TOPIC: The Mercury 1000 Twins Compression

I'm going to back-burner the twins for a bit, i need to bring in the MFG and put in the new floor.

But before i put them in the back of the garage I thought i'd look at compression and get some questions out for me to think about...

I removed all the spark plugs on each motor. Had the choke open and tied the throttle open. Both engines cold. Sprayed DeepCreep in all cylinders before test. Ran the flywheel around until i felt 4 compression strokes. This is what the manual said to do. It did not have what the compression should be, only said that it should not be more than 15 lb different between cylinders.

Stuck Pivot Twin:
cylinder compression
1 22
2 22
3 22
4 30
5 30
6 22

OK Pivot Twin:
cylinder compression
1 0
2 41
3 42
4 0
5 40
6 60

One of the cylinders with the 0 was expected, but not 2 of them. I was told when i bought them that this one had a burned piston. He sold me set of 6 NOS pistons and rings.

Thoughts?

When i get my MFG in for the now floor i'm going to run a compression test on that one for comparison. It's a '68 1000.

cheers!

dan

120 lbs would be good both motors must come apart. jmho.

Well I wanted to get some experience with these beasts. Looks like I got what I want.

Thanks.

did you spin the motors with the starter ? or with a wrench? just wondering. frog

With a wrench on the lift ring. Is one or the other better?

I don't know if its the proper way to check the compression, but I will crank the motor until the compression gauge stops climbing. The readings for both motors almost seem to low the believe for all 12 cylinders. Just my two cents.

use the starter to spin it. then give us the #####'s

"I don't know if its the proper way to check the compression, but I will crank the motor until the compression gauge stops climbing."
That is the correct way to do it, and as Dave B added, use the electric starter to spin the beast.
Also, use the type of compression gauge that threads firmly into each spark plug hole - not the type that is simply held firmly against the spark plug hole.
The numbers should all come up a TON doing it that way.
Even the zero results might come up some, but probably not enough if you were initially told that once cylinder was low or dead, etc.
Keep us informed.
Doc

Fog the motor before you test it too. It will give you more accurate numbers and protect it from scoring.

Pull the plugs out.----Use a starter cord to pull it over for compression test.----Use a hefty pull as if you were trying to start it.----I do not think is is possible to score cylinders on a cold motor when doing a compression test.

Here comes a newby question...

Can i just hook the starter up to a battery?

One of them does not have a wiring harness on it. The other does, and i think i have the mate to it in the boxes of partes that came with the motors.

Now don't laugh too loud!

Shouldn't hurt a thing. Battery and jumpers, Negative to a good ground motor, touch the positive to the starter hot wire lug. Watch yer fingers and be careful..

Thanks. Maybe i'll get around to that this weekend before i pull in the boat to start on the floor.

I have jumpers and cables. I'll be careful. Wouldn't be the first time i got a shock though.

Not any shock, just spinning parts to stay away from. Wear safety glasses too.

you need to turn the motor over with the starter it can be done on a big six with a rope but use the starter and watch out for spark/gas and the flywheel

Well I been thinking this a bit again. Even ran my thinking by the famous "Doc Frankenmerc..(stop and BS ya soon Doc)I simple want to CLARIFY on a Comp check, a 2 stroke vs a 4 stroke. Been messing any type motor since the A+ in "auto shop", at 14, and thats 50 years ago!

Anyway on a 4S? Yes, carb butterflys open for free air intake. And as with them valves, bet even (need to experiment this) a fast spin with a flywheel wrench on the comp stroke "might" get ya a decent reading? But thats NOT the way to do it anyway..(not beating up on any here, heck we all learn as we grow more knowledge with experience and age!

But on a 2 stroke? Comp check? Yep hard pull a rope works..(and like Doc said, screw in plug gauge, yank until the (GOOD) gauge says thats it.

But heres a vid of how a 2S motor works. Thing is if ya do a slow turn any crankcase charge it simply goes out the exhaust port fast. BUT! Even throttle closed? Don't matter. The millisecond that EX port is open, atmospheric pressure fills ANY vacuum INSTANT!

On soaking any 2S motor first with "Oil"? Its gonna give ya a false reading too. Most I ever did (even sitting years) was spray some WD in and rotate and distribute some lube, then the fastest spin ya could do, rope or starter is more a real reading. Hope some will understand what I'm saying. Dude, ya got a nice "set 1000 Twins!"

Get those beast running. Besides the "Doc"...?..anything needed to know?..ASK, as great bunch here! Wish the best! Robby..(the vid now)

Thing is, any engine that old needs to come apart for a freshening up regardless of compression, unless it is going to be a sawhorse queen. Age and sitting around takes it's toll as much as a lifetime of hard running and abuse, so if you want to be able to rely an the twins to get you and your family out AND back, grab a Mercury manual and get at those two with a vengeance and lay them bare. Don't kid yourself that they will be fine without a rebuild... they won't. At minimum you will need a gasket set, crank bearings, rings, seals and carb rebuild kits. That's if the innards are in good shape and nothing is rusted, bent or scored. Upside is that you will be rewarded for all your work and expense with an engine that will give you a set of emotions that will really stick with you.

"Age and sitting around takes it's toll as much as a lifetime of hard running and abuse, so if you want to be able to rely an the twins to get you and your family out AND back,...Don't kid yourself that they will be fine without a rebuild... they won't."
GeoFGarland hit the mark on this subject!

Compressions are only an alluring, but false comfort.
So are 'leak-down' results.
I've found stuck rings, broken rings, failing pistons at the pin that keep the ring from rotating, & lots of &scored skirts with what would be considered good or even great compression readings.
145, 145, 150, 145, 145 compressions with 2+% leak-down on all (4) would suggest the thing is pretty decent inside. So could 120, 120, 121, 122, etc, but there is seemingly always something that should be corrected to have a chance of getting another (5 to 40) years out of the beast.
Doc F

So what I'm hearing is that I might as well plan on a total rebuild. Either I do what I can and take them to someone to do what I can't. Or I need to learn how to do it all.

Pick up a genuine Mercury service manual for your model of engine and read it cover to cover before you touch a screw. There is no trick to rebuilding these things, just be meticulous and patient and ask LOTS of questions. That is what we are here for after all.

Ask questions before you use a hammer.

---

Sometimes folks will do something and ask questions after they make the oops that cost $$$$$$

Thanks. I'll have to look for a manual. I have a non merc service manual. Forget the name. Like a Chilton book for a car. Just checked. Its a seloc manual.

I could see me taking a year or 2 to get hem done. I don't think I can wait that long to get the powercat on the water!

An original manual on amazon is $85. Anyone buy the CD version? Or have a spare? My twins are 65 and I have a 68.

Sent it again..copy and paste the address both times.
It will be from yellowjacketboats.com