Changing powerheads

Hello again, I was on earlier this month about a bad piston on my 1987 force 125. I pulled the head and sure enough the piston was melted and a penny size hole melted out of it. The wall is scared bad. I have purchased a used power-head with everything on it entire top end is coming. My question is do I need to do anything special to switch them out except buy a gasket? I intend to get the carbs rebuilt also. I was just wondering if I needed to do anything else. As always I appreciate the advice I get here. Thanks, Chris

Re: Changing powerheads

Chris: I don't remember your last post but I'm sure I replied to it.

Anyway, first thing you need to do is to remove your old powerhead. to do this, you must remove all wiring and carbs. Take off anything you can to make it lighter. Bare, it weighs around 100 pounds. So head and flywheel , stator and all electronics come off first. Then you must remove the lower pan. To do this, remove the six 7/16 head bolts holding it to the rear shroud, and I think there are six additional ones holding it to the front shroud. Lift it off. Remove the six bolts holding the rear shroud on then pry out the two pins at the bottom of it and remove it. Now the nuts holding the engine and match plate to the mid leg are visible. remove the six 9/26 nuts and six more in the front of the match plate. Disconnect the shift and then start to pry up the block and match plate.This is frequently difficult and time consuming. It would be best to have a little help at this point.

Once you get the block and match plate off the leg, you will see four more 9/16 nuts inside the match plate. remove them and using a rubber mallet, tap off the match plate and exhaust tube for use on the replacement engine. this gasket will tear and need to be replaced.

Once you get the match plate and exhaust tube on the replacement block, then you need to lift it onto the lec. You will need help here because you must tilt and turn it for the flange on the exhaust tube to clear the water tube mount inside the top of the leg. No gasket necessary between the match plate and mid leg. A little silicone RTV is all. Don't get too generous with it as it will squeeze out and partially clog some water passages in the match plate--you will see them.

If you have a manual, everything is pretty much straight-forward--except they don't tell you just how difficult it can be to pry up the old block off the leg.

Before you even attempe to mount the replacement, spin the flywheel which you will have temporarily put on for this step. Put your thumb over the spark plug holes and see if there is compression. This is just a rough go/no go test. If nothing here, then contact seller for a refund.

Once you mount the powerhead, assemble all subsystems and again first thing, check compression. Might not be a bad idea to remove the head when you firse get the block and eyeball the cylinders and pistons. If you do that, then you don't need to check compression with your finger.

After everything is assembled, --oops, gotta go. more tomorrow.

Re: Changing powerheads

You guided me on my original Post Frank. I'm with ya so far on this, I will print it out and have it with me. I have a manual on order and a place with forklift to pull it off and put it on. I'll check the compression and see if it is what was advertised right around 146 on all four. I'll order the gasket. I'll look forward to the rest of your advice when you come back. Thanks again Frank.

Re: Changing powerheads

Oh, I forgot to mention Frank The power head looks like it still has the spacer plate on it. I'll know more when It gets here. Chris

Re: Changing powerheads

Once you get the replacement powerhead on, the best advice I can give you is: Assume everything that can be out of adjustment is. Leave nothing to chance and set everything to factory specs. You would be surprised at some of the things I have found on engines I have bought.

Be especially paranoid about the carbs. The original melted piston was most likely due to a lean run on one carb for whatever reason. So, be sure to put a disposable filter AFTER the fuel pump and and BEFORE the carbs. Be sure the carbs are not too lean--the example used in the Clymers (5/8 turn) is too lean for my taste. My opinion is that it is far easier and cheaper to change fouled plugs due to a rich mixture than to replace a piston or powerhead. Of course, I would routinely change the fuel pump diaphragm no matter what its condition to ensure good fuel supply.

If the engine comes with the adapter plate and exhaust tube, leave it on and don't monkey with it. That way you won't need to buy the base gasket (they'll nail you for 10 bucks). If you need to change the exhaust tube be very careful--because of the acidic nature of exhaust gases, very few of them come apart without snapping off at least one of the four bolts that hold it to the match plate, or breaking one of the mounting flanges. So, if the engine comes without the exhaust tube, in that case, you would be better off to replace the whole match plate assembly and buy the gasket.

By the way, the exhaust tube comes with a foam gasket under the bottom flange. The purpose of this gasket is to seal the leg so that at idle exhaust gases do not pass up the midleg, and to have a quieter idle. Since some of the midlegs have several holes in them (sometimes plugged with plastic, sometimes not) It is not critical and if broken (the gasket) it can be left off. For the same reason, do not worry if the lower flange itself is broken. As long as the exhaust tube is sound, it's OK.

You don't really know the history of the block you bought so just in case, run the first tank (6 gallons) at double the oil and take it easy on the engine. After that, you can be reasonably certain that there is enough residual oil in the block to properly lubricate it so run it like you regularly would. After all, it is already broken-in.

Re: Changing powerheads

Ok Frank. I think I got it. I will let you know how it turns out. Thanks for taking the time to let me know how to do this. Chris