cause the car to simply fail to spark period or would it be more irregular than that?
 
  #15  
Old 07-20-2016, 04:08 AM
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That oily stuff may be the fluid from the dashpots that have been overfilled.

The odd looking fuel will have ZERO to do with spark or no spark, but the ability to "burn" would be seriously reduced, and cause some very erratic running events.

Water, or any contamination usually enters via the fuel cap, and/or the cap seal. Normal condensation inside the steel tank/s can build up over years of little use, AND the fuel we get today does NOT store so well, and if it is Ethanol blend, that is known to cause wierd stuff as the watery content of it seperates. I would drain the tank/s and replenish with fresh fuel, and ensure the carbies are drained also, and I would also renew the fuel filter.

This small write up may help. I also have the LARGE version (28mb) WITH PICTURES and that will have to be sent to your private email address, as it is way too large to upload here. Send me a PM if you want it.

SU HS8 Tuning and setting up.doc
 

Last edited by Grant Francis; 07-20-2016 at 04:14 AM.
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Old 07-20-2016, 05:16 AM
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Old 07-21-2016, 06:42 AM
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thanks, have looked at these documents and helps a lot, have looked at some basics on carbs too to clarify.
rough diagram attached of the black bay pipe that rights along the engine side and onto which the carbs are attached. just trying to clarify in my head what I figured against what is fact, I'd always thought this pipe carried water and the fuel went in beneath the carbs but it seems while there are encased water/air pipes at either end and it is bolted at the right to the filler neck for the coolant the main bay for the fuel to make its way to the cylinders? or are these merely air ways and the fuel travels beneath the bridge part of this bay.? the red ovals are the gaps onto which the carbs are affixed. going to look it up a bit more as not sure I have the impression clear yet.

 
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Old 07-21-2016, 08:44 AM
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Don't know for sure. but, the diagram suggests a water cooled or warmed intake manifold. slick.


Carl
 
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Old 07-21-2016, 09:16 AM
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Yep.

Double walled casting, incorporating the inlet manifold with coolant passages around it.

The spigot at the rear is coolant TO the heater tap.

The 2 spigots underneath are brake booster vacuam, and a/c system vacuam, from memory.
 
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  #20  
Old 07-27-2016, 05:29 AM
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Well, cleaned to shine the carbs and re-set but still no action.
If something huge like seized pistons or timing chain what noise would I hear?
I mean thing is the fuel is getting to carbs and the plugs are sparking as best I can tell. Doesn't leave very much to go wrong in between those points.
 

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