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Author Topic:   Disable cylinder cut-out on Merc 115
Plotman posted 09-05-2003 08:52 AM ET (US)   Profile for Plotman   Send Email to Plotman  
I have been told that the low speed cylinder cut-out on ther merc 115s is a mechanical system - is this something that can be disabled?
Clark Roberts posted 09-05-2003 09:07 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Plot, here's how it works: two of the four carbs have no idle jets (but have small orifices for oiling purposes)therefore when speed (read air flow through carb venturis) lowers to point where high speed jets no longer pick up fuel, that cyl just cuts out. At idle it is a two cyl engine and when speed increases to point where high speed jets take over the engine runs as a four cyl. Simple system and seems as though you could replace the carbs which have no idle jets or add idle jets to existing carbs... These engines idle on timing only so some linkage adjustments may be necessary... A look at the parts breakdown will give you the answer... Happy Whalin'... Clark.. Spruce Creek Navy
lhg posted 09-05-2003 01:48 PM ET (US)     Profile for lhg    
Clark - would you be willing to give us a comparison, and your opinion, on overall performance and power output, gas mileage, etc between this 4 cylinder 115 and the 3 cylinder 90? I know that the 115 weighs about 55#'s more.
Clark Roberts posted 09-06-2003 10:46 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Larry, since I tend to be very long-winded, I will try to be brief and following are my impressions from actual experience with several classic 17's with 84 cubic inch Merc 90's and 106 cubic inch 100's and a 115 (same block as 100) ... engines are same except that 90 is 3 cyl and has a 2.33:1 lower unit vs. a 2.06:1 ratio for the four cyl models. So not apples to apples really... Here are my opinions/impressions... the 90 will outperform the 100/115 in acceleration and top speed and get better mileage. I have run them against each other and the 90 will pull away every time... now these were not test boats but my 17 newport with 20" hi-five against a Montauk with 100hp four cyl. The 115 was on a 17' (RAnger I think) bass/ski boat and same results... In any case there was no performance advantage to the 100/115. Once I tangled with an 18' CC (SeaFox) with a 125hp four cyl Merc and it was decidedly faster than my 90... Interesting that the max WOT RPM for the 90 is 5500 and for the others (including the 75hp 3 cyl version) is 5250rpm. So the extra rpm sort of cancels out the lower unit ratio differences... As to fuel efficiency I think Mercs data says about 11 gals/hr at WOT for the four cyls and 8 gals/hr for the 3 cyl 75/90hp engines. Like I said, the above is emperical and others may observe different results????? I have a hunch that the exhaust tuning on the 90hp is very, very good (only difference in 75 and 90 is exhaust tuning and timing curve I believe... maybe reed lift is less in the 75... not sure). A three cyl loop charged 2 stroke lends itself to some very good harmonics and you can really feel/hear it when it gets on the "pipe" (a little motor cyle jargon,, heh, heh). Hey, I remember the first 3 cyl looper, the 1958/59 Evinrude Triumph 55hp, and how it scaulded everybody with that little 49 cubic incher... showing my age now! blah, blah, blah.. happy Whalin'.. Clark. \ Spruce Creek Navy
Peter posted 09-06-2003 09:11 PM ET (US)     Profile for Peter  Send Email to Peter     
Hey Clark...that would be the 1968/69 Triumph 55 with, as I recall, a light olive green paint job.

I've recently become acquainted with what I understand is 3 cylinder loop charged harmonics with a 70 hp Yamaha on a 15 Super Sport. It seems to get off the "pipe" at about 4500 rpm where it sounds horrible. A couple of hundred rpm above or below and everything sounds fine although above 4500 it sounds like the engine has kicked into "warp" drive. First few times I heard it I thought there was something wrong with the motor. With the 2.33 gears turning a 19" propeller, that engine winds up to the 6000 rpm redline almost instantly. Everytime I run this motor, I'm very impressed with its performance. It certainly makes the 15' hull dance.

Back to the original topic...why does Mercury disable the cylinders on this model? Low speed fuel economy, improved emissions? I believe that the Yamaha EFI's do this to improve emissions. Is this a feature that it has always had?

Clark Roberts posted 09-07-2003 07:45 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Peter, you are correct...1968/69! And I have no idea why Merc disables the cyls...maybe someone out there knows the answer. I know what you mean about the "sweet spots" when you crank on the power to the 70 yamaha and I am a fan of 3 cyl loopers as they seem to perform very well and get fantastic fuel mileage for a 2S. I just put a 60hp (51 cubic inch) Merc on my Montauk and it is a great combo for cruising... runs out at about 37mph at WOT and gets over 7mpg at 3800-4400 steady rpm. Exhaust melody hits at 3800, 4400, and 5000rpm and sounds like a bugle on a "C" note. Happy Whalin'... Clark.... SCN
dgp posted 09-07-2003 08:45 AM ET (US)     Profile for dgp  Send Email to dgp     
...and sounds like a bugle on a "C" note. Happy Whalin'... Clark.... SCN

Hey jimh, you oughtta chronicle all of Clark's advise and analogies onto a special page of the Reference section.

jimh posted 09-07-2003 10:39 PM ET (US)     Profile for jimh  Send Email to jimh     
A couple of weeks ago I was wandering around the docks at Friday Harbor, WA. I was still on Eastern Time, and the rest of the gang were already on Pacific time and still sound asleep. I struck up a conversation with another early riser who was admiring the two Boston Whaler classics were were cruising on.

