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Author Topic:   Mercury 115-HP 2+2

Capt_Tidy posted 07-22-2009 05:26 PM ET (US)
Anybody have comments on the Mercury 115-HP [with the displacement on demand or 2+2 arrangement of cylinders]? From what I understand these shut down to 2 cylinders at idle and then kick back to 4 when throttled up?
Sounds complicated - Comments.
JMARTIN posted 07-22-2009 07:08 PM ET (US)
My buddy has a 100 hp Merc which supposedly does the 2 to 4 thing. It does idle smooth, and runs smooth, but there is a noticeable transition when powering up from idle to plane. It is not as noticeable when powering down. John
L H G posted 07-22-2009 07:16 PM ET (US)
I understands it's remarkably simple. The carburetors on two of the cylinders have no low speed jets, so no fuel is delivered to those cylinders below 1800 RPM.
JMARTIN posted 07-22-2009 07:57 PM ET (US)
So, do they still get oil?
John
jimh posted 07-22-2009 10:36 PM ET (US)
In this day an age of advanced motors, these Mercury 115-HP motors are a throwback to the stone age of outboards. When running on only two cylinders they are not particularly smooth at idle speeds. As you advance the throttle the other two cylinders kick in and BANG, off you go with suddenly double the power.
I think it is somewhat telling that Mercury only used this technique on one motor, and essentially no other manufacturers followed suit.
If you want a throw-back motor, the Mercury 115-HP 2+2 is a good one.
I have also heard that getting the carburetors set up properly can be a problem. The two cylinders that shut off at low speed don't have a fuel mixture that is rich enough to ignite.
Tom W Clark posted 07-22-2009 11:04 PM ET (US)
I thought it was only the Mercury four cylinder 100 HP and 125 HP that operated this way. Do all the four cylinder 115s operate this way too?
Clark Roberts posted 07-23-2009 07:18 AM ET (US)
John, yes those two cyls get oil via a small orifice. Never heard of anyone having big problems with idling etc. on these motors...in fact most I have seen seem to idle quite well. As to being "throw back" motors ... I just don't get it! Why TRASH a motor for being very simple in design? Well, I guess I'm the "throw back" but can live with it!Happy Whalin'... Clark... Spruce Creek Navy
jimh posted 07-23-2009 09:39 AM ET (US)
Moore's Law says the cost of transistors is decreasing very rapidly. There is a corollary which says as the cost of transistors becomes lower, you ought to use more of them in your product. I think a Mercury 115-HP 2+2 motor probably has about three transistors in the whole motor. When that motor was designed, those transistors probably cost about $10. A Mercury VERADO has several million transistors, and they probably cost about $10, too. In designing a motor for 2010, you can't really ignore the benefits of cheap transistors.
I don't find "throwback" to be a trash term. It means from an earlier epoch. The Mercury 2+2 motor is from an earlier epoch of outboard motors. It is a low-technology motor, and a high-emission motor. I don't think Mercury even makes them any more, and if they did, they probably could not sell them unless the sales volume was a tiny fraction of their total product mix so as not to affect their overall emission limits.
A good friend of mine has one of these 115-HP 2+2 motors. When it was running, he described the transition point using the term "rocket launch." The motor is on its second or third stator, and currently is out of commission due to some other fatal flaw.
Another Boston Whaler owner that I bumped into on a dock somewhere--so you can put this into the category of "dock talk"--told me he had a 115-HP 2+2 on a MONTAUK. The said adjustment of the carburetors was a frequent maintenance procedure.
I think the Mercury 115-HP 2+2 has plenty of power. That's probably why the need for reducing to two cylinders at idle. They probably could not tame down the motor enough to get it to run at idle speed on all four cylinders.
Peter posted 07-23-2009 10:11 AM ET (US)
Hey Larry, did Yamaha "copy" this feature on its 3.1L Ox66 EFI V6 motors that idled on 5 cylinders, ran on 4 cylinders to 2000 RPM and then 6 above 2000 RPM?
