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MG MGB GT V8 Factory Originals Technical - Misfire When Warmed Up

I've just completed my V8 conversion and done about 1000 miles since fitting my SD1 standard motor with its original Lucas Opus ignition system. I,m running new HIF6 carbs set up just like the factory V8 with K&N filters. Cooling fans set up as factory V8 but with RV8 manifolds so engine temperatures have been fine so far.
The problem is a misfire develops once the engines had a decent warm up run, I,ve had the carbs tuned for optimum running ie not low emissions, so I'm happy the mixture is fine, but something seems to be affected by temperature, any thoughts?

Had the same problem. Would run fine till it warmed up then start to misfire. After a while it wouldn't run at all. Come back later after it cooled and it would start right up. It is the Optronix or Optimax or whatever control box. In the states, you're halfway to a Mallory distributor by the time you buy the Lucas control module.

Pulled the Lucas distributor and put in a GM HEI and the problem went away($5 at the local bone yard). Fitted a Buick distributor ($28 at the local auto parts store) with points for sheet metal clearance and will eventually fit a Petronix.

Could you find an earlier model of the Lucas distributor that has points? The other option is to spring for a Mallory from RPI.

Best of luck,


What do your spark plugs look like? I have SUs as per factory and had a plug fouling problem after a while.

With HIF 6 carburettors, the mixture needs to be confirmed on a dyno in my experience.

If it helps I had a thread a couple of months ago on this, called SU carburettors - rich. You could check it in the archives.

Ian G Buckley

MG junked the Opus unit after only a year as it was so troublesome. You don't need it or any other electronic system in the UK, it was only used for North America to meet emissions requirements. If it misses when hot on the drive use a timing light on each plug lead and the king lead as diagnosis (watch the flashes).
Paul Hunt

Another option for replacement of the Opus system is from Crane. It used to be the Allison system but was bought out by Crane Cams in the mid 90's. Moss USA has them for $120 I think. I have used one in an 1980 SD1 for 5years. Got tired of replacing the Opus system every year or two.
Hope this help.

If it's not the module (the most likely) - could be an HT lead breaking down. Also what about fuel vapourisation (my favourite topic)- check the routing of your pipes.

Dave Wellings

Thanks for all your responses. I've fitted a set of new silicone ht leads which seems to have solved the problem.

This thread was discussed between 07/10/2002 and 13/10/2002

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