Hot Rods running electronic speedo with magneto

Hey guys

Iv installed a electronic speedometer on my model a running a sbc with a magneto.
The speedo goes crazy fluctuating all over the place. Iv done process of elimination trying to work it out and all i can bring it down to is the field coming from my magneto/leads.

Anyone else had any similar problems?

cheers Alex

 

Yep http://motoredbikes.com/threads/massive-electromagnetic-interference-from-magneto-or-cdi.11251/

 

Rewired a local 50 Chevy a year or 2 ago, usual small block with HEI, Dakota Digital cluster. Everything worked fine till it developed a ignition issue sometime later. He replaced the complete HEI with one from ebay, now the speedometer is jumping around. My only suggestion at the time was shielding the wires coming from the transmission sender since they did come through the engine compartment but I don't think that was ever tried. He did borrow a used GM HEI last week and that cured his speedometer issues.

 

^^^^^ Point ignition and solid core plug wires will do it also. Can't use a digital multimeter anywhere to test electrical.

 

I planned to use GPS signal Speedo and realized I had magneto, I think I'll just replace the magneto with dual points ignition.