Old merc gurus - fine tuning '73 mercury 40 single

[fstarocka]

Hi all.. Just recently got a mercury 4hp single. New water pump, rewired the head - cleaned points and set gap at 0.020"..

Got a question regarding the ignition timing, it says thunderbolt ignition on the front cover. Im not 100% sure what a good timing curve is for these things? I made a vid showing what I currently have here (https://youtu.be/C1qAk1XXdVA) -

I dont have the manual for this thing either.. It runs first pull - At the "shift" setting my ignition is about 10-15 ATDC! My Start marks begin at about 0 degrees and end at 10 BTDC.. WOT I get about 32 degrees BTDC.. My carbs open around 10 BTDC.. Any tips on improving the tune?

I marked the absolute TDC using the piston stop method - It seems the word TDC is used without clarity - as TDC dwells for a few degrees! I marked mine at exactly the middle position -

Also - here I am turning the carb idle screw in from 1 turn out - see how its starting to climb in revs? All the best lean idle adjustments i do work, but this runs highest at 1/2 turn out - but sneezes (lean)..

https://youtu.be/_EN5t0D7Grc


I find it starts best around 1-1/4 turn out, and has little to no delay in accelarating as compared to 1 turn out (or a little less) depending..

[Lepke]

Find a manual. My last Merc was a 2.2hp. Only runs about 6 months before a problem. Like most 2 cycle outboards. I went to electric. No problems.