Jeff posted 07-23-2009 10:53 AM ET (US)
Well Mercury must have copied it from Cadillac who first introduced this type of cylinder shut down in their L62 motor for 1981. The motor was built to help met CAFE standards and would run on 8 / 6 / 4 cylinders depending on the load.
There is a conundrum. Mercury ripping off Cadillac?
Peter posted 07-23-2009 11:00 AM ET (US)
Forgot about that engineering marvel.
Capt_Tidy posted 07-23-2009 07:01 PM ET (US)
My local mechanic suggests that he remove the controllers and simply re-jet all carbs resulting in a pretty solid but older engines.
If I purchase the boat, I will report the results of re-jetting. The 115s are currently on an older 25 outrage which just needs more hp to really get up and out. I plan to up moving the engines onto a couple of 17s and repower the outrage with twin 150s or 175s. A 115 would be nice on a vintage 17 or even as twins on a 19/21 outrage. They are a good looking compact outboard. Again - if I purchase the boat. Lots of choices out there right now.
Cheers
Ian
jimh posted 07-23-2009 08:05 PM ET (US)

quote:
"My local mechanic suggests that he remove the controllers...."
Please elaborate on the controllers. What are they?
L H G posted 07-23-2009 08:38 PM ET (US)
Clark - I think we should start an elite new club around here at CW.
"THE DINOSAUR CLASSIC WHALER OWNERS CLUB"
Membership would be limited to those who don't own Evinrude E-tecs, with special honors to those who own "throwback" Mercury 2-strokes. 4-stroke and Verado owners welcome, of course.
jimh posted 07-23-2009 10:57 PM ET (US)
Where does that leave the classic Boston Whaler with OptiMax owner?
L H G posted 07-23-2009 11:18 PM ET (US)
If I understand it correctly there are none!
Capt_Tidy posted 07-23-2009 11:53 PM ET (US)
Do mean a club for those who don't own an engine that has not been rebuilt?
As for as the control units - I was lead to believe they are electronic - simply unplug?
Jeff posted 07-23-2009 11:58 PM ET (US)
Larry, Jim McPadden has the 21 dump truck with a 150 optimax. There is one.
jimh posted 07-24-2009 09:22 AM ET (US)

quote:
"As for as the control units - I was lead to believe they are electronic - simply unplug?
I am more confused than ever. Are you saying electronic control units are involved with the displacement on demand function? This is completely new to me. All prior discussion about the displacement on demand function has indicated it was accomplished in the most simplistic and non-electronic manner possible: the omission of low-speed fuel jets on two carburetors.
Please elaborate on these electronic control units that are going to be removed from the Mercury 115-HP 2+2 engine to revert it to operation on all four cylinders. Thank you.
Tohsgib posted 07-24-2009 10:18 AM ET (US)
I wish they did have a dinosaur club here...then we could have decent conversations without people who never owned a modern engine jumping in and telling us we are crazy, liars, foolish, overweight, higher maintenece, etc. Nirvana it would be.
bill705 posted 07-24-2009 01:03 PM ET (US)
I've had a 1991 100hp 2+2 on a 1968 Nauset since 1991 and haven't had the rough idle or transition problems with it.
If you get the link and sync set up right and use the NKG plugs instead of the surface gap plugs as per the service manual, it does fine. Cranks right up with no throttle advance and idles smooth about 750.
As far as re-jetting, the two lower carbs have different part numbers and may not allow an idle jet to be installed.
That plus early on there was a caution of some kind about crankshaft breakage if it was tried. (don't remember where I heard that but I do remember it)
The only controls that I know of are the throttle plates on the two lower carbs, when they open far enough to start sucking gas.
Bill
jimh posted 07-24-2009 07:46 PM ET (US)
I am somewhat surprised at the reaction to my assessment of the Mercury 115 2=2 motor. You'd think I was criticizing Dick Fisher and the classic Whaler 13-footer. The motor does have a lot of raw power, but it was not Mercury's finest hour, and pointing this out should not really be taken as some sort of radical opinion.