The fellow had a Montauk and had one of the first of the 4-cylinder 115/125 HP Mercury outboards on it, the first year they were available.

To make a long story shorter, he said the engine was rough and eventually it actually caught itself on fire when the tilt switch on the cowling shorted and caused the tilt pump to come on, over heat, and start some smoke and fire under the cowling. Mercury gave him a new engine.

According to his experience, the tuning of the carbs was critical, and if not set up correctly the engine would not run smoothly as it changed from 2-cylinder operation to full 4-cylinder running.

Also, the low limit on the crankcase speed for the 4-cylinder (5250) has been reported by some to be needed because of some resonance in the 4-cylinder engine that gets hasty above that speed. The advice I heard, which is third or fourth hand now, was don't over rev that engine.

Also, with regard to the Yamaha 70-HP 3-cylinder, my pair start to honk with some crazy sound or resonance at 4100-RPM. At 4000 or 4200 it is gone, but at 4100 they sound like they are coming off the setback brackets. Of course, the engine seem like they always want to settle in and run at that speed! What's a fella to do?

I guess from the sound of this discussion I should move up to a pair of 90-HP Mercury outboards.

Clark Roberts posted 09-08-2003 07:44 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Jim, retighten all engine/transom mounting bolts and check the engines' shock mounts upper and lower. A bad shock mount (these are glued rubber pucks) could possibly be the cause... just wild guesses here but worth a checking into! Is that a dangling participle or what? BTW, you're the man and thanks for providing us all with this web site and putting up with our ramblings... Happy Revengin'... Clark... Spruce Creek Navy
lhg posted 09-08-2003 04:25 PM ET (US)     Profile for lhg    
Clark, thanks for the very valuable information, as usual.
If I am understanding you properly, it sounds like the Merc 90 is the way to go, even if you're in the 115 Hp market, or to step up to the 4 cylinder 125, which also sounds like a good performer. I know that the 3 cylinder 90 is basically half of the Mercury 2.5 liter V-6, so it makes sense that it would be "running machine".

We have speculated for some time why the new Montauk 170 came out with the 90 HP rating, instead of the 115, and your experience seems to give the answer. Same performance output, less money, greater economy, tougher for a competition 90 to power the boat as well.

One last question. JimH here, owner of Yamaha 70's, and I have speculated on how the 52 cubic inch, 3 cylinder Yamaha 70 measures up to the 59 cubic inch, 3 cylinder Mercury 60. (Talking 2-strokes here). Since you have experience with both, it would be interesting to hear your results. From your description above on your new engine, it would sound like the Merc 60 will at least keep up with the Yamaha 70? One would even expect the larger "cubed" Merc 60 to run faster?

Clark Roberts posted 09-08-2003 06:36 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Larry, I think the 90 Merc is a match for the 115 if set up properly of course. The 115 uses some horses just to carry the extra weight and the higher rev limit on the 90 means more rpm throughout the range. BUT, these are just my observations... and the 125 really lights up. Now I have two of the 51 cubic inch Merc 60's and the best thing is its quietness! Almost silent except for the tone of the exhaust tuning... this quietness is due in part to the one-piece block (cast as one piece) with no gaskets at head gasket, no water jacket gasket and no exhaust baffle gasket... this is a remarkable engine!!! I am looking for a 59 cubic inch version for my Montauk even though it runs great with the 51cubic inch version (change was made from 51 to 59 cubic inches in 1998 models). That's almost a 20% increase in power and still weighs in at about 205lbs. This is an engine that is so simple and quiet in operation and I hope they never quit producing it... but alas... you know the drill... no more 2.5L EFI 200. As to performance of the 60, it will hold its own easily with other similar displacement engines... well, you can see that my mind is made up and I will qualify my comments as "opinionated" except the comments about the one piece block, etc.. This puppy can be overheated with no resulting problems like warped head, leaky head gasket, water intrusion..etc...Fantastic engine. ooops there goes that opinion again.. Happy Whalin'... "Opinionated but Lovable"... Clark.. SCN
PS>As to 60Merc vs 70Yam I can't say for sure but the 70Y readlines a 6000rpm and has the Merc 2.33:1 lower unit while the 60 Merc readlines at 5500rpm through a 1.64:1 lower unit. Do the math and the Merc is spinning the prop at 3353rpm at max rpm while the Yam is spinning prop at only 2575rpm. The Merc lower unit case is slimmer and smaller. The 59 cubic inch Merc with the 1.64:1 lower unit can deliver some quite respectable/exceptional performance!
lhg posted 09-08-2003 08:39 PM ET (US)     Profile for lhg    
Thanks, again Clark. That Merc 60 2-stroke is the standard engine on the new 150 Sport, I believe. Sounds nice, and you are the first person here who has much experience with it. Sounds like you prefer the regular gearcase to the Big Foot.

