'84 Merc 115 options

[procraft89]

We have an '89 ProCraft fish & ski that originally came with a 115 Mariner. When that motor blew, it was much more economical to buy a used boat and have the motor transplanted rather than buy a new motor. Now this boat has a '84 Mercury 115 I6.

From what we have been told, there was a change in the way hp was rated between the years, in '84 it was done at the flywheel and in '89 it was rated at the prop.

This motor is having some carb issues, trouble coming out of the hole without feathering the throttle but is ok once it gets on plane it seems to run good but isnt turning more than about 4200 rpm. Oddly enough when I first put it on the lake the other day I was able to get it up to about 5200. The lake was much calmer so it is possible I trimmed the motor out more to get the rpms. In addition, it will NOT push the boat on the trailer and fuel is obviously leaking out of at least 1 carb.....so they need rebuilt.

Since we are going to be into anyways we have decided this would be a good time to look at upping the power of the motor. Is it true that the block on this motor is the same as 125, 140, or higher? We have heard that with some basic changes we can effectively increase the hp.

Does anyone know what changes would need to be done? Is it just jetting or do we need new carbs? Does this motor have reed valves?

I am a pretty competent 2 stroke mechanic, my son races motocross so tearing into carbs and motors doesnt scare me at all as I do it all the time.

Thank you very much in advance.

Ed.

 

[Chris1956]

Re: '84 Merc 115 options

Ed, The main difference between the HP of the inline sixes is the port timing. Some of the blocks have port timing that allows the use of a power port piston.

You need to remember that after 1978, the inline six was never rated higher than 115HP. The 125-150 HP Inlines sixes were older than that.

I would think that converting your block would be cost prohibitive, and the gain realized is small/none. Why not pick up a used V6 engine. I grabbed a '93 135HPV6 for $1K, last fall.

BTW - You need to give your carbs some attention so that motor will run and accelerate smoothly.

 

[procraft89]

Re: '84 Merc 115 options

I am really not intersted in doing any machine work on this motor, i do have a friend that is a machinest and could probably make this motor sing with a little porting and polishing but I really dont want to take the whole motor apart.

so outside of a carb rebuild and a good solid tuneup there really isnt much we can do with it? Are there aftermarket reeds available that could help? My son is sponsored by Boyesen and although marine applications are not listed with our sponsorship i could probably get our discount by calling them.

honestly, this boat only sees water a handful of times per year...it is almost 20 yrs old and with the exception of a few battle wounds it looks almost new. but when we do run it, we run the snot out of it. so as little as it is used replacing the motor isnt really much of an option....however, a prop change might be considered.

i have read on here about drilling the prop? what is involved in that? please pardon my ignorance, i have been around boats most of my life but other than carb rebuilds i havent done much wrenching on them and havent done any modding.

 

[jimmbo]

Re: '84 Merc 115 options

Ed, The main difference between the HP of the inline sixes is the port timing. Some of the blocks have port timing that allows the use of a power port piston.

You need to remember that after 1978, the inline six was never rated higher than 115HP. The 125-150 HP Inlines sixes were older than that.

Other internal difference are the number of reeds, and exhaust tuning

As for ratings over 115hp.... the 140 was made until the 1982 miodel year when merc started prop rating these engines.

Procraft... for all intensive porposes your 115 is almost identical to the 1981 140hp. The pistons in the 1984 are what are called "low dome" and have slightly lower compression.

I had the same engine for 14 yrs. I still think it was the best outboard I ever owned. It did do the bog thing in hot weather. careful adjustment of the carb linkage and idle mix went a long way to fixing it. Drilling the prop will help by unloading the engine.

 

[procraft89]

Re: '84 Merc 115 options

Other internal difference are the number of reeds, and exhaust tuning

As for ratings over 115hp.... the 140 was made until the 1982 miodel year when merc started prop rating these engines.

Procraft... for all intensive porposes your 115 is almost identical to the 1981 140hp. The pistons in the 1984 are what are called "low dome" and have slightly lower compression.

I had the same engine for 14 yrs. I still think it was the best outboard I ever owned. It did do the bog thing in hot weather. careful adjustment of the carb linkage and idle mix went a long way to fixing it. Drilling the prop will help by unloading the engine.

what is meant by "drilling the prop"

so it does have reeds.....are performance reeds available or are they worth messing with?

 

[jimmbo]

Re: '84 Merc 115 options

what is meant by "drilling the prop"

so it does have reeds.....are performance reeds available or are they worth messing with?

drilling the prop refers to the drilling of a hole on the low pressure side of the prop blade just behind the leading edge of the blade. The purpose is allow a controled amont of exhaust gas to temporairly permit the blades to ventilate thereby unloading the engine which will allow it to accellerate faster. Once the boat is moving forward the water pushing past closes off the vents.

The reed valves on an inline six are buried very deep in the engine. They are located on the crankcase bearing supportd/blocks. To get to them requires pretty much a complete teardown. Performance reeds are usually made of thin fibreglass which have short lifecycle, so they aren't a good choice

 

[procraft89]

Re: '84 Merc 115 options

do you have a picture of where to place the holes? sorry for my ignorance

 

[jimmbo]

Re: '84 Merc 115 options

http://img.photobucket.com/albums/v235/perrycl/ventingprops.gif

 

[emckelvy]

Re: '84 Merc 115 options

Drilling "ventilation holes" in the prop certainly does help improve hole shot. I used to do this with my old ski boats and you could really tell the difference when pulling heavy loads like a deep-water-start slalom skier.

What the picture doesn't show is a size for the ventilation holes. Don't Drill Too Large!

Best to start out small, 3/16" holes will produce a noticeable result. If you try those and think you need to go larger, try 1/4". Don't go larger than 5/16" or the slippage may become uncontrollable, to the extent that the prop slips all the time like a bad automatic transmission.

Just drill 'er out to 3/16" and take your cordless drill with you next time you go out. You can increase the size of the holes at the beach or dock if need be.

Note: the correct prop size is determined by W.O.T. rpm with a light load in the boat (just the driver) and would include trimming the boat out for maximum performance. It sounds like your boat is overpropped since this motor with a light load on a trimmed-out boat should run 5500 rpm.

A prop with 2" less pitch, or a performance stainless prop, will probably spin 'er up into the power band. Not good to lug these motors. BTW, many stainless props come with ventilation holes (or even slots) from the factory.

HTH & keep us posted......ed

 

[procraft89]

Re: '84 Merc 115 options

its got a stainless prop on it now, so i will look for the holes in it.

we also have the stainless prop off the original motor, are these things worth anything used? if so, what would my chances be for trading 2 for 1?