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Mercury XS850

W

Wotan

Contributing Member
Hi there,
I accepted the challenge to install on my vintage 13´ Hannibal Deep-V Racer the old original motor it used to be challenging with sometimes.
I acquired now an XS850, which was originally installed on the boat in the 70´s.
This motor has experienced some changes during time, so it has now a common 850 lower unit, but the power head appears original to me.
It appears the need to overhaul the cylinderhead (it leaks coolingwater into the cylinder) and a few seals which require to strip quite a considerable bit of the power head.

Now my question, -I was told from contemporaries of those years that there were rather simple tricks, clues and parts involved in getting this bit more power out of the motor.
I was told about over 90Hp at the propeller easily..!
Since I am that deep involved in the motor overhaul, I thought it reasonable to improve its performance probably that little itch...
-Is here anybody around who has any experience with this motor and possible tuning issues?


Thanks a lot!
Stefan
 
Pull the powerhead,bore it if needed,replace ALL gaskets and seals,install a set of fiber reeds (not boyensons). If is a true xs just clean up the exhaust(match gaskets) as it already ported to max spec and there are no bolt on items except race pipes which are extremely hard to find.
 
Thank you faztbullet. A true XS I believe.. I must say. Obviously some people were busy on the motor over time, and the powerhead has no readings anymore for a SN. There is a serial number on the lower part, but it does not match the motor, which I am told is original.
Is there a way to ascertain from the outside that it is a XS? Until now I have only a manual from an 850 and I get along with it fine. I do not even have the proper specifications of the motor ...
 
Thank you racerone. But there is pressure, so shouldn´t there be a proper headgasket? It is not just a lid, is it, like the exhaust cover?
 
If motor serial don't match midsection it make be just a 850. The 850XS has high dome power ported pistons and compression is usually around 140+psi. The exhaust and intake port timings are a little taller,one size bigger jet in carbs,low water intake so it can be raised higher on transom for RPM increase/drag reduction and motor make about 92HP at powerhead and 80HP at prop. Before you spend $$ on it test compression first and ignition system 2nd. If you rebuild it(re_ring is not rebuilding) do the bleed restrictor mod to block.
 
That is real information, -thanx a lot!
I fear you may be right, its a 850... Through the holes of the spark plugs I clearly see nose cone pistons...
The water take in was not original anymore anyhow.. but the powerhead is all that counts.. So the wrap around is original and I do not like it anymore.. pity
Anyhow its a great motor once it is timed again and restored to running condition..
-But it is somewhat disappointing.
 
Now, taking for granted that I bought an egg.. an 850 in an XS shell..
Still, or even more remains the question what can be done to make the fisherman a vintage racer?
I will replace all seals and gaskets, - since the ports will be original I rather refrain from carbon reeds I believe, and if compression proofs ok, I should not change pistons for fun certainly.
Do you have any suggestions to make the 850 a little wild?
 
Thank you very much. There is supposed to have happened a rebuild some years ago, with not many running hours since. Did they install the wrong pistons??
The cylinders of an 850 will likely not be capable to house a high dome piston, but vice versa it may work, from what I see at the pictures of that thread you send me.

My plan is to take a visual after I have removed lids and covers, but before I start I will check compression to have an idea where I actually am and how to forge a plan of action.
Compression tester is ordered and on the way.. so just a few days and I will let you know some details of the motor.
I appreciate very much the support from your wealth of experience.
 
Bleed restrictors?----------------I install an 8-32 stainless set screw with proper hole drilled in it.----Peen the thread so it can not come out.
 
The cylinders of an 850 will likely not be capable to house a high dome piston,
Click to expand...
Yes it can as it the design of piston...nearly all the early model motors had high domes from factory. Piston on left is low and right is high.

piston.jpg
 
I see! There is the light.. so also no telling from looking at it through the peephole sparkplug...
This becomes very interesting
 
Hello Faztbullet,
I would appreciate a judgement..
Today I managed a compression test on my 850.. probably XS :)
The motor is not ready to run, so there is no warm up, and I got readings of exactly 100psi all four cylinders identical. WOT of course and I turned the motor with a strong battery, ignition grounded spark plugs removed.
In my understanding 100psi is low, but all identical. After the age.. I suppose somebody has done a job already. I think to postpone to pull the motor apart and try to get it running first of all.
What is your opinion?
Thanx a lot!
Stefan
 
Compression readings vary depending on the gauge used, who's doing it, barometer reading, altitude, cycles of the moon, sunspots, etc etc etc. :rolleyes:

Very Very Very rarely will you ever find an 2-stroke outboard (especially an old Merc) that has evenly worn all cylinders to a uniform low value. This is almost impossible unless the engine has thousands of hours and absolutely nothing has ever gone wrong with the piston/ring interface or cylinder walls.

On the contrary, the fact that all the cylinders are perfectly even is a good sign that it should be healthy inside.

So, in a nutshell, run the darn thing and see how it does. If it makes any nasty noises, knashing or thrashing, you'll know right away. It obviously hasn't blown itself up so far by cranking the engine, so you're ready for the next step: preps for, & Light Off.

Let us know how it goes..........ed
 
Thanks Ed for mental support!
The old owner told me that a rebuild was done.. some 10-12 years ago :rolleyes: but it has not made many hours since.
So I take it as a good sign!
Ignition works perfect, just the carbs.. another story.. but then is time for test run!
I keep you updated! Thanks
 
Very Very Very rarely will you ever find an 2-stroke outboard (especially an old Merc) that has evenly worn all cylinders to a uniform low value.
Click to expand...
This I gotta know......please explain how 1 or more cylinders in normal operation can wear more than the other on a 2 stroke???
 
faztbullet said:
This I gotta know......please explain how 1 or more cylinders in normal operation can wear more than the other on a 2 stroke???
Click to expand...

OK, so how many times have you seen an old outboard completely worn out with even compression readings, versus the more common way that old outboard dies, by having problems with one or more cylinders? What I'm saying is it's more likely that a motor with even compression readings is OK internally, vs one whose readings are all over the place, obviously indicating a problem.

I'm not saying it's theoretically possible for all cylinders to wear evenly, but statistically it's an anomaly and not what you'd find that often in the real world.

Please enumerate on how many worn-out multi-cylinder Mercs you've found with completely even, low compression, vs the ones you've found to have problems, with uneven compression readings.
 
Please enumerate on how many worn-out multi-cylinder Mercs you've found with completely even, low compression
Click to expand...
A truckload!!! And all inlines as 100psi is low for a inline Merc. This engine if a XS came from factory with high dome pistons and compression new was 145-150psi and if rebuild should be around 125psi with low domes replacements.
When I bought my T2x used years ago it only had 115psi average and would not idle worth a crap and didn't have enough umph to pull a greased straight out of a cats butt. After a 015 overbore and freshing up it jumped to around 155psi and would jump on plane in about 10ft . I sold it after crank broke as to hard to find replacement but a I6 sounds good at 7500rpm.
 
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Thank you for your discussion!
Considering that I just gave it a shot, without warming up the motor and no fuel going through the cylinders for some time, I was happy with even numbers.
Probably I should do another test after adding a drop of oil through the spark plug hole? The motor used to start but with nowhere near idle, -what I put down to the ruined carb.
Due to no idle speed I was not yet able to confirm the proper ignition timing.
Another contributor here suggested that I probably even have the wrong carburettors. So my plan of action is to get the new carb(identical to what I have) get the idle and timing right, and to try it out.
From the Propeller/weight/hull ratio/revs obtained it is my profession to figure out the power that actually gets into the water.
That might be the starting point to decide what to do with the winter...
 
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