Re: 1974 Mercury 200 (20HP) Troubles
First, all of what 8075 says above is right on. Second, regarding tuning, there are two different approaches to it on this motor, which in terms of timing is really a matter of setting the relationship between the linkage and the trigger. I think (I'd have to check this) that the factory manual only gives the procedure for setting with a timing light, and the aftermarket manual gives the procedure for setting it by clearances at various settings with the motor off. I use the factory manual for most things, but liked the aftermarket for simplicity on this. You also need to look carefully at the whole linkage as you move it--there are bushings in there that tend to get lost or not replaced when the motor is worked on (especially when you remove the rewind, look where the front bolt for the rewind goes, on the side away from the shifter, it holds part of the linkage there). Also, the plastic pin that runs in a long curved slot (look for this) can wear and make that part of the linkage loose. Both affect the trigger movement, and the trigger, rather than the carb, is what slows the engine at idle (the butterfly does not open until you have already begun to advance the timing, at idle retarding the timing further is what slows the motor after the butterfly is closed). I hope this makes sense, you'll have to be looking at the motor to see what I'm talking about, I think. I'll agree with the post above on one more thing--the 20's of this vintage are indeed nice motors in my opinion, pretty easy to work on and very solid. Collecing spare ignition parts on eBay helps with troubleshooting (swap and try), otherwise ignition parts can be tricky to diagnose and pricey. But it sounds like you've got good fire, so on with the show!
