Merc gurus & techs please help with mercury 3 cyl mods

[skipper6567]

Hi All, I hope this post will be able to help me and many others that want to modify smaller Mercury 3 cylinder engines. This site and S&F site are the top dogs for hot rod boat builders, racers.. The information on these sites is priceless!!Best in the world! I/we need your help because 99% of technical info for mods is for V4-V6-6L outboards, plus Fastfreds Mod 50 thread. Maybe this could become like fastfreds mod 50 thread but for mercury 3 cylinder. This post is for technical help so please don't bother posting, not much to do with blind bore engines, mercury small engines are already at the most hp or buy a new engine, no replacement for displacement ect. Please post just the facts of how to mod merc 3 cyl If there isn't any mods then this will be a blank thread.. Oh well I won't know if I don't ask.. I need your help with building 1 engine from 3 I have in pieces. This engine is for a slt inflatable tunell hull DUX. I'm not racing, just want to go fast, no skiing, just top end fast. I want to use what I already own. All engines ran before disassembly. I will buy new reeds,non metal.
The engines: 1 merc blueband 850, 85hp, 4cyl, 2 carb, dist/mag ignition
2 merc blueband 650, 65hp, 3 cyl, 2 carb, dist/mag ignition
3 merc mid 80s clamshell cover 70 hp, 3 cyl, 2 carbs, cdi ignition
Also merc 40hp 2cyl, 1 carb, cdi ign, short shaft. mount the 70 hp powerhead on the 40hp short shaft, when engine done that's my plan.
Here is where you experts come in..
1 All the pistons are the same except 2 or 3 rings? any preference? reasons?
2 INTAKE?Front half of case where to grind or add? side intake plates mods? pics?
3 Type of reeds, pedals? the 85 had 10 pedals 2x5,70 hp 4 pedals 2x2 alum holder, 650 4pedals 2x2 but steel holder, any pics of mods to these types? where to grind to "smooth" air flow? Which one to use with 3 cyl block? 4 or 10 pedal?
4 cylinder porting? where? intake? exhaust? approx. how much? copy of original cyl port map? pics of stock cyl walls with some guides to where and how much to grind or add?
5 exhaust tuner, tuner tube, remove tuner tube? keep just short tuner that's attached to plate under powerhead? Mods to that area?pics?
DRY Stacking? how to on small engines? where to cut,size?
6 Carbs? I know this depends on porting and mods but any general rule of thumbs specific to 3 cyl?
I have many more questions but some clarification in these areas would be greatly appreciated.
Please pics of any mods is worth a thousand words...
Thanks all,
Mark

[Fastjeff57]

I can help you with a few of these. The older type triple PH can be mounted on the "40 hp" short tower with some simple mods. Let me know if you want to do it.

This same power head makes excellent power with external reeds. I've done a couple of these mow, using Yamaha and OMC reeds, and they work well. Again, let me know if you're interested and I'll supply details.

The Merc 850 PHs came with two different exhaust arrangements: the ADI has three big ports that make good power. The earlier ones used 4 small ports and anemic timing that make good torque. Which 850 are you working with?

Jeff

[baldad45]

Can you post more pics and details of the intake setup of the 650's in the pics above ? Thanks Glen

[Fastjeff57]

Do you mean the 4 cylinder 850, or the homemade triple carbed Merc 700?

Jeff

[baldad45]

The homemade triple carbed Merc 700.Thanks Glen

[Fastjeff57]

Carbs are stock Merc triple--three of them instead of two.

Reeds are stock OMC triple stuff.

Construction is a welded steel box bolted and epoxied to the crankcase front--no welding (or warpage). I use in-line six main bearing blocks (instead of the stock triple reed blocks), and these bolt the steel box to the crank front--I'd never trust the epoxy alone.

A bunch of filing and weld filling is required to get the mounting flange flat (since I lack a machine shop). Multiple layout fluid applications mark the high spots for filing.

The carb mounting plate is a sandwich of 1/8 " aluminum plates with body filler between (for thickness). Nuts between the plates hold the carbs on (and provide equal spacing during the body filler addition).

The rest is just busy work; carb linkage, fuel runs, etc.

Jeff

[baldad45]

How did this version run compared to the two carb version you built 1st ?

[Fastjeff57]

Not sure yet--long story--but the first, two carb external reeds motor is a gem! Made at least another 10 to 20 hp and reved a happily thousand rpms higher (while also having better mid-range torque). Idle was not all that great, however; the three carbed one does idle better.

Jeff