Twin plug weak spark after dizzy ?
 

Gents I have a curious fault on my twin spark 2.8 motor. Ever since I first ran it I've had problems getting it to run cleanly, I went through all the usual checks for air leaks, over weak, over rich, ignition timing, advance rate even cam timing. The motor has excellent comp at 170 to 180 psi across all six and very low even leakdown across all. I'm totally confident in the cylinder sealing.

So I was working with a race car guy to dig deeper. We suspected ignition so tried coil substitution , no change, cdi sustitution, no change.We earthed the coil to dizzy "king leads" direct to the engine case and got massive..massive sparks.

He has a handheld msd unit which can be used to generate sparks so we fed the dizzy cap to the upper plugs then the lower with that instead of the cars ignition The motor was set on TDC compression number one . We then used a dummy lead and plug to check that the sparks were being carried from the generator through the rotor and cap and out to the plug. Result nearly zero sparks.

Just to check we put it all back together and ran the car connected to a Krypton analyser. Spark voltage was way low.

Now I have new caps, new leads all solid copper, and new plugs. We are utterly confident in their integrity.

So the strange thing is the spark power loss in the dizzy cap. I am running a TP dizzy I designed which runs the Jag Marelli Cap and custom 120 deg rotor. We disassembled the rotor and bearing assembly and fired our spark generator through it looking for earth leakage and there was none.

Any ideas out there or experience of problems with the aftermarket dizzy caps??

Need more information here. :)

Precisely, what ignition boxes & coils do you have on the car?

How & where are they power & grounded?

That kind of trigger does your distributor have?

How is it wired?

Need details about all your grounds,....number, location, gauge wire, etc.

Electrical issues are all about the smallest detail and thats whats needed here to give a clear picture of what you may have going on.

Some aftermarket caps do not have great conductivity through the carbon button coil terminals and I've seen that.

I agree with Steve...we used to replace the carbon button with a copper slug in most cases...just to get rid of the extra resistance.
Bob

I'll be watching this thread, I just picked up a Jarvis dizzy and I think the PO may have been having spark issues. He gave up and ditched a 930 turbo custom CIS/Jarvis/MSD setup for a SDS crankfire. This solved his issues so it was for sure either the CIS or the Jarvis/MSD
He could get it to run fine on start-up but as soon as he hit the road it would crap out.
I will only be using the Jarvis and the matching plug wires, I have different MSD and I'll be running N/A with Webers. I'll also be using a WB meter for tuning.

Thanks Bob, I could easily make up a copper slug, I've also read that if you stretch the carbon spring a bit that helps as well.

I'd love to find a "real" Jag cap, but so far no luck with that.

Hi gents, a bit more detail, the car has an accuspart trigger unit wired via two doides to trigger two cdi units. Each cdi also has an ignition switched feed, a perfect earth, and a perfect connection to its relative coil. Each coil is perfectly earthed. I cannot remember the coil type but I think MSD and am certain compatible with the cdi's.

Regarding cable size I've used 2.5 mm cross section multistrand heat resistant cable its a full new race loom with all connections 100 properly crimped and tested prior to install I'm confident in that..

This lot produces perfect meaty sparks easily jumping an inch as far as the distributor that's where it gets lost or weakened.

The dizzy I designed works just like all the other adapted bosch ones except its mine I am certain of its mechanical integrity and importantly that the rotor is lined up on cap pin centres and is not earthing through any known route. I suspect the carbon brush in the cap top may be an issue but did do the spring stretching trick. It also doesn't explain why the brass ring in close proximity to the rotor for feeding bank two sparks doesn't seem to be working.

I've read up on problems with these caps and rotors in jaguar applications. The advice seems to be to fill the voids in the rotor base with RTV clear silicon to prevent tracking through the relatively thin plastic so Ill do that as insurance. Also they have some more tips to improve the brush set up. I like the idea of improving the connectivity at the brush so will try that on my old cap. I will also look at the radial clearance on the brass ring to rotor pin to see if I can tighten that up

I've just ordered a new known source cap and rotor since the one I had was an e-bay special I suspect.

Any more for any more?

http://forums.pelicanparts.com/uploa...1366621032.jpg

Another cap mod along the vein of spring stretch is to drill the centre and press in piece of brass or aluminium tube.
It's about 3/4 page down on this <a href="http://www.jag-lovers.org/xj-s/book/Marelli_pix.html">Jag site.</a>

http://www.jag-lovers.org/xj-s/book/...CheapFixed.jpg

Your piece looks like nice work, I was going the same route till my friend offered me the totally sweet deal on the practically new Jarvis and wires.
http://forums.pelicanparts.com/uploa...1366631456.jpg

Thanks for that, I am lucky to have some capability that saves me money but is time heavy and when I hit a snag its all mine to deal.

I had actually seen the same Jaguar site. I just checked by basic continuity from the king lead connection to the carbon brush . I'm not convinced its reliable so will look to engineer a solution like the one proposed. Its odd that the other bank of plugs rely on the spark jumping to the rotor, I note that genuine porsche marelli caps have two carbon brushes and a different rotor.

