My Mk 25H conversion...

[LakeRacer99]

Mk25 powerhead, KG7H tower, Quicky lower.
mota is in the process of restoration. Runs like a Banshee, i just need to finish painting the cover.
Anybody got an old hydro laying around?
LR99

[Mark75H]

There is a "MK25H" and there are "conversions" and there are "H's".

Antique purists would prefer to name your motor a Mark25Q indicating a Mark25 with a Quicksilver lower unit and not one of the other motors.....

Ron Leitha has the Mark25H. It was a prototype front loading B Quicksilver lower unit...sort of like a Speedmaster. In Antique Speek "conversion" means the tuned exhaust mid section Kiekhaefer made for the Mark20H to correct for the original's very embarassing out of tune mid section that actually killed power.

I'll try to find time to make links here to show the Mk25H

Here is a link to some Mk20H and conversion stuff:

Charlie Williams' site Warning: Heavy Drool Factor for antique junkies!!

And yes, I do have an old B hydro laying around that would suit your motor!
Wanna buy it?

[Laker]

Yep Got me going to with that Conversion listing....
but...
Love how the Toilet bowls sound!!

[WharfRat]

back in the '70s they used to hold some small boat races on Clear Lake in Texas that started out as all fun..............at the start you had to guzzle a full beer, jump in & start your rig (20hp limit) go out to a bouy & turn around & come back in to win, then after about 4 years some dude shows up with one of those 20H motors with a full conversion on it...................
OMG I loved that sound.....................the next year the rules were all changed & it got all serious because everyone was gunning for the guy LOL
That same boat was clocked at 62mph at Lake Liberty the next year...............

[LakeRacer99]

Thanks for the correction Sam, I am limited on knowledge of these motors, they are 20yrs older than me. The only knowledge I gain from these antiques is mostly from the internet. Thank you for your site, I have spent alot of time there.
Do you have any pics of any other Mk25q? Do you think that case would last long with this power?
This motor is louder than my 2.5!!

Thanks,
LR99

pic deleted

[Mark75H]

It worked for me the first time.

I overlooked the stacks the first time. Don't get your hopes up on the power. First both the Mk20H and Mk25 are 20 cubic inches, no power increase from being bigger; secondly, compression ratio means more than tuned pipes for the first power increase on the small deflectors. Mk20H's have cylinder head clearance of 17cc's, about 10:1; Mk25's usually are in the 22 to 23 cc range, about 8.5:1.

Are you using the little AJ carb that came on the Mk25 or a larger KA size carb? The 20H uses the Tillotson KA or Carter N, both bigger than the AJ. You have to relocate the carb mounting studs for either. I usually find a pair of aluminum 1/4-20 bolts, thread them in and cut them off flush because the relocated holes hit in the old holes a little. The 20H also uses a bigger throated reed cage with bigger reed openings that may also be used in the Mk25. Look for the big reed cage in junk Mk58's and Mk35A's. (Not a real 20H cage, they only work with the real 20H crankshaft which is a little different.)

Your tower and lower unit are the second generation "Q" length, not the third generation, shorter "H" .

[LakeRacer99]

Hey Sam,
I don't really know what to expect for power, it is a wall hanger for me that I put new rings in and painted, I have less than $100 in it.
It sits on a stand amongst my fishing motors.
I have no clue what carb is on it, it is a Tillotson and has the numbers 010 739 near the fuel inlet.
If you look above the "Q" cav plate on the mid you will see a weld line where it was shortened, there was also a plate that came down the middle of the mid in the back for strength. I cut it off, as I knew I wouldn't run it much and hopefully the mid will stay together.
I don't really know if it has any value or not but I sure like it. It runs and when I get the cowl painted, it can go in my living room.
LR99

[David_L6]

Some of the Mark 25s had a stainless cage with bearings. The bearings were a larger diameter than rod bearings. The cages from a 4 cylinder engine won't work on that style crank.

I used and case and crank from one of those Mark 25s to build a B mod. It had a Quincy cage with a bushing instead of the bearings. Quincy didn't like the stainless cage and built the above mentioned cage with bushing while they were opening up the carb hole and squaring p everything.