1983 70 HP compression

"So I have no compression on c

"So I have no compression on cylinder 1. Engine runs fine but not much power under load. My thumb test indicates there is no (next to none) compression in cylinder 1 (top one). Is redoing a piston a big job for someone who has worked on cars?

The spark plug is wet, but had a piece of metal on it - I have to take the thing apart but I think a reed valve must have broken and gotten into the cylinder or the motor overheated. The water pump was spitting water, but who knows till I take it apart. There was a water leak around the spark plug...

Again, is redoing cylinder 1 a big deal? Can I just do the rings? I would figure that if I have it apart I should just do the piston as well."

"Hi Derek,as a qualified car m

"Hi Derek,as a qualified car mechanic who has worked on just about everything,i treat most things as a basic motor/g/box/etc etc.so a 2 stroke outboard is still just a 2 stroke motor,except for a few differences.if its been near salt water[like most motors in new zealand]then u have possible corrosion of the casings,eating out the walls.but get a good manual,and ull be fine.theres always this site to help out.very good site it is,too.9am sun "

cahnces are it isnt just the r

cahnces are it isnt just the rings the general rule of thumb with outboards is if you do one you do them all . and as far as a car mechanic doing it there will be special tool involved that a auto tech wont have or be able to get without seeing a dealer.

"Hi,you havent seen my tool bo

"Hi,you havent seen my tool box then.most auto techs can usually make any special stuff we need.The late model outboards are a bit different,but the early stuff is reasonably easy to work on.ive rebuilt 8 of mine no worries"

"Well the top cylinder has bro

"Well the top cylinder has broken a ring. I have only looked in from the side so far, so I don't quite know the extent of the damage. Interestingly, the top piston only has two rings while the others have three, also the machining on the piston is different than the others, sooo I'm guessing the piston has been done already.

Can someone tell me if the cylinder is steel lined? I'm guessing it is since the pistons are aluminum.

Is a broken ring a common thing on these? Any suggestions for causes?"

"Broken ring is most commonly

"Broken ring is most commonly caused by 1) lack of fuel/oil to that cylinder caused by dirty/blocked carb or injector 2) faulty cooling system - impeller or thermostat toasted resulting in an overheat 3) the rings just got old and checked out - about 2000 hours of use, or significant underuse (sitting for many seasons and rarely being fired up) seem to dictate the lifespan of a powerhead.

If you are going to rebuild her I would have all the cylinders done - not necessarily bored, but at least honed out (if they don't need a bore)- the machine shop dude will tell you what's best.

The biggest costs are your time (or shop time) plus the powerhead gasket set - so at minimum re-ring ALL the pistons, replace any bearing or seal that's even minimally suspect - since you have her cracked, do it then or do it all over again next year.

Plus, absolutely rebuild the carbs and service the waterpump. Ya that adds a few bucks to things, but since it was "most likely" one of those that caused the problem in the first place, if you don't address it you will just be killing your "like new" powerhead...

Doing most of the work yourself you can do a rebuild of the powerhead and carbs for under 1500 bucks, and that's if you have to go so far as to replace all the piston/rings/bearings and purchase OEM parts from a local dealer, significantly less if most components can be re-used or you can find a source for quality/discount parts...

I have a 4 cylinder tore down in my shop right now - costs are (and I never reuse pistons on a motor I'm doing for myself) - powerhead gasket set $120, 4xCarb kits ($35ea), 4xPistons/Rings ($75 each), rod bearings ($45ea/but does both ends), crank bearing/seal kit ($135 - don't think I needed that but since I'm in there why not do it and sleep better at night) - so parts about $875 and 50 bucks a hole to have the machinist punch them (bore or hone). So just over a grand, plus maybe 10 hours of my own time and presto, practically new motor that should last another 20 years (with care)..."

"Well here is the piston.

G

"Well here is the piston.

Graham L.: I had done the water pump recently and it did need to be done. I'm not sure if this motor has a thermostat. The motor has sat for some time so perhaps. I will rebuild the carbs.

I will obviously replace this piston. The others look OK. This one is different from the others so maybe it was done at an earlier date."

"It would not be uncommon for

"It would not be uncommon for say a carb clog that feeds that cylinder to have caused an earlier piston to look just like that. So the previous owner probably replaced the piston but did not correct the underlying problem.

When only "one" piston goes that is almost certainly a carb problem, otherwise ALL the pistons would be toast..."

How can I tell if I have LOW o

How can I tell if I have LOW or HIGH dome pistons?
The "dome" on the top of mine measures close to 3/4" where on the net I have seen low dome numbers of 0.65". Also the other pistons have three rings should I match them??

"I sorted it out and got a thr

"I sorted it out and got a three ring high dome piston. I would rather match the piston already in the motor than fool around with different ones, even if they are "weight matched". I'll replace the rings on the other pistons and hone all three cylinders. There appears to be some aluminum stuck to the side of cylinders 1 + 2 so I will clean that up prior to honing."

"Did mine the same way and it

"Did mine the same way and it worked out well.

Here's what the # 1 piston on my 650 triple looked like!

Jeff
"

"I did some more reading and d

"I did some more reading and discovered that detonation may have been the cause of the broken ring. I can't say I have ever heard pinging, but that may not be that easy to hear on these things. I noticed some pitting on the tops of all three cylinders, which may be from detonation....

I will have to DVA the ignition when it is all back together. I would love to get to the root of this. I have a new switchbox here so I will throw it on for kicks and giggles."

Be sure to time it as speced o

Be sure to time it as speced out.

Jeff

Detonation is a valid cause of

Detonation is a valid cause of a toasted piston - again could be caused by a clog in the carb or too much prop pitch...

"And, in this case especially,

"And, in this case especially, a lack of cooling for the top cylinder. The standard fix for old Merc triples is to add a Tell Tale to the top of the block.

Note the one I added in the photo.

Jeff

"

I can't find a thermostat

I can't find a thermostat for this motor. I am waiting for the shop manual but being of the impatient sort can anyone comment on whether this thing is supposed to have a thermostat/poppet valve?? There is what looks like room for one on the port side of the engine head.

"IF it has a t-stat, it will b

"IF it has a t-stat, it will be in the front of the exhaust cover on the left side of the motor, about halfway up.

Jeff"

"I think it should be in the r

"I think it should be in the red circle, poppit too, but there was nothing in there!"

Correct! An obviuos case of a

Correct! An obviuos case of after thought engineering.

Jeff

So there never was a thermosta

So there never was a thermostat or poppit valve??? I can't come up with a part based on web searches.

But they cast a spot for them anyway???

Does slow speed cruising with this motor cause it carbon up badly with the engine running too cold?

I've put 5 gallons through

I've put 5 gallons through the motor out on the lake and all is well. Thanks for the help.