Mk28 'Sleeper'

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Mk28 'Sleeper'

Postby mercaholic » Wed Nov 22, 2017 9:06 pm

I have always been intrigued with the Merc 250's, one year offering.

Lots of good stuff is presented here, especially by Daytona John however not much in the way of water tests.

Stan's post of his Mk28 he is selling got me thinking so I took one of the Mk28's I have and bolted on a decent Merc 250 powerhead from a Rat Racer project in the past.   I do have baseline water tests of the original Mk28 powerhead on my Mirrocraft so will run this one in the spring.  Interesting, when I unbolted this Merc 250 I notice that the butterfly valve was not fully able to open due to a maladjustment of the linkage so makes me wonder how much better that old Rat motor would have performed had I noticed it a few years ago... :eek:

So essentially, the same motor (less Mk28 powerhead) and the same boat.  Hope I don't gain weight over the winter.... :lol:
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Re: Mk28 'Sleeper'

Postby JimWildgen » Thu Nov 23, 2017 2:03 pm

Curious to know if the motor bolted right on?  I ask because I once intended to bolt a '63 M200 PH on a '60 Merc200 and discovered several of the long bolts were the wrong length.  Sure, I could have removed and swapped them but since I had a spare '60 PH I just used that it instead.

As for my prediction of your test results next spring, I suspect the only difference would be something you probably didn't measure: acceleration!  The motor should top out at the same mph because you didn't change the propeller right?  This also assumes you top it out at the same max rpm.

I remember back when I owned a 10hp Johnson on a small boat and then bought an old 18hp Johnson (same prop).  I was expecting a much faster boat and instead all I noticed was much quicker hole shots.  Makes sense because both motors top out at 4500rpm and use the exact same prop.  I could have looked for a higher pitch prop but there was no point because the boat is a one seater so the 10 was a better choice for weight reasons.
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Re: Mk28 'Sleeper'

Postby Fastjeff » Thu Nov 23, 2017 4:14 pm

How's this for an idea: Have someone (?) build three motors using similar LUs and prop and see how they compare on the same boat (kinda what I've done all year long).

The three motors (power heads, actually) would be:

1. Merc 250

2. Merc 200

3. Mark 28

The speed/ planning of performance of these three jewels should prove rather interesting, right?

Jeff
Words spoken in two stroke heaven: There's no fuel like an oiled fuel.
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Re: Mk28 'Sleeper'

Postby jesse c » Fri Nov 24, 2017 12:39 am

hey Jeff,you want to volunteer to be 'that someone'? Maybe you know someone with parts and a test boat ,lol.
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Re: Mk28 'Sleeper'

Postby DaytonaJohn » Fri Nov 24, 2017 5:16 am

Someone posted some time ago that than an aluminum boat with 2 fisherman went noticeably faster with the 250 turning a 3 blade. I think it was Larry Roberts.

If the loads were the same I think a sensitive tachometer would show some different readings with the 11 inch stock propeller compared to the 10 inch 3 blade and the 2 blade 12 inch. Vald told me he never had much luck running the 12 because the boats wouldn't allow the power head to wind up .

Also the Mercury and Michigan propellers LOOK noticeably different, the blade shape, even for the same pitch. Would that make any difference ? Only testing would tell. Just to complicate things, lets play with the spark plug gap as we test !

Then we'll add my Mark 10 "H" to the mix. Exhaust ports machined up to the water jackets just like in Brinkman's book with the 8 reed Mark 15A reed block and drive shaft housing  :lol:
John Timmins, Ormond -by-the-Sea, Florida.  Retired licensed marine engineer from steamship companys under contract with the Marine Engineer's Beneficial Association, a union of merchant marine engineers since 1876. Ship your cargo on  U.S. flag ships !
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Re: Mk28 'Sleeper'

Postby lakerat » Tue Feb 20, 2018 4:24 am

Interesting topic.  I took a Merc 250 power head and mounted it in a 1963 Merc 200 gearshift lower.  I did have to change the studs.  The hard part was I had to use all of the ignition assembly from the Merc 200.  I concur with the post earlier about the props and reeving the engine up.  This combo is a bit faster than my 1966 merc 200, but not much and only if the 12' prop was used.

I also made a Mark 28 rat motor in the usual way, KG7 lower and KE7 middle.  I tried a series of good props.  It wasn't any better than the Mark 25.

The Merc 250 gets it's boost from higher compression.  Another interesting motor like this is the late 60s Evinrude Sportster 25.  It is a high compression version of their old 18.  It is the fastest Rude I've ever known, a match for the Merc 250.

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Re: Mk28 'Sleeper'

Postby Bob » Tue Feb 20, 2018 4:59 am

Running the three M200 M250 and a Mark 28.  
It’s a bit of work switching these power heads tower to tower.  But there is no difference other than the location of the water pump in the towers so one could easily run each on there own and just switch out props.  I could check part numbers but I think all the gears are the same.   I have all these autos and could do this experiment.  The only question is which of the 2 Mark 28’s to run and which of the three Merc 200’s to run then when it comes to the Mark 250’s which of the three would I run ?    Or do I have to run each one ?   
These things get heavier each year to lug around. Think it may have something to do with my age of 71.   My Might Mite I can hold in one hand and my Super 5 is easy too.  
I would put my money on the 250 as the fastest.

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