Crappy running - Coil choice suggestions?

[The Original Tom]

Had the gas system tuned and all was hunkey for about a week, ran really well on petrol and gas.
It's had a very slight tick from the off-side bank towards the back (around no. which I took to be a small manifold leak, but I never bothered to sort it as I'm getting tubular headers at the end of the week.
Recently it's started jerking when it warms up (after about 20 mins driving) and is most noticable when moving slowly in forst gear at constant seppd, it's always jerking forwards and backwerds.
Cooling is fine as I'm running 3 leccy fans and the whole coolant system was checked and flushed before I rebuilt and the gauge never goes higher than it always has.
When I got to work this morning it was worse than ever, and when I left, it started jerking from pretty much cold. It idles ok, but when you apply the power it is completely gutless and almost undriveable. Limped home and just quickly checked the dizzy when I got back, as it does the same on both petrol and gas and both were running great so to both de-tune so much so quickly is unlikely.
Dizzy shows signs of arcing on both the rotor and the inside of the cap, both were new less than 500 miles ago. Very marginal side-ways slop in the dizzy but maybe enough to cause this?
Also I get intermittant ignition in the inlet manifold if I apply and power in too low a gear.

I need it sorted because right now it feels like driving with half the valves missing!

[ChrisJC]

Have you got a rev counter? If you have, then that's a good diagnostic on the low tension side of the ignition. If it's steady then LT is probably OK, if it kicks, then you've got an LT problem.

Chris.

[The Original Tom]

JC, it appears you're right! The Roadside Asistance man today reckons it was that too. That's right, it died spectacularly... in the rush hourn , now it's a no-go at all so I'm in the market for a new coil. I've currently got a standard lucas one (my second one actually) and an RPI A&R spark booster amplifier, so are any coil makes really worth the extra dosh?

[katanaman]

I cant make out from your reply of your saying Chris is right on the low tension part. If its low tension then it has nothing to do with your coil, your amp or dizzy is busted. Given comments I have read on the RPI amp I would start with that first.

[ChrisJC]

I always say this - you can't beat the std. Rover amp + distributor. They might not be perfect, but they're well tried and tested!

And a spark-booster?, presumably to get a bigger spark it gives the coil more of a hard time...........???

I would just go back to that if it were me.

Chris.

[The Original Tom]

I cant make out from your reply of your saying Chris is right on the low tension part. If its low tension then it has nothing to do with your coil, your amp or dizzy is busted. Given comments I have read on the RPI amp I would start with that first.

Right, I thought it was the secondary winding on the coil that was shrinking when cold and allowing a current flow, then expanding when hot, opening up a break and giving an erratic / non-existant spark.

I replaced the coil today (only £17 so no biggy). This yeilded a huge spark (able to jump about 1" from the end of the king-lead when the engine's turned over), also proving the correct functioning of the ignition amplifier, because there's no middle ground with these right? They're either working or not.
Now, I crank the engine over with the kinglead next to an earth - huge spark.
I plug the kinglead into the dizzy cap and spin the engine over - no firing.
Remove the leads from 3 separate plugs on the engine, and crank it over - not a single hint of a spark anywhere. So I'm now thinking rotor or cap? Guess that serves me right for buying those from Halfrauds as they're only a month old if that.
But as the whole thing is now beneath contempt, sod it, I'm buying a new dizzy!

[x.l.r.8]

Whoah, reread your post, there's nothing wrong with the dizzy. You have a spark to the cap, but it's not transmitting fromt he king lead to the rest of the HT leads, thats not the fault of the dizzy. 2 case sceanrios, 1 the rotor is the wrong one and is not contacting correctly, 2 the rotor has lifted and compressed the carbon spring loaded contact and it's stuck, this would mean the spark has no where to go and the rotor looks bad when it's the contact in the cap.
So make sure the rotor is seated properly and it's the right one, the best method I have is to find and old identical cap andcut the center out on the sides leaving some material so you can see the rotor and look to make sure the rotor is close to the contact points and the top conector is actually making contact. If your getting a timed spark i would leave the dizzy there and buy a new cap and rotor, change the rotor first and see what happens, make sure it's seater and the cap contact is not stuck and is free to move on the spring.[/i]

[The Original Tom]

The cap contact was one of the first things I checked when it died, and it looks fine. The rotor is the correct one (well... according to Halfrauds any way) There aren't any arcing marks on the rotor or the inside of the cap, but both were a bit dirty. Cleaned them up with brake cleaner and it ran rough for about 10 seconds, during which it wouldn't idle, and then it died just as before.