Regarding the 200 EFI, I was under the impression that the 3.0 liter version was only for the 20" shaft, red decals, basically the bass boat market, or maybe a notched transom 25 Outrage with twins. That has got to be a hot engine for 200HP, since Mercury also gets the 300 HP "Darth Vader" engine out of that block.

I was under the impression that the Counter rotating 25" Saltwater versions, 25" shaft, were still using the 2.5 liter block. Any truth to this?

I've got to say that it really bugs me that all of these low(er) cost 2-strokes, practically trouble free ands Highly refined, are bing forced out of the market place, while the big diesel powered boats and trucks continue to pollute the air with black smoke at will. I'm sure their time will come, however.

andygere posted 09-08-2003 08:43 PM ET (US)     Profile for andygere  Send Email to andygere     
The 3cyl Merc 90 sounds pretty good when stacked up against the 115, so I gotta ask this question: Would a pair of them, in your estimation, be a viable set up on an Outrage 22? Do you think that motor provides enough power to plane a "lightly loaded" 22 hull? You know where I'm going here...
Clark Roberts posted 09-09-2003 06:31 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Larry, don't know about the 25" 200EFI! Surely hope they still make the 2.5L... Andy, easy way to figure it out... Take square root of HP divided by gross weight and multiply by hull factor of 180 and this should give you the theoretical top speed assumming clean hull, proper trim/prop etc... example: if gross weight is 4000lbs take square root of 180(hp) divided by 4000=square root of .045 = .212... then .212 X 180 equals 38mph. So if I'm high/low on my weight WAG then speed will be higher/lower... seems plenty of power unless gross weight is much higher than my WAG (wild-ass-guess). Using 4K lbs. and plugging in a single 90hp results in estimated 27mph on one engine (provided that other engine is tilted clear of water and change to ideal prop)... maybe someone out there has real world experience with this set-up and can verify or debunk above! Happy Whalin'...
George Harkness posted 09-09-2003 04:55 PM ET (US)     Profile for George Harkness  Send Email to George Harkness     
For Larry and Clark - I'm back after an extended absence - glad to see you're both still here. Found this thread very interesting as we're looking at what's available in the 90-115 hp range for a flats boat project. Larry - when you can, can you update your e-mail address for me. Thx . George
bsmotril posted 09-09-2003 10:24 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
The emission restrictions for off road large diesel engines goes into affect next year. Caterpillar GE and GM Electomotive all have new technology engines running on testbeds or prototypes that greatly reduce emissions output. I have family that works in the fuel system engineering group at Caterpillar and there is some really neat technology that is going to start making it's way into the big engines. Let's just say that you'll see a lot more computers and electo-hydraulics, and a lot fewer camshafts in the future.
KDW posted 09-14-2003 06:17 PM ET (US)     Profile for KDW  Send Email to KDW     
Clark Roberts,

Do you have a performance opinion on the Mercury 2 stroke 90HP 3 cylinder vs. the Yamaha 2 stroke 90HP?......or are they practically the same motor?


Clark Roberts posted 09-15-2003 08:08 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Kenny, I assume you mean the Yamaha 3 cyl 90! These are two very different engines. The Merc is 85
cubic inches/Yamaha is 71; Merc weighs 305lbs/Yamaha 265(I think); Both share same Merc lower unit (2.33:1) and can share props although the Yamaha unit has hole for cotter pin and Merc doesn't. I have owned several of each and find that the Merc is a stronger engine throughout the rpm range but the Yamaha is a great engine for performance also and seems to get a tad better fuel economy as expected. Both are great engines and since Yamaha has changed to the Mercury paint system corrosion resistance is same. BTW both engines are sensitive to the idle/air ratio adjustments (one for each of the 3 carbs) and the Merc idles on timing only while the Yamaha has adjustment via single screw to carb linkage. It is not uncommon on both to have low idle roughness and some caughing and sneezing but when properly fussed over and adjusted (in water/in fwd gear/tied to dock) will idle steady and very, very smoothly. These are two great engines IMHO! Happy Whalin'... Clark.. Spruce Creek Navy
KDW posted 09-15-2003 01:40 PM ET (US)     Profile for KDW  Send Email to KDW     
Thanks Clark Roberts.

You are correct in that I mean't the Yamaha 90HP 3 cylinder engine.

Thanks so much for the information. It really helps my decision-making process.


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