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Maximum Spark Advance vs. Full Throttle Advance

M

Matt Butzer

New member
I've got a 1969 Merc 20 hp and the SELOC manual. It explains the Full Throttle Advance setting (.275 BTDC) fairly elaborately and I've got a good handle on setting that, but it also references the Maximum Spark Advance (.375) in the appendix, and I'm not sure what they are talking about. I'm doing a complete overhaul (more or less) on the motor and would like to check as many aspects as I can so if someone could explain how to check and set this that would be great. Thanks for the help and the time.
 
Confusing, ain't it!

Not sure what that book of yours is attempting to say, but my 1975 Merc 200 book says 33 degrees max advance. If you set it there you should be okay.

With today's crappy ethanol gas, a few less degrees is a safe bet and doesn't hurt perfomance.

Jeff
 
The book says (summarized) to put the engine to Full throttle advance (reference appendix for BTDC), back out the Full throttle screw, got to WOT on the tiller handle, turn the full throttle screw in until the points just begin to open, lock the full throttle screw into position. I'm confused why the appendix was referencing 2 measurements. What is supposed to be happening at this other measurement?
 
You got me! Some people love to confuse things! Ever read PC documentation?

The bottom line is this: Get the timing set where it does not ever exceed the max under any conditions and you won't drill any pistons. With today's crappy ethanol gas, you have to be careful.

Jeff
 
fastjeff said:
You got me! Some people love to confuse things! Ever read PC documentation?

The bottom line is this: Get the timing set where it does not ever exceed the max under any conditions and you won't drill any pistons. With today's crappy ethanol gas, you have to be careful.

Jeff
Click to expand...

PC documentation??
 
..."PC documentation?? "

Okay, Personal Computer software. Anyone other than a geek can't figure out what the hell it's talking about.

Jeff
 
kimcrwbr1 said:
Do you have a dial indicator? If you havent messed with the max timing advance it should be good I normally wil just put it in forward gear and give it WOT and check the clearance between the throttle cam and the throttle lever on the carb the throttle butterfly should be fully open but you should have about 1/16-1/32 inch clearance when you push on the throttle lever from the cam you dont want the throttle shaft to bind on the carb. There is no pickup setting on that motor. The only way the max advance changes is if someone messes with it not knowing what they are doing. The book says to turn the flywheel counterclockwise to check the max advance do not turn it counterclockwise with the gearbox in you wil need to pull it anyways and fix the impeller. Find zero with the dial indicator and turn it clockwise almost one full turn to the proper position then check the points opening. Just be aware you dont want the flywheel to move and the magnets in the flywheel grab onto the coil when it gets close and can move the piston position.
Click to expand...

Yeah, the plan is to check and if it's close not to mess with it. I was just curious what the difference between Maximum Spark Advance and Full Throttle Advance is. Maybe they're one in the same, but I've got no clue. I was just confused why they referenced 2 different piston locations. I'm taking the flywheel off. I've set the points before, but since I've rebuilt the carb I thought I should check the timing to be sure it's still in sync. I'm assuming that the butterfly in the carb should be dead nuts centered when at full throttle. I know it will over rotate if you manually work it, so I'm going to set the full throttle screw so that the carb is centered when at WOT, with .010 clearance in the linkage, and then check the timing/points position from there. Thanks to everyone for the help, and any other assistance is more than welcome.
 
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