[ramon alban]

Tom, I've read thro the thread twice and its not clear to me if the system has totally failed yet. It seems it is still an intermittant fault.

If so then the symptoms indicate either a wiring/connection/earthing problem or the amplifier is on the verge of total failure. The amplifiers have a finite life in the region of 100,000 miles and they invariably display intermittant faults before they crap out.

If you have already changed the amp, is it possible you omitted the special paste that aids heat transfer between the amp and the dizzy mounting area? If there is poor heat sinking then the amp will quickly overheat, fail and cool down, ad nausium. Bear in mind that there are some pretty high amperages going thro the coil, all having to be switched by the output stage of the amplifier.

The same would be true if you still have the original amp but it has been removed for inspection and refitted without the paste.

The heat transfer issue could be even more crucial if perchance the wrong coil with a lower primary resistance has been fitted.

Another possibility is a intermittant fault inside the dizzy, where there are a couple of elecronic components and some wires.

I have heard this is pretty easy to inspect the dizzy for broken connections.

[The Original Tom]

Cheers folks - I've just been doing some error-finding and I think I've narrowed it down to a most complicated of components - the rotor arm (well done x.l.r.
Firstly - ramon, the amplifier I'm using is an RPi one, that is mounted in the cab. It's done far less than 100k miles and I'm leaving that as the last thing to change as it's expensive! I would follow Chris JC's advice and put the oroginal back on, except that I have the dizzy type with the amplifier mounted behind the coil - and I never had that. It's my aim to get hold of a GWO dizzy with amp on the side to swap out. Someone on here offered me one from my wanted ad, so I'll contact them soon.

How I deduce it's the rotor:
Coil fires big sparks from the end of the kinglead. Coil fires big sparks from the carbon tip inside the cap (tested with a croc clip on the carbon tip, and the other end on my finger!) Carbon tip touches rotor arm (put a very thin smear of grease on the inside of the cap and put it on - grease transferred to rotor)
Then check all 8 ht leads with a strobe, and... nothing. Even sticking my finger on them yeilds no results.

Now, I can't see that could be anything other than the rotor. The amplifier manages to make the coil fire when the engine turns over, and it's also completely un-affected by engine temperature being inside the cab and not mounted to the fire-wall, so it must be something else.
Also the dizzy was checked for connections a very short while ago.

If anyone can see any omissions in my logic I'll happily receive them!

[IanT]

Make sure height of rotor arm is matched to dissy cap. I've had a few rotor arms and they vary in height - some are about 4mm shorter than others.. the shorter ones are not tall enough to transmit the spark.
I think the rotor arms can also fail to work even though no problem is visible..

[The Original Tom]

Dad just got home with a new rotor in hand. 30 seconds to remove the old one and slot the new one home, and the engine fired first turn, which it hasn't done in weeks. I didn't check the heights actually, but the thickness of the brass on the new one is about half that of the one it replaces, and it's supposed to be a genuine one
The new one's about 0.3mm longer too - it's the littlest things sometimes...
Cheers chaps!!!

The Original "Looks like I get to go out tonight after all" Tom

[The Original Tom]

Also worth noting!
Whilst I was skulking around the connections for the amplifier to double-check it was wired ok etc, I noticed something:
On the dizzy (35DM8) there are 2 wires for the amplifier, one ubove the other.
One is orange, the other black. Now when fitting the A&R amplifier, I went with convention - the black joins the black, and the red to the orange.

WRONG!!!

The red input connects to the black wire off the dizzy and the black input connects to the orange wire on the dizzy!
Side-effects of incorrect fitting are: problems achieving static timing, and increased timing scatter.
I' have been having problems with rough idling, so my idle sits very high.
I've also been having trouble with intermittant back/front firing, hence scatter, and I also reckon I'll have accurate RPM readouts now...

It runs the smoothest it ever has!!!

[x.l.r.8]

Well done, now jsut remember to throw away the old rotor before it lands back in the spare parts box Not that I have ever done anything like that

[The Original Tom]

I ran the other one over in celebration