Slow Cranking with Electronic Ignition

[carsbroke]

Car 1977 rubber bumper MGB
I have fitted a powerspark distributor fitted with electronic ignition new HT Leads and 12 volt coil.
The white and l/green resistive cable has been disconnected from the coil and replaced with a direct 12 volt feed.
The car will start and runs ok. When starting from cold engine turns over at normal cranking speed. When hot the cranking speed is intermittently slow (more often slow). Almost like the battery is flat.
If the coil is disconnected or the ht leads are removed from the plugs then the cranking is at normal speed.
If the original distributor and leads are re-instated then cranking is normal.
Appreciate any suggestions.

[Charles Farran]

Have you checked the condition & connections of the earth strap from the engine block to the chassis as a weak contact here under load from the starter motor may affect the route the current flow takes , i.e. the route back to the battery may in part go through the ignition /plug route/ throttle cable/ choke cable.? Temperature can affect contact points.
Cheers, Charles

[Vic Butler]

Where have you taken the 12v supply from for the coil?
The starter/ignition circuit on a late rubber bumper B is a fairly complicated set up. During running conditions the power to the coil.via the resistive wire in the harness comes from the front of the 2 black relays on the inner wing on the right hand side of the engine bay. When the ignition switch is in the start position the front relay is deactivated and the other relay is activated via the starter solenoid and supplies 12v to the coil. When the key is released when the engine starts the 12v supply is cut off and the front relay is activated supplying power to the coil via the resistive wire in the harness. Because it works when you disconnect the wires it points to something in the wiring.
I fitted Lumenition to my 77 BGT and a Lumenition coil too but I fitted a ballast resistor one and have had no problems. The distributor was rebuilt by the Distributor Doctor to suit the state of tune of the engine.
Did you get the correct distributor for a late rubber bumper car? Because the vacuum advance is taken from the inlet manifold instead of the air filter side of the throttle disc on the carburettor the distributor, no 41610, is the only one suitable for rubber bumper B's.
It would be simpler to fit the correct ballast resistor coil than fiddle about with the wiring to cure the problem.
. Vic Butler