mercury Mark 25 hydro motor

[weippert49]

Hello all just recently acquired a 56 Mark 25 hurricane motor that was used on a vintage hydro built in Michigan in the late 50's early 60's also have access to the hydro but it needs to be recovered. Here are my curiositys it has been changed to a aj50a carb instead of 47 and has a few odd brackets I was wondering about. Any information on the odd setup would be greatly appreciated. And holy cow this thing has a lot of compression

[weippert49]

It has the tiller still as well as the wing bracket for steering but have no idea what that bracket on the transfer port cover is. Any information would be highly appreciated the motor will be going on a 8ft aluminum boat until I get my little hydro built. Any idea of what kind of speed I could expect and if the aj50a is in fact an upgrade. I've worked with quite a few different 2t engines but these outboard are new to me.eventually it will get a tuned pipe

[Instigator]

The MK 20H's were the race motors the 25's were the fish motors. If it's the stock block/pistons, it has less (believe it or not) compression than the 20H's.

There was a bazzilion carbs that would interchange so easy to go from stock/small carb to larger.

Not sure on the steel bracket?? Wrong side to be part of a pipe bracket.

As to the steering bar mount, they all had that.

Looks like a fish motor to me.

BTW, I raced them for 9 yrs and putz'd w/them for twice that time.

[weippert49]

Right on I figured it was a fishing motor when it had the regular lower unit but I think it will be a fun cheap way to get something vintage also looking at a Chris craft commander 10. The man I got it from said the original owner put new pistons in it in the 80's . I was wondering is the exhaust plate the same as the earlier motor i.e. can I build a pipe to the specs of a few pipes I have seen for the mk20 ? I know the tuning will be different to drop the power band for the engine. Also and thanks in advance what would the max rpm I should run this engine?

[Instigator]

Right on I figured it was a fishing motor when it had the regular lower unit but I think it will be a fun cheap way to get something vintage also looking at a Chris craft commander 10. The man I got it from said the original owner put new pistons in it in the 80's . I was wondering is the exhaust plate the same as the earlier motor i.e. can I build a pipe to the specs of a few pipes I have seen for the mk20 ? I know the tuning will be different to drop the power band for the engine. Also and thanks in advance what would the max rpm I should run this engine?

Everything is interchangeable between the (KG-4/Mk15 to KG-7 and Mk 25's)2 bangers from that era.
Exhaust plate is the same.

Another trick w/the old Mercs is the 4 and 6 cyl's are just 2 bangers stacked on top of each other. So you can use their pistons, rods, reed cages and carbs. Not necessarily any better but a cheap way for spare parts.

Biggest differences are the carb opening in the case (and reed cage, for bigger carbs) is twice the size of the fish motors and the block has what looks like a weld bead in the bottom (what would be a head on other motors) of the cylinder which raises compression.

Another trick to make them run is to actually "time" the ignition instead of just setting the points at .018".
I ran 2nd in a race w/a bunch of fast guys but couldn't get around the 1st place boat.
We timed the motor that night and I ran away and hid from him the next day w/no other changes.

To do it, you put a dial indicator in the top cylinder w/a light or buzzer connected to the points. I made one out of a plastic butter container to hold the batteries and screwed a small light to the top of it w/2 leads w/alligator clips going to the points.
You set your points to .018" then adjust/rotate the mag plate until the light just starts to go out at your setting on the dial indicator.
.285" BTDC comes to mind???
Once set, lock the mag in place. You'll have to devise a lock.

Then move to the dial indicator to the bottom hole and your light to that set of points.
Now, just adjust that set of points so the light goes out at the same position, regardless of actual gap.

When you do that you can then remove the screw ball set of linkage, fulcrums, bell cranks, and other gizmo's which saves some weight but really cleans up the powerhead for ease of maintenance.
You're then only opening the carb butterfly for your throttle.

BTW, the pipes look/sound cool but do zero.

Another trick is to replace the base gasket w/a thinner than stock version. I think mine were .060".
This raises compression a bit and advances port timing.

