merc c-mod help??

[vislaguy]

before I even start yes I know yamatos win most of the time but I'm building a merc for other reasons(someone always chimes in with that). what sort of porting has worked good 20,30,40 thou ??? or higher? exhaust side? intake side?? what about crank beveling? I heard that the crankcase pressure is already low and that beveling just hurts that more?? what gear ratio's work best with this motor?? any help would be great .thanks mike

[David_L6]

Stay conservative if raising the ports. I wouldn't raise either side. Most people are running 16:21 gear ratio but Merc gears/units may be hard to find. I would bevel the crank on the side next to the reed cage (top picture) but some people bevel both sides.

[David_L6]

Ports

Two pics - wild ports and mild ports.......

[Bob Dunlap]

Hey David, which picture is your good motor!! We don't want to copy the slow one!

[David_L6]

Hey David, which picture is your good motor!!

Both of 'em.

[Sonny]

"C" mod merc

best bet is to get a copy of Harry Brinkmans book. Dont raise the ports too much...they are best widened...do you have a two port or four port cover motor? a two port cover motor has more meat to the block and can be ported with little fear of carving into the water passage. Round ports will give you a wider power band were square ports give you a target rpm were the motor will perform best. When beveling the crank....it is best to not bevel the very top and very bottom counterweight....they become almost impossible to balance. As far as atmospheric pressure in the crank case goes...when you put megaphones on it it will syphon air through it trading off what you lose. You will have to put a 10 degree champher on the edges of the new port work or you will break rings. You can also do some work on the leading edge of the intake passages inside the block. when you backlash the crank set it up loose for the rpm's. It is best to have a machine shop take a few thousanths off the back of the intake manafold...(front cover) this will clamp your reeds down tight so you dont break them or the cradles in the block...Best gear ratio for a "C" mod is a 12/16 gear ...this will help you come out of the corner strong....Hope this was helpfull

[vislaguy]

thanks

thanks everyone for the input I think I have a good game plan for my motor now . we'll see if the new omc 31 cubes get voted in and blow me a way?? thanks mike

[STEVE FRENCH]

31 cube OMC?

Has someone got them to perform?
Thought porting was very mild for fishing use. Heard the 22cube/25MOD motors were much stronger. If they (31CI) can be made to turn up, would be interesting----ton of em out there!----CHEEP!!!
Plus I think the OMC-A foot will almost bolt on with out much trouble!

I have an extra C-MOD block avail if you want to look at it.
One from Dan Coleman marked "by Carl Matheny". all ports are squared on the top side. .015 O/S. Stock carb flanges. Sorry no pistons. $75+ postage.

The C-Merc I run has only the center hole squared in each port, two outer openings look stock. I believe this was originally built as a runabout/marathon motor cause it was on a Bayer tower with a large tank built into it. Motor seams to have good pull out of corners and turns up well. Ton of fun to run at club races. Also I can still start it myself in water! Bayer 40 cube exhaust, Kg-51 carbs. Timed .380 BTDC. 16/21 foot. Same wheels I run on my 25MOD OMC----Most 7 to 71/4" dia, 13 to 14 pitch.
Good luk with your build--------Hope to see it out there----IN MY WAKE!!!

19P