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MG MGB GT V8 Factory Originals Technical - Mallory condenser problems.

Hi all
My Rover 4.0L engine is fitted with a mallory dual point distributor. The problem I have is that the condenser keeps failing causing a missfire. On replacing the condenser it runs fine for a couple of hundred miles and then I get the missfire.
The Ballast resistor is showing 6volts with the engine off, but with the engine running im getting 10 volts.
I have a 6 volt coil ( lucas DLB 120 ), could this be the problem, as its being supplied with too much voltage?
I bought the distributor as a kit from RPI but they arent very helpful on the phone, they said to take the car up to them but its a 400 mile round trip!
I have replaced the cap, plugs, leads (magnecor), coil, points, ballast resistor and so far 4 condensers, even replaced the fuse box.
The battery connections are good and the engine is well earthed.

Help appreciated,

cheers

Mark
Mark Rawlins

A 6 volt coil could very well be the problem, 10 vdc on the primary side of the 6 vdc coil could add up to too much secondary voltage/current and subsequent failure of the condensor would be a result.
In all the changes you've made so far, are you sure you have the correct polarity of the coil still?
John Morgan

RPI, well well well. A friend had some work done by someone with a very similar name and ended up with *two* ballasts in series with a 12v coil.

What you see on the coil +ve of a 6v coil with external ballast depends on whether the points are open or closed. If they are open you will see 12v, it is only when they are closed that you see 6v or so (depends on the relative coil and ballast resistances). When the engine is running you will see an average between these two voltages because the points are continually opening and closing, of course. The exact average depends on the points dwell, which is higher on a dual-point, one assumes, than normal. What also has to be factored in is that with the engine running the 'points open' is in fact closer to 14v and the 'points closed about 7v. Hence a 10v average would be about right when running, but that would be for a standard distributor, which makes me wonder whether you are even getting the dual point action - what is your dwell? Coil voltage is not likely to be the cause of condenser failure, when the points open there is a 250v or so spike across the condenser anyway, they are designed to cope with it. How many have you had fail? If only a couple you could simply have been unlucky. Are you using Mallory replacements or something else? If 'something else' try Mallory. If Mallory try 'something else', and if it doesn't physically fit I would jury-rig it outside the distributor if neccessary.
Paul Hunt

Thanks for the input.
My knowledge of the ignition and tuning is basic to say the least.
Paul
I have been using genuine mallory parts for the distributor, dont know the origin of the ballast resistor.
I have been using Circuit Motors at Castle Combe for the tuning work on their rolling road so I couldnt tell you what the dwell is.
I am getting a lot of conflicting info.
Some suggestions Ive had are to use a 9v coil and use two condensers.
Im on my fourth condenser with the last one only managing about 200 miles before problems.
My local landrover dealer said to fit a capacitor to the coil?
There must be a simple fix, its just knowing what to try next.
Ideas anyone?

Cheers

Mark
Mark Rawlins

I'm no expert on mallory systems-surely something for the US boys to comment on-but I do have the following comments:
One of the cars in my local club is a V8 conversion with a Mallory Dual point dizzy and initially it was nothing but trouble-poor starting(very unusual for a V8) and they had several breakdowns which seemed to be burnt out points/condenser failure.
They had a totally non-standard wiring loom and Malcolm Beer eventually traced it to the ballast resistor/coil combination. I haven't checked their current combo visually since then but I understand that it is now running a standard Lucas 12v sports coil without ballast resistor.
Mallory dizzys make sense when you are faced with 250 recon costs for the standard item-but not when you are broken down at the side of the road in the UK and the only appointed Mallory agent is Real Steel. That's when Lucas starts to make sense...
That said it now semms reliable and they now carry around a complete set of points and condensor.
I also seem to remember that the condensor was on the outside of the dizzy and very close to the front of the block(heat soak?)- could this be part of the problem?

Hope this helps

John Bourke
John Bourke

You can convert a Dual Point to a Unilite setup ... much more reliable ... parts available from Summit.
Ted

Capacitor/condenser same thing. The usual reason for fitting a capacitor to the coil is to the +ve terminal for radio suppression. I've tried adding less and more capacitance to the points in the past just to see what happens on an oscilloscope and surprise surprise, the standard setup is best. An incorrect ballast/coil combo usually burns the points if the ballast is too small for the coil, and poor running if it is too big. Putting a 9v coil on a 6v ballast will worsen running and performance. The advice you have been given is just poking in the dark. Condenser failure is very rare (I've not had one in 35 years and many hundreds of thousands of miles). For you to have four go so quickly there must be some other wiring or component problem that is causing them to fail. I have just rewired a conversion with a Mallory distributor and coil, and that has done over 5k without problem (in that area anyway). But that had three wires coming out of the distributor rather than the usual one, never looked inside to see why.

If you have a digital ohmeter measure the coil primary and secondary resistances and ballast resistor and tell us what they are. For the ballast measure from the coil +ve to the white or white/brown on the fusebox with the distributor disconnected.

As I mentioned before I would get a 'standard' condenser next time and jury-rig it outside the distributor. If that fails too then I think I would start getting desperate and lash some completely new wiring in between ignition, coil and distributor.

Whereabouts in the UK are you? Presumably not too far from the Midlands if you go to Castle Coombe.

Paul Hunt

I've run Mallory dual points for several years and have found a tendency for them to fall out of tune fairly easily. In that time I had the condensor fail just once. It started with a 'was that a misfire or did I imagine it?'. Over the next 200 miles (returning from the Nurburgring)it became progressively worse, from a misfire every mile or so to continuous and a flat-bed recovery back home. Yes, I do carry a spare now! I use a 12v Moroso coil (chrome version) bought from Real Steel and their recommended ballast resistor. Based near Uxbridge they're very helpful blokes and will sort your system over the phone. Bell them on 01895 440505. For a pound or so they'll send you a copy of their latest catalogue. Loads of stuff in there and there's a nitrous section for bedtime reading!
DC.
David

On further thought it's just possible that the coil is starting to break down internally and is feeding some of the HT back to the condensor i.e. giving it a far greater 'back emf' than it should and breaking it down. I think this would be difficult to check without an oscilloscope, but new coils are not that expensive so you could try a different 6v coil. But then again, that is likely to make the condenser go short-circuit which would stop the engine dead, not cause a misfire. A misfire implies an open-circuit condensor, which is more likely to be caused by internal mechanical failure, which if it were a manufacturing defect could apply to a whole batch.
Paul Hunt

Thanks everyone.
I have been in touch with Real Steel and it seems that RPI have given be the wrong ballast/coil for my distributor. I have ordered new ones from Real Steel, inc. a spare condenser. Hopefully this will be the end of my problem.
Thanks once again

Mark.
Mark Rawlins

This thread was discussed between 13/04/2002 and 16/04/2002

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