They also made sand cast and die cast cases. The sand cast were tighter (less volume) and faster.

[Mark75H]

Reeds are the same between the early twins, 4's & 6's but the cages are different because the crank journals are different sizes to support the different overall lengths.

A pipe will do nothing but draw negative attention to your use.

I think there is a Quicksilver tower and foot that will fit the Mk25 on eBay right now

You should not run the cast iron flywheel more than 5,000 rpm. The real race twins used a forged flywheel that is stronger at high rpm.

An extra thin case gasket raises compression but retards timing (ports are lower/closer to crank).

Charlie Strang experimented with the various volume crankcases and decided they made no difference with the variation available (even with the special extra tight case he custom made).

[Instigator]

Reeds are the same between the early twins, 4's & 6's but the cages are different because the crank journals are different sizes to support the different overall lengths.

A pipe will do nothing but draw negative attention to your use.

I think there is a Quicksilver tower and foot that will fit the Mk25 on eBay right now

You should not run the cast iron flywheel more than 5,000 rpm. The real race twins used a forged flywheel that is stronger at high rpm.

An extra thin case gasket raises compression but retards timing (ports are lower/closer to crank).

Charlie Strang experimented with the various volume crankcases and decided they made no difference with the variation available (even with the special extra tight case he custom made).

My bad.

I thought I'd used those cages before. I know the biggest thing was finding the ones w/the large openings like the 20H's had.

You are correct on the port timing due to gasket thickness. Was standing on my head when I wrote that.

Didn't realize that on the flywheels either.

I know most of the fish motors came w/the Bendix ignition/flywheels and we always ran the Phelon.

[RogerH]

Just for girns.......my eclectic combination of KG7H / MK25 / Quincy that I put together. Nothing but fun.

[olboatman]

Just for girns.......my eclectic combination of KG7H / MK25 / Quincy that I put together. Nothing but fun.

Hey Roger did ya ever run that beast......reminds me of my old KG-7 I had when I was 14... on a 10 ft CastaCraft U-boat. Gary

[Instigator]

Back in the day.

[RogerH]

Hey Roger did ya ever run that beast......reminds me of my old KG-7 I had when I was 14... on a 10 ft CastaCraft U-boat. Gary

Nope - Just decided to build it from parts I had and I don't have a boat to run it on any more. Also did a 20H just for the fun of it. Like you, I had those motors years ago, and enjoyed re-creating them. My kids will wonder what to do with them some day LOL

[weippert49]

Thanks for the info guys I'm actually thinking of an expansion chamber exhaust. On the flywheel are their any other models beside 20h that will work ie other models? Also will putting a cone on my lower unit help with drag kind of how the quicky is? Thank you guys so much. I just got a vintage hydro built in the 50's

[weippert49]

Has vintage Keller throttle and steering I love the thru deck steering cable guides

[Mark75H]

Looks more like a 1980's racer ... definetly not 50's. 50's boats of this config were required to have a forward cockpit and seat. She's got good bones and can be restored.

Expansion chambers will not work with crossflow porting. Been tried about a million times by guys who were experts at 2 stroke racers.

A cone won't give any improvement to the std lower unit, the speeds the prop, gear ratio and power involved have just aren't critical beyond the original good shape. The only good thing is that the original lower unit is faster than anything else less than 30 hp made up to about 1970.

There are some freak Western Auto Wizard models with the forged flywheel but just about everyone who has one of these, knows what it is and within the WA models, the forged flywheel was not on all of them.

[53w]

Thanks for the info guys I'm actually thinking of an expansion chamber exhaust. On the flywheel are their any other models beside 20h that will work ie other models? Also will putting a cone on my lower unit help with drag kind of how the quicky is? Thank you guys so much. I just got a vintage hydro built in the 50's

That is a runabout. It could be a wood Race Craft? The remains of the nose looks like a Race Craft?

Expansion chamber will run on a cross flow, They run very well once you get them dialed in. I run them on my A-mod Hot-Rods.
53w