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View Full Version : new to me Mark 20, mods?



RathaBsurfin
01-16-2005, 03:53 PM
I just bought a mark 20, i havent even looked up the year yet. But its bone stock. I was wondering if there were any easy mods I could do without comprimising the dependability of the motor. Im thinking a prop change, larger carb?, cut down the flywheel. I really dont know what I am doing, any help is appreciated, thanks yall :D

Andrew

Mark75H
01-16-2005, 05:17 PM
Cutting down the flywheel won't help a bit. Changing to a larger Tillotson KA carb or Carter N carb will make some difference, but by the greatest margin, the best way to increase the power of a Mk20 is to replace it with a Mk28 powerhead with the KA carb already on it and the bigger reed cages. With the relatively low compression of the old Mercs there isn't a lot of "magic" that they respond to.

And since they haven't been made since the mid 1950's finding good props for them is a real challenge. If you want to go faster, its much better to use a current motor.

RathaBsurfin
01-17-2005, 10:31 AM
thanks, ill look into the larger carb. I really dont want a new motor. I wanted this one, Ill look into putting a larger carb on it and Ill talk to a prop shop and see what they say.

Chinewalker
01-17-2005, 11:54 AM
What are you running the motor on? Why do you want to modify it if you haven't even tried it out yet? I can tell you from experience that on the right hull, a Mark 20 (or any of the 20-cube Mercs) is plenty, even in bone stock form on a tiller. I run a KG7 on a 12-foot tin can and that's PLENTY fast! High 20s on the GPS and it's a handful! If you're running it on a small tin can, then it will be fast enough to make you wet your pants with no modifications whatsoever. If you're running it on a hydro or something "hot", then go for a true racing motor, whether it be new or old. If you start cutting up a stock Mark 20 on a fishing unit, any mods you make likely won't be noticed anyway due to limited propeller selection and drag from the larger unit...
In order to install a larger carb, you'd have significant machine work to do to make it fit - it's not a bolt on fix.
- Scott

RathaBsurfin
01-17-2005, 11:28 PM
No, I havent. It is currently not running. I need to put a new water pump in it and probly rebuild the carb. Its on a 11 foot wooden runabout and I doubt Ill even be able to run it at full throttle. I have run a late 50s 10 hp OMC on it in the past and performance was crappy to say the least. I guess carb swapping isnt an option if it requires machining, i want to leave it fairly stock. Not tryin to start a battle, i just like to tinker and I want to know if there is anything I can do to this motor. Im sure yall can understand not being able to leave things alone.

Mark75H
01-17-2005, 11:37 PM
Not machining would have precluded cutting down the flywheel anyway. If the work to be done has the limits you have just described, then, no there is absolutely nothing that you can do to this motor. I can't think of any direct bolt on changes for any motors from that time or type, other than the Quicksilver lower unit. Can you Scott?

RathaBsurfin
01-18-2005, 12:14 AM
I should have pgrased that better. I dont mind cutting down the flywheel, I can get another one. I just dont really want to cut into the outside of the block like that. Are oversized pistons available and is that a feasable mod? sorry for all the questions. I guess I can pull it apart an balance and check all the clearances on everything. i just want a project, i really dont have a goal for this thing.

Mark75H
01-18-2005, 08:27 AM
For about $400 you could have it bored out to 22ci with 22ci porting or better.

Here is a project: Mark20H tech (http://www.boatsport.org/pettyfinal.html)

Do all this work (it is quite a bit). Follow the instructions to the letter and it is likely you will make the 20 run better than new. :)

Chinewalker
01-18-2005, 04:55 PM
... is to make sure it pumps out all 16 ponies. All to often I get folks asking me how they can hop up whatever motor they've got, and when I take a look it's leaving a slick on the water, sounds like crap when it's running, the prop is mangled, the motor is bottomed out on the transom and the trim angle is all wrong. Set-up is everything. Make sure the motor is timed right, all linkage is good and tight with no extra slop, the prop is the right one for the rig, the propshaft is set dead-on level to a little out, and that it's set as high on the transom as it'll go without ventilating. Set up right on an 11-foot runabout you should have a fun ride! Might want to consider installing a kill switch, too...
- Scott

RathaBsurfin
01-18-2005, 07:15 PM
thanks guys. I am going to rebuild the transom for this motor and I think the boat will end up growing a few inches in the process. I plan on tearing the motor down and making sure everything is tight and right.the prop is pretty mangled, i tried to get another one off the guy but he said it was the best one he had so I need to find a good prop shop to talk to. I was told that this motor has to run 50 to 1, do yall agree with that? I dont have a problem doin it but I wanted to hear what yall thought. thanks for the help.

Mark75H
01-18-2005, 09:36 PM
I would use more oil than 50:1

RathaBsurfin
01-18-2005, 10:54 PM
oh shoot, just jokin, i need to stop posting while im in class, dang wireless. The guy i bought it from said 16 to 1. Can i run 32 to 1 or am I going to kill it? sorry about that

here is the boat its going on

Blue Prints
http://www.glen-l.com/designs/outboard/outboard-images/dsn-utll.jpg

Boat
http://www.glen-l.com/picboards/picboard8/pic465a.html

Mark75H
01-18-2005, 11:24 PM
32:1 is fine

Chinewalker
01-19-2005, 11:03 AM
Should go like a bat outta Hades on that boat! Looks cool!
- Scott

lilabner
01-19-2005, 10:16 PM
As a 15 year old kid, in 1957, I took my block to the machine shop and had them cut about .030 off the crankcase mating surface..took a couple of hand files and raised the ports about as high as I could and made the bridges smaller, put her back together without all the stuff in the article Sam posted, (good info by the way) and proceeded to bust up the bay. It ran as good as my buddys Mark 25..I think the article should have included that after finish sanding with 600, to use the other side of the paper for a while...lol :p :D