12 v alternator conversion troubles

[Breconbus]

hi all,8 weeks on an engine rebuild, single port 6 volt to twin port with 12 v alternator conversion.
loads of help from the forum on the build.

engine is in and runs beautifully.... When you start it with a cable direct from the battery.i can't get it to start on the key.
once it's running if I disconnect the direct cable, it cuts out.

the alternator is checked to be outputting ok when the engine is running.
when the engine is running I,m only getting 11 volts to the coil

A few other bugs to sort, no indicators, oil pressure or elec choke
Headlamps, horn wipers ok.
I am thinking maybe a damaged cable back to the coil, draining the power to the coil. would 1 volt drop stop it running?
the electrics were ok on the 6v pre engine rebuild as far as starting was concerned
Done the usual earth checks.
Does anyone know a good classic sparky in West Wales or Brecon area.

I don,t want to go down the route of a new harness without checking this out. Maybe something fundamentally wrong.

thanks

[Breconbus]

ps
When I start on the key the engine turns over but I have no voltage from the coil

[vwJim]

Do you have 12v at the coil when the key is turned on?

[Breconbus]

Hi there, I’ve got 11v at the coil when I turn the ignition on

[vwJim]

And what at the battery and what at the fuse board?

Sounds like serious volt drop.

[Breconbus]

I,ll. check them out tonight
Thanks