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Author	Topic: Mercury 115-HP 2+2
Capt_Tidy	posted 07-22-2009 05:26 PM ET (US) Anybody have comments on the Mercury 115-HP [with the displacement on demand or 2+2 arrangement of cylinders]? From what I understand these shut down to 2 cylinders at idle and then kick back to 4 when throttled up? Sounds complicated - Comments.
JMARTIN	posted 07-22-2009 07:08 PM ET (US) My buddy has a 100 hp Merc which supposedly does the 2 to 4 thing. It does idle smooth, and runs smooth, but there is a noticeable transition when powering up from idle to plane. It is not as noticeable when powering down. John
L H G	posted 07-22-2009 07:16 PM ET (US) I understands it's remarkably simple. The carburetors on two of the cylinders have no low speed jets, so no fuel is delivered to those cylinders below 1800 RPM.
JMARTIN	posted 07-22-2009 07:57 PM ET (US) So, do they still get oil? John
jimh	posted 07-22-2009 10:36 PM ET (US) In this day an age of advanced motors, these Mercury 115-HP motors are a throwback to the stone age of outboards. When running on only two cylinders they are not particularly smooth at idle speeds. As you advance the throttle the other two cylinders kick in and BANG, off you go with suddenly double the power. I think it is somewhat telling that Mercury only used this technique on one motor, and essentially no other manufacturers followed suit. If you want a throw-back motor, the Mercury 115-HP 2+2 is a good one. I have also heard that getting the carburetors set up properly can be a problem. The two cylinders that shut off at low speed don't have a fuel mixture that is rich enough to ignite.
Tom W Clark	posted 07-22-2009 11:04 PM ET (US) I thought it was only the Mercury four cylinder 100 HP and 125 HP that operated this way. Do all the four cylinder 115s operate this way too?
Clark Roberts	posted 07-23-2009 07:18 AM ET (US) John, yes those two cyls get oil via a small orifice. Never heard of anyone having big problems with idling etc. on these motors...in fact most I have seen seem to idle quite well. As to being "throw back" motors ... I just don't get it! Why TRASH a motor for being very simple in design? Well, I guess I'm the "throw back" but can live with it!Happy Whalin'... Clark... Spruce Creek Navy
jimh	posted 07-23-2009 09:39 AM ET (US) Moore's Law says the cost of transistors is decreasing very rapidly. There is a corollary which says as the cost of transistors becomes lower, you ought to use more of them in your product. I think a Mercury 115-HP 2+2 motor probably has about three transistors in the whole motor. When that motor was designed, those transistors probably cost about $10. A Mercury VERADO has several million transistors, and they probably cost about $10, too. In designing a motor for 2010, you can't really ignore the benefits of cheap transistors. I don't find "throwback" to be a trash term. It means from an earlier epoch. The Mercury 2+2 motor is from an earlier epoch of outboard motors. It is a low-technology motor, and a high-emission motor. I don't think Mercury even makes them any more, and if they did, they probably could not sell them unless the sales volume was a tiny fraction of their total product mix so as not to affect their overall emission limits. A good friend of mine has one of these 115-HP 2+2 motors. When it was running, he described the transition point using the term "rocket launch." The motor is on its second or third stator, and currently is out of commission due to some other fatal flaw. Another Boston Whaler owner that I bumped into on a dock somewhere--so you can put this into the category of "dock talk"--told me he had a 115-HP 2+2 on a MONTAUK. The said adjustment of the carburetors was a frequent maintenance procedure. I think the Mercury 115-HP 2+2 has plenty of power. That's probably why the need for reducing to two cylinders at idle. They probably could not tame down the motor enough to get it to run at idle speed on all four cylinders.
Peter	posted 07-23-2009 10:11 AM ET (US) Hey Larry, did Yamaha "copy" this feature on its 3.1L Ox66 EFI V6 motors that idled on 5 cylinders, ran on 4 cylinders to 2000 RPM and then 6 above 2000 RPM?