Still we know the set up I and others have does work... I just need to get it right with regard to rotor and cap.

TTFN

Andy

Please keep us in the loop and let us know how you make out.

Cheers SmileWavy

Something to think about.
If the spark...as you say...is strong going from the coils to the cap...but from the cap out is not as strong...then unless there is a lot of sparking inside the cap (would see the evidence of it) then you probably have a resistance in the circuit.
The reason I say this...without a spark jumping around inside the cap...the only other way to diminish the spark is to pass it through a resistor (wire, center button, traces inside the plastic, etc.).
The rotor could also have resistance built in as well.
Unless you need to reduce the spark interference (radio noise?) I would suggest a non-resistance circuit.
This would entail metal type spark plug wires..a non-resistor rotor...non resistor plugs...and as plain a cap as you can find.
If this doesn't do the job for you...the next step would be Coil-on-Plug...(I like spending your money..LOL).
Bob

Good advice, I'll check for in built resistance in the cap and rotor , I know some have it in other applications, and any internal tracking in the cap. Gotta say the quality on the current cap is pretty poor.

Andy

I'd suggest that you check the register of the rotor to confirm that it is properly aligned with the appropriate terminals when firing.

regards,
Phil

Hi Phil, yes a good point; I am extremely confident in that factor but again worth checking. Obviously the dizzy body moves independantly of the rotor so within limits (set by the slot at the base to adjust intial timing ) I can eliminate that. However if I'm drastically out I should be able to re-index the gear drive by one tooth or whatever.

I'll check SmileWavy

Success..nearly

I had some time today so I made a brass contact to replace the carbon one and got consistent continuity. I also filled the base of the rotor with RTV.

I was checking the rotor to cap pin position and found that the rotor was poorly moulded and the two copper contact were not parallel. This led to the "bottom contact" not aligning with the cap pins and the top likewise . So as a temporary try it and see I heated and reshaped the rotor so the contacts are parallel .

re assembled and fired it up...6 cylinders and smooth:):)

So I went for a drive...I forgot to fasten the bonnett rubbers and lost the hood over the roof:mad::mad: Body shop time....

Sorry too hear about the bonnet mishap, but I'm glad you got the cap sorted.

When you pulled out the carbon contact was there any trick to removing it? Such a simple mod I think I do this as a pre-emptive measure.

Hi Mark, no its easy just rotate and pull I think its ant clockwise but if it doesn't come try the other way. Its not exactly well fixed so no need for big force.

cheers

Folks a bit more input or advice needed as a spanner jockey ignition leaves me pretty cold but I do my best. I have been struggling to get my r car to run properly on all its cylinders and thought I'd solved the problem but its just not right, So here is my question:-

I have twin Bosch 3 pin CDI units. They are triggered by a single Accuspark unit split via two diodes to feed the dizzy in terminal on the cdi. I have two MSD blaster coils each one fed by its own cdi. These connect to the twin plug dizzy which I designed.

Now I have big fat blue sparks to the dizzy cap and big fat blue sparks at the bare lead end . BUT when connected to a Beru cap (all measured at within spec for resistance and repeatable) then a Bosch BP6ES plug the spark is very weak. So the issue is now I think with the "spark strength" at source. I had it on a Krypton analyser and the spark voltage was low ?

To be clear the car runs pulls with intermittent missing and is drivable I have 6 deg BTDC static and 28 degrees BTDC at 3000 rpm, I know its running very fat (rich) but cannot isolate a poor burn from a carburation issue. All cylinders have excellent compression 170 to 180 psi hot so are definetly sealing there is no ring by pass or crankcase compression to speak of,

So question is are the Blaster coils compatible with the original type CDI units or any other hints I can persue. I'm desperate to get this car running properly to help celebrate 50 years >>>> after my 10 year build.

Help appreciated.

Andy

Andy,

While their quality control is suspect, the Bosch CD coils are a better match for your Bosch CDI units and that's what you should be using.

I would also confirm rotor phasing in the cap as this is a problem I've seen before with some aftermarket distributors.

Hi Steve,

regarding rotor phasing I have checked that the rotor tip for No1 top plug aligns with the correct cap post and like wise that the opposite tip aligns with the correct cap post for No 1 bottom plug. I can easily set the timing at an indicated 6 deg BTDC and get the correct max advance.

So my question is ..is there more to rotor phasing than I currently appreciate ?

The really odd thing is as I said I'm struggling to identify if the weak spark is really an issue since the right hand side cylinders run really clean with the carbs crossover disconnected and just that side revved not so on the left which stutter like a 70's mini in the wet :D

I could be barking up completely the wrong tree...and its a carb issue ?

Haasad,

That is only part of the phase question. Where is the rotor pointed when the spark happens. You have a timing light so you know it fires near the correct cap pin but is it directly in line? Does it trail away from the cap pin as advance occurs?

Yes you could have a carb issue too, start with float levels. Are the plugs on the one bank fouled with fuel? Wet? Lots to check.

I would put this car on a scope. That would immediately show the ignition situation at each plug and give you some direction for troubleshooting.

regards,
Phil