Re: force 120 tuning question
As far as detonation, ALL outboards do not like to run lean. A lean run will melt a piston faster than you can say sh--. For this reason, you never lean carbs to the point where the engine idles well. Because the low speed circuit is always exposed to manifold vacuum, it always delivers fuel. The high speed jet is sized to compensate for this. So, if you set the low speed needle too lean, the full throttle will also be too lean. Always have the low speed adjusting needle open at least 3/4 turn (preferably 7/8) from lightly seated. When adjusting idle, start at about 1 1/4 turn out.
Force engines are a very old design--started as West Bend, then bought by Chrysler, then AMF Brunswick (Force) , finally Mercury.
The 50 plus year old block design is as simple a crossflow design as you can get and without massive rework will only deliver adequate horsepower. As an entry level engine with a price point, it did its job but don't ask it to race--not any more. In the old days it could be made competitive but now, with short rods, poor head design, poor block packing, and no boost porting, it would be very difficult
Since you have messed with 2 cycle engines, you are probably aware of porting. These engines can be ported (squared, enlarged, and trumpet shaped) and will deliver satisfactory power increases with the following exception: Unless you are going full race with no exhaust restriction, do not port the exhaust on 4 cylinder models. Port bypass side only. Port timing is about 120 degrees open with 90 degree crank throws for an overlap of about 30 degrees. Couple that with the fact that #2 and #3 cylinders fire right next to each other and there is a common exhaust chest, and exhaust porting causes enough blow-back that power will be reduced to less than stock.
You can fully port three cylinder models and on the older Force/Chrysler you could get a 10 hp per cylinder increase.
There is not much you can do with the heads and because of their design, timing is set to 30 degrees BTDC. This was reduced from 32 degrees BTDC in the '80s as a result of poor quality gas available in marinas. So, if you were careful, you could re-time to 32.
Older models benefit from more responsive reeds (Boyeson) but I don't know about newer ones.
Earlier force engines used an exhaust tube that only directed the exhaust into the lower unit. The exhaust tube on the Mercury models is smaller in cross section and may have been an attempt to increase scavenging by tuning the tube (gas mass and velocity versus tube area and length). I never got into that.
If you really wanted to get into it, and had machine shop facilities, you could make special pistons to use longer rods--like from a Rude, Groove the head for O-ring seals instead of a head gasket, bigger rollers on the rod big end, relieve the block for the longer rods then pack it. Change gas flow from bypass to through piston skirt, and isolate cylinder exhaust. Change porting to the maximum size consistant with reliability and ring catching. But---is all that work worth it? Only if you wanted to prove something.
A premium example of this would be the 1984 Chrysler 55-60-65 series engine. It is without a doubt the best engine Chrysler ever designed This thing was a 2 cylinder 2 carb BEAR, and produced at least its rated HP. The engine is so strong and torquey that if you prop it to rev up too quickly, it develops HP too fast and tears out the lower unit gears. I had one on a 14 foot glass runabout and it topped 45 mph with an aluminum stock prop, without any concern as to jacking the engine to acheive maximum speed and without power trim/tilt.
As far as idle, they use a surface gap plug which while touted to reduce fouling is actually more prone to oil fouling than a bent electrode plug. That's part of the reason these engines don't idle well. And when I see someone on the river having trouble with starting his Force, I lend them a clean set of plugs--usually fires right up after that.
