Compression 1974 50HP Evinrude

" Top cylinder wasn't prod

" Top cylinder wasn't producing power - checked compression: about 50lbs top and 140lbs bottom!! What underlying problem should I assume caused this failure? Is full rebuild the only way, or what about just pistons, rings, gaskets?? hone?? bore?? I guess I'm wondering how much to spend on this age of engine. Thanks for any guidance. "

"AStrong.... Whatever that eng

"AStrong.... Whatever that engine requires will have to be your judgement call. However, I'd suggest that you first remove the cylinder head to inspect the piston, cyl head and block surfaces etc. Hopefully you'll be lucky and find that you've simply got a blown head gasket. Get back to us with what you find.

Joe "

" My last post got lost, so he

" My last post got lost, so here it is again:

Joe: I am opening up and checking pistons, rings, etc. Except, what is the trick to getting the power head off the rest of the unit? I took off the 4 bolts on the block brackets (with rubber grmmets), but can't budge the power head off the drive shaft etc. Any suggestions?

And is it usual to replace all rings and/or pistons (if necessary) when one cylinder still has good compression, or just the affected cylinder?

And is signs of purple potting compound on the block below the aft pick-up coil always indication that a new coil/stator assembly is required? What should the ohm readings be on one of these coils? I was pretty sure my spark was good but now am wondering......

Thanks for your (and others) opinions/help here...

A.S. "

you need to remover the exhua

you need to remover the exhuast cover and there you will find some long bolts that go through the adaptor to the power head remove these and it should come off

"AStrong... As Bill mentioned

"AStrong... As Bill mentioned above, if you haven't removed the eight (8) long bolts and the 1/2" nut at the rear of the powerhead, do so. Also, make sure that you have disconnected the shift linkage at the forward bottom area of the powerhead. On the other hand, if you've already removed the bolts/nut and have disconnected the shift linkage, I'd suspect that the driveshaft splines are stuck (rust/corrosion) in the crankshaft splines... in which case you will need to try whatever you can think of to pull them apart.

The stator readings I don't have in front of me at the moment. Remove the flywheel and inspect the stator. If it's cracked/melting down, it doesn't matter what the readings are, just replace it. If you need one, I may have one left in stock. Let me know via my e-mail address.

Never replace a good piston, but do install new ring sets. Pistons are expensive and there's no reason to replace a good one.

Joe"

" Rebuild Time!! I finally got

" Rebuild Time!! I finally got around to opening this engine - top piston was damaged around top, looks like the ring broke/burnt up. The head also has pitting, and there are some obvious scores on this one cylinder wall. So, I have priced out a bore job to .030 oversize, new pistons/rings, etc. Everyone knowledgeable I speak to say I have to bore out BOTH cylinders inorder for balance, etc. But, really, on a 1974, with limited hours of use each year, is it essential?? I know your experience and trust your advice on this Joe - if I can save the old piston and just replace rings, it sure saves some dollars. And the head, I'm planning just to clean and gently grind as smooth as possible, but re-use as well. The pits have left a roughish surface. Crazy??

Also, do you recommend replacing the rod bearings (needles in split cage) at this time, or again, considering the age, re-use the old?

Also, what is the underlying cause I should be looking for with regards to the this top ring being burnt up/destroyed? Carb rebuild to fix lean mixture? Improper timing? Bad cooling causing overheating? Or was it likely just worn out rings? Likely, a bit of everthing, eh?

As always, Joe, Bill and others, I appreciate all of the help and knowledge here....

AStrong "

"Astrong.... There will be no

"Astrong.... There will be no weight balance problem if only one cyl is bored. If the other piston is okay, reuse it.

However, if any metal (steel) has been forced into the cylinder head, I strongly suggest that you replace the head. You will never get all of those small steel pieces out, and at least one of them will burn steady like a glow plug of a model airplane engine. That will create pre-ignition and a repeat of your problem.

Clean those carbs thoroughly and rebuilt them to eliminate any lean running possibility.

If you haven't moved the full spark advance screw (the one with the rubber stopper on it), I'd assume the timing is okay. You can check that when you're checking the ignition (spark) as you finish up the overhaul.

Replace only what is damaged or obviously worn. If you start replacing everything simply because it's old, you'll just be wasting money.

Joe"

" Joe: Thanks for your timely

" Joe: Thanks for your timely reply. I was hoping to reuse the old piston - a new .030 over with pin is about $210CA, another $45CA for rings.

I haven't priced a head - I'll check at a shop to see if they can do a grinding/surfacing job on it.

What's your thought about reusing those bearings though? They appear O.K. (still look like needle bearings).

One last question: is it tough to press the new piston and pin onto the old rod at home? Can I drift-punch it using a shaped wooden cradle or do I need to take it to a shop with special press/piston cradle?

Thanks again for your time on such an old beast. Hope my next post is once I hear it running..... "

"Astrong.... If those needle b

"Astrong.... If those needle bearings are still shinny and have a chrome like appearance, have no blue (burnt) coloring look to them, in short they look like new..... use them.

The wrist pin question is a toss up. Some of those pins are a pressed fit, and if so, you'll need to look at the bottom of the piston, at the areas that house the pin on each side, to locate the letter "L" which will indicate the loose side.

In those cases, I've always secured the connecting rod/piston in a vice, with the "L" side facing away from me, heated the piston with a hand held propane torch, then tapped the tight side in until it cleared the piston.

When installing the wrist pin, I'd easily tap it into the loose side, through the wrist pin bearing, then as it got to the tight side, secure it in the vice, heat the tight side of the piston and tap the pin the rest of the way in.

When you're through, check the shirt of the piston for slight damage due to possible rocking in the vice (it happens) and touch it up (file) as needed.

If, on the other hand, those pistons have a floating wrist pin, it creates no problem and it could be tapped out and in while being held in your hand.

Joe"

" Joe: A little late, but I t

" Joe: A little late, but I thought I'd mention that 2 weeks ago I had my 1974 Evinrude back together and in the water, running great. Compression both at 120 right now, having put in one new piston, new rings on both, new gaskets and oil seals, and carb kits. I used muriatic acid on Q-tips to etch the top cylider walls where the bad piston had left aluminum deposits - then used a car hone and honed both cylinders - no bore job required as it seems to have worked fine. Anyway, after some minor carb adjustments, everything seems to be running fine.

So Thanks Joe (and some others) - your advice was very much appreciated. Now I just hope it keeps running....

AStrong "