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MG MGB Technical - 73 won't start

I have a 73 B that had been running Ok just a week ago, the carbs are rebuilt, new kit. Electronic distributor added last year and the valves have been adjusted. I've also added an octane booster. When you retard the timing it would start but as you advance the timing to between 11 and 13 degrees the No 1 cylinder starts to miss fire and the idle steadily decreases. The motor stops and it won't start. Looking for some advice?
jl leclaire

I am not a great fan of octane booster, but it may be the best you can do there depending on available fuel. Fuel does lose volatiles over the winter althoughmy car always starts and runs in the spring, after some encouragment. Some fresh fule added may help, or even a change. You could also try going back to points and a known good condensor.
Stan Best

This isn't a 123 electronic distributor is it?

If so, read the instructions for setting the timing.
Geoff E

It's possible the points phasing of the distributor is out i.e. the spark is occurring with the one edge of the rotor near the cap contact instead of it being more central. As the distributor advanced the spark could then occur with the edge further away from the cap contact and the spark is taking an alternative route. Normally the phase only changes with vacuum advance not centrifugal (nor with twisting the distributor, I don't think, but it's a bit mind-boggling), but as timing affects idle speed it could also affect vacuum.

Can't see this being anything to do with fuel if only one cylinder is affected, but I'd also go back to points and condenser at least for diagnosis, substitution is pretty-much the only diagnostic with electronic ignition. However if you have a timing light clip it on the *coil* lead, and see if you start missing every 4th flash as you advance to the point where No.1 starts missing. If it doesn't then the problem is with the rotor and cap. If it does then it's with the trigger.
PaulH Solihull

Thanks for the advice and info, I'll check a few things and respond.
jl leclaire

The distrubutor is a flame thrower petronix and the octane of fuel here is 91 Premium, so that is why I put in octane booster.
jl leclaire

I had a distributor built for my supercharged B. There were some odd problems until I followed what one of their techs advised. I cut a hole, in the side of the distributor cap, and started the engine with a timing light hooked up. Sure enough, as Paul mentions, the rotor was firing well past the the distributor contact. I removed the base plate and enlarged the mounting holes, allowing the base plate to be re-aligned. When I re-tested the rotor to cap terminal alignment, it was dead on. The company that built the distributor offered to fix it for free, but I didn't want to have to deal with the down time and figured that I could accomplish the fix on my own. And this is a $400 distributor! RAY
rjm RAY

This thread was discussed on 18/07/2011

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