I use my boat for fishing in fresh water and salt water, tubing in the summer and all around joy riding and playing on the water. It's a 20 foot flatop with a Mercruiser 120, and it doesn't have the power I need for pulling a tube behind the boat and having 2 or 3 people on the boat also. I have purchased a mercruiser 4.3 liter engine and plan on putting it in instead. The exhaust change from the left side on the 120 moves to the center with the 4.3 liter. Is there a modification for that? If not, would it do anything to the engine compression if I had an exhaust made to connect the the left side of the old exhaust port? Do I have to run the exhaust out through the prop like the old 120 or is it ok to run it out the back? According to the manual I have a model I outboard drive. Will it be Ok to use it with the bigger engine and a 19 pitch prop? Should a different prop be used? I plan on repacking or replacing the universal joints while it is apart also, anything special for that or is it a simple fix? The engine looks like the drive shaft will work and the old engine mount also, anyone have recommendations since this is my first time with this? Thanks ahead of time. KING DYNER, Thanks for your info on the subfloor. that is working out well.
The best solution is to get a whole 4.3 drive with the proper gear ratio and exhaust hookup. The 4.3 uses a "y" pipe that might bolt into the center of your unit. The down pipes have a flapper in them to prevent backwash. I would not hook both into the existing down pipe, but it might work. Just don't cut off the part with the flapper valve in it. The exhaust could be run out through the transom, but since the run will be short it will be very loud. Use flapper valves on the transom to prevent back wash into the cylinders. The 4.3 uses a taller gear ratio so with the 4cyl gear ratio and the extra power you will need considerably more prop. I would start with an aluminum 23 and see how it runs. For skiing the boat should reach at least max rpm wide open. The u-joint should have grease fittings in them. The engine is aligned by raising or lowering the front mounts. It is right when the drive shaft will slip in easily. They make a special tool for this but an old drive shaft will work or if you are very careful the existing shaft will work. If you remove a u-joint for replacement you could use your shaft by itself for the alignment. You are going to put considerably more power through that unit than it was designed for so for-warned is for-armed. jim
