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I have a couple of issues with my new Mk1 Cortina. First of all Mk1's are supposed to be + ground. When this car arrived to me the battery was connected as - ground. The car ran ok so I presume it had been converted to - Ground but I would like to know for sure as it may be some of the cause for what I've got going on further down. Going forward to today. I have been having trouble with the car missing and backfiring under both light and heavy load. Some days it is better than others but never perfect. It had old dirty gas in it and was starving for fuel, so I pulled the tank, cleaned it, resealed it and blown out the fuel line as well as pulled the carb and dismantled and cleaned. It does not have a fuel problem now. I have replaced Points, Plugs, Leads, Dist Cap, Rotor, checked timing and still no improvement. I have noticed that the Coil was connected wrong. The Low Tension Lead was connected to the + which is different to the way the battery is connected at the moment. I did switch it around but not noticeable improvement. Can anybody shed some light on any of these issues?

Thanks Brent

Not easy to diagnose from a distance, and while a trip to New Zealand appeals, I don't think so!

If the car is wired -ve earth the the coil -ve terminal should connect to the distributor and the +ve terminal be fed from the main loom. I notice that you've replaced most of the ignition parts, but I have little faith in many replacements since these are often cheap Chinese crap, so might not cure an existing problem. Unless you know their origins, you might be better with the originals.

Likely suspects for backfiring, on the ignition side, are the rotor, cap and condenser, but it's worth checking all electrical connections from the ignition switch forwards and backwards. Also check the exhaust system has no leaks, nor are there any leaks into the induction system, such as a blown manifold to head or carb to manifold gasket.

I didn't see you mention the condenser in your list of parts replaced and as Jan mentioned, some of these parts can be defective right out of the box. I know that I had a similar poor running situation on my side valve at one time and it was the condenser, I ended up using an older Bosch condenser that would not fit in the distributor so I removed the original part and installed the Bosch unit at the coil end away from the engine heat. The engine has run well since.

It sounds like the previous owner and/of their mechanic was grasping at straws trying to get this car running right,

I'd park it next to a proper running car of yours and compare all of the differences....
It sounds like they are shooting in the dark with some of the choices they made.

You'd be shocked in how few common day mechanics know about pos-gnd cars.

And by backfiring, are you talking exhaust pipe backfire? or carb sneezing?
Confirmation on that one will be a huge help.

Brent, have the rocker arms been adjusted. Maybe that the ex valves receded a bit if no hardened seats installed.

Jan, I wish I was in NZ but I'm in California. All the parts I sourced thru the Mk1 Club in the UK so they are pretty much the real deal. I did replace the condenser as well.

Joe, it is backfiring thru the Carb.

Pete, yes I adjusted all the rockers, the exhaust side was adjusted a little to snug so I adjusted to required specs. Did notice a little change but not enough to stop misfire.

Joe O'dMoors wrote: ↑Mon Nov 22, 2021 1:34 pm
You'd be shocked in how few common day mechanics know about pos-gnd cars.

How does the polarity come into this?

Genuine question (coil connections aside)

Ah well, I understand that California is quite nice too!

Anyway, this additional information confirms thoughts of an air leak on the induction side. It might be as well to check that the base of the carburettor is truly flat - they pull down at the ends of the mounting flange as the nuts are tightened against soft gasket material. Easily corrected with a file, but be certain to thoroughly remove all swarf.

Another possibility is wear in the distributor, either in the bushes allowing the spindle to waggle around altering the ignition timing; or the springs for the bob weights are stretched, again allowing the timing to vary.

And as Joe O'dMoors, says, today's mechanics - sorry, technicians - don't understand contact breaker ignition and carburettors. Since I had to teach young apprentices for fourteen years, I know this!

Yes, as JAN mentions, it's likely a carb mount problem. (but it sounds like there were multiple issues going on at the same time) which makes it very frustrating to diagnose.
How are the plugs looking ,when you pull them out? What temp (plug) are you running?

If the carb is popping/sneezing, it's highly likely it's not sealed correctly, and/or there's a lean situation going on.
My car sneezes just a few times while it's warming up, and clears away soon.


And polarity issues can be a headache, as things like ignition spark, etc can be vastly degraded if the pos/neg is following the wrong conductive path.
Which reminds me!
You should check to ensure all of the ground straps are in good order, with clean/tight ends to where they are attached to. A loose/oxidized ground strap end will cause much chaos.

Keep us updated!!

Finally got the motor running properly. I rebuilt the carb and it now runs great. Thanks for all your input.

Brent