1971 mercury 500 thunderbolt (50 h.p.) ignition

[dougredinger]

I recently acquired this old mercury outboard and am trying to get it running well. I was able to get it started with some difficulty although it would only stay running at higher rpm (2000 or more).
I completely disassembled the thing and cleaned and serviced everything I could! Lightning energizer distributor was disassembled and completely cleaned of dirt and carbon and repaired the crumbling wire insulation on the 3 wires coming out of it ( red , white and blue).
Cleaned up switchbox and terminals and replaced rectifier which tested defective!
Cleaned up primary coil and terminals and attached solenoid!
Cleaned rust out of fuel pump and replaced check valves, gaskets, and diaghram.
Removed carbs and cleaned or replaced any iffy looking parts and reinstalled.
Plugs and wires looked fine! Removed and rebuilt lower unit with all new oil seals and pressure tested fine ( 10 psi for 24 hours )
After putting it all back together it still only wants to run at higher rpm.
I am trying to narrow down the problem!
I know a bad rectifier can damage the switchbox so I am suspect of that! Or could it be the low speed trigger coil in the lightning energizer distributor is bad since it won't run right at lower rpm!
Sorry about the long post, but I wanted to cover all the bases.
I know there are some very knowledgeable people on here who have experience with these older outboard motors!
Thank you for all responses!

 

[racerone]

Belt timing checked?

 

[dougredinger]

Belt timing checked?

I will check on that! I could not see a white mark on the flywheel as it was so greasy and dirty! There were 2 marks stamped into the flywheel and I set the base timing to one of those! #1 piston was at tdc and the tiny arrow on distributor pulley pointed at mark and center of crankshaft! I need to remark this line with bright white paint and check the dynamic timing as engine is running!