rj Posted July 5, 2009 Share Posted July 5, 2009 A friend has an MGB 1.8 with Lumenition ignition. At 3.5k it cuts out, but recovers when he lift off. He has replaced the coil, cap and HT-leads. No difference. Car idles perfectly. As the tacho returns to zero when the engine stalls I'd expect the problem to be on th LT side. Any suggestions? TIA /regin Edited by - rj on 5 Jul 2009 20:50:55 Link to comment Share on other sites More sharing options...
Peter T Posted July 5, 2009 Share Posted July 5, 2009 Has he checked the air gap? If it is rpm related and not load then it suggests that the Lumenition is losing the signal. Is it the magnet and chopper version or the light version? Also are the bobweights ok, ig a spring has broken allowing one of the bobweights to expand it could throw the trigger out of alignment causing the cut at 3.5K. Also is the dizzy tight worth checking . *thumbup* Link to comment Share on other sites More sharing options...
Peter T Posted July 5, 2009 Share Posted July 5, 2009 PS also worth checking is the breather on the car, because if the crankcase is pressurising and causing blowback through the dizzy drive and contaminating the sensors, check also the rubber 'O' ring seal on the shaft. Link to comment Share on other sites More sharing options...
Peter T Posted July 5, 2009 Share Posted July 5, 2009 I take it that he has removed the ballast resistor 🤔 Link to comment Share on other sites More sharing options...
rj Posted July 5, 2009 Author Share Posted July 5, 2009 Peter He reckon it's the light version, he hasn't fitted it himself, but there is no ballast resistor. No blow back at idle - or at 1500 RPM. I would most certainly expect it to be the signal being lost for some reason, either from the sensor or internally in the amplifier. Would you happen to know the expected amplitude of the output from the sensor? Link to comment Share on other sites More sharing options...
Peter T Posted July 5, 2009 Share Posted July 5, 2009 *wavey* Blimey mate.................... you dont ask much do you? Give me a couple of minutes to cross reference my old data books to see. Link to comment Share on other sites More sharing options...
Peter T Posted July 5, 2009 Share Posted July 5, 2009 here might help Link to comment Share on other sites More sharing options...
Peter T Posted July 5, 2009 Share Posted July 5, 2009 Is it fitted with twin SU? Link to comment Share on other sites More sharing options...
Peter T Posted July 5, 2009 Share Posted July 5, 2009 Positive or negative earth vehicle? Link to comment Share on other sites More sharing options...
rj Posted July 6, 2009 Author Share Posted July 6, 2009 Thanks Peter, Link is most useful. Twin SU and negative earth, spot on! Link to comment Share on other sites More sharing options...
Peter T Posted July 6, 2009 Share Posted July 6, 2009 You could also check the voltage is 12 volts + when driving as it could be getting a spike supply causing the failure. Usual earths and alternator checks. Link to comment Share on other sites More sharing options...
h_____ Posted July 6, 2009 Share Posted July 6, 2009 Worth checking the wiring on the LT circuit, I have seen situations where the little brown wire in the dist. has the shielding come away as the dist advances, but goes back again when you look for it. Could be something odd like that. Surely ballast resistor relates to the coil fitted? If you have a balasted coil without a ballast resistor, then it will simply overheat causing missfire, not related to RPM. Link to comment Share on other sites More sharing options...
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