Jeff	posted 07-23-2009 10:53 AM ET (US) Well Mercury must have copied it from Cadillac who first introduced this type of cylinder shut down in their L62 motor for 1981. The motor was built to help met CAFE standards and would run on 8 / 6 / 4 cylinders depending on the load. There is a conundrum. Mercury ripping off Cadillac?
Peter	posted 07-23-2009 11:00 AM ET (US) Forgot about that engineering marvel.
Capt_Tidy	posted 07-23-2009 07:01 PM ET (US) My local mechanic suggests that he remove the controllers and simply re-jet all carbs resulting in a pretty solid but older engines. If I purchase the boat, I will report the results of re-jetting. The 115s are currently on an older 25 outrage which just needs more hp to really get up and out. I plan to up moving the engines onto a couple of 17s and repower the outrage with twin 150s or 175s. A 115 would be nice on a vintage 17 or even as twins on a 19/21 outrage. They are a good looking compact outboard. Again - if I purchase the boat. Lots of choices out there right now. Cheers Ian
jimh	posted 07-23-2009 08:05 PM ET (US) quote: "My local mechanic suggests that he remove the controllers...." Please elaborate on the controllers. What are they?
L H G	posted 07-23-2009 08:38 PM ET (US) Clark - I think we should start an elite new club around here at CW. "THE DINOSAUR CLASSIC WHALER OWNERS CLUB" Membership would be limited to those who don't own Evinrude E-tecs, with special honors to those who own "throwback" Mercury 2-strokes. 4-stroke and Verado owners welcome, of course.
jimh	posted 07-23-2009 10:57 PM ET (US) Where does that leave the classic Boston Whaler with OptiMax owner?
L H G	posted 07-23-2009 11:18 PM ET (US) If I understand it correctly there are none!
Capt_Tidy	posted 07-23-2009 11:53 PM ET (US) Do mean a club for those who don't own an engine that has not been rebuilt? As for as the control units - I was lead to believe they are electronic - simply unplug?
Jeff	posted 07-23-2009 11:58 PM ET (US) Larry, Jim McPadden has the 21 dump truck with a 150 optimax. There is one.
jimh	posted 07-24-2009 09:22 AM ET (US) quote: "As for as the control units - I was lead to believe they are electronic - simply unplug? I am more confused than ever. Are you saying electronic control units are involved with the displacement on demand function? This is completely new to me. All prior discussion about the displacement on demand function has indicated it was accomplished in the most simplistic and non-electronic manner possible: the omission of low-speed fuel jets on two carburetors. Please elaborate on these electronic control units that are going to be removed from the Mercury 115-HP 2+2 engine to revert it to operation on all four cylinders. Thank you.
Tohsgib	posted 07-24-2009 10:18 AM ET (US) I wish they did have a dinosaur club here...then we could have decent conversations without people who never owned a modern engine jumping in and telling us we are crazy, liars, foolish, overweight, higher maintenece, etc. Nirvana it would be.
bill705	posted 07-24-2009 01:03 PM ET (US) I've had a 1991 100hp 2+2 on a 1968 Nauset since 1991 and haven't had the rough idle or transition problems with it. If you get the link and sync set up right and use the NKG plugs instead of the surface gap plugs as per the service manual, it does fine. Cranks right up with no throttle advance and idles smooth about 750. As far as re-jetting, the two lower carbs have different part numbers and may not allow an idle jet to be installed. That plus early on there was a caution of some kind about crankshaft breakage if it was tried. (don't remember where I heard that but I do remember it) The only controls that I know of are the throttle plates on the two lower carbs, when they open far enough to start sucking gas. Bill
jimh	posted 07-24-2009 07:46 PM ET (US) I am somewhat surprised at the reaction to my assessment of the Mercury 115 2=2 motor. You'd think I was criticizing Dick Fisher and the classic Whaler 13-footer. The motor does have a lot of raw power, but it was not Mercury's finest hour, and pointing this out should not really be taken as some sort of radical opinion.