2003 Merc 115hp 2cyl to 4cyl feature

[Boatin Bob]

My Merc is carbed and it has a feature that starts the motor off running in 2 cyl mode until you hit around 1800 RPM and then it switches to 4 cyl, supposed to make it more fuel efficient when running slow. Does anyone know if you can disable this feature so it runs in 4 cyl mode full time? We use it mostly for skiing and wakeboarding and could use the extra torque at take off. Engine has lots of power and when the other 2 cyl kick in it tries to rip your arms off. Thanks

 

[quantumleap]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

No, the only way to override that feature would be to replace the bottom two carbs with ones like the top two. The engine is designed around the 2+2 feature so I don't know If the rest of the electronics would be compatible or not. <br /><br />The practical solution to your problem is to try repropping up or down in pitch to get your motor to kick in the bottom two carbs sooner or later depending on at what point during takeoff the other two are kicking in. If you want extra torque at take-off, go down in pitch and it will raise the rpms sooner. <br /><br />I have the same motor and have mine propped where you don't even feel the transition to 4 cyls. You might want to have the carb sync checked out to make sure it is set-up right also, as this could make the transition more pronounced.

 

[Spidybot]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

The 2+2 concept is maintained by the fuel system. The principle is to make cyl. 3 & 4 run so lean, they will not fire. So 1 and 2 makes the job and as the firing order is 1-3-2-4 it runs smoother thn one could fear.<br /><br />These motors incorporate a mechanical accelerator pump (supplies fuel to 3 & 4 only) to cope with fuel demands on sudden acceleration. A slight difference when it swithes to 4-cyl will always be, but at least the pump should be checked for proper operation.

 

[achris]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

Another option is to change to a 'ventilated' prop, like a Laser II. They have vent holes in them to allow the engine to spin up really quickly before producing any significant thrust. Just be wary, when the prop bites and the boat accelerates, that skier is going to be up soooo fast! <br /><br />Chris...........

 

[Boatin Bob]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

Thanks guys....didn't realize it was pretty much controlled by the fuel system. I think I'll try Quantum's suggestion of the prop, we have a 19 in on now, I'll go down to a 17, don't really care if I lose a bit of top end so long as I make it up at the bottom.

 

[steam_mill]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

Boatin' Bob<br /><br />I have a 17" prop you can try if you want to. If you remember I went from a 17" to a 21" on my boat with a 90 (same boat as you).<br /><br />Actually, If you wanted to do an even trade for the 19, I can let you have the 17, that would be great.<br /><br />The 17 I have was used for 6 weeks last year with no dings or even a scratch.

 

[Boatin Bob]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

Hi Joe....thanks for the offer! I'm going to see if a buddy of mine has a 17 I can try and if it works OK I'll get back in touch with you to do a swap.

 

[steam_mill]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

Just being nosey.....<br /><br />Since we have the same boat and you have 25 more horses than me.<br /><br />With the 17, my hole shot was great. It has slowed a little with the 21. I went to the 21 because I could turn over 6K with the 17.<br /><br />Now with the 21, with me in the boat I turn just over 5500. Loaded with 5 passengers on board (2 women 135 lbs each, 1 half man 155 lbs., and 2 men 200 lbs. each) I was turning maxed out at about 5300 rpm.<br /><br />Tried skiing, tubing with the 21, again a little less for hole shot but no problem skiing or tubing. This was with 2 onboard (the man and half man) and the other man skiing.<br /><br />If I'm doing this with 21 and 25 less horses you are going to be able to pull a cruise ship with a 17....<br /><br />Comments?<br /><br />Joe<br /><br />P.S. On another note, when you hit NAV lights, you can select the bow AND stern lite or just the stern lite? <br /><br />I ask because I had a problem with it and it must have been a loose wire. Noticed that there is diode on the switch to make it work....<br /><br />Joe

 

[Spidybot]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

I have a 115 Merc on a 16' Sea Ray and had severe issues with the std. alu prop.<br /><br />The hull is notched from factury and the motor height is approx 1" above keel line. I had no bite at all, ventilation on every attempt to get ut of the hole and on even the softest turn.<br /><br />Switched to a 22" HighFive SS and got more than I thought possible. A tad to much pitch but suits me fine for cruising etc.<br /><br />Might want to test a Laser some day....

 

[Mercury140-I6]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

Boatin Bob:<br /><br />All you need to do is disconnect the accelerator pump and get #1 and #2 carbs and replace #3 and #4 with the new ones. #3 and #4 just don't have idle jets. #3 and 4 cylinders get just enough fuel/oil to keep them lubed, but not enough to make any power. Put the carbs that have the idle jets and it will run on all 4 all the time.<br /><br />Craig

 

[Boatin Bob]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

Thanks guys for all the feedback and suggestions.<br /><br />Craig...thanks for the info it sounds easy enough (of course I know I'm going to cringe when I hear what Merc wants for a carb)<br /><br />Joe...with the 19 on there now and 3 of us in the boat it turns around 5400 and does about 55 mph (boat speedo) not sure what the WOT rpms will be with a 17 but I expect near 6000 and I'm not what the Merc recommends?<br /><br />As far as the nav/anc lights go, yes both bow and stern are on with the switch in Nav and just the stern on with Anc. The diode is a cheap way around putting a more expensive switch but it works.

 

[steam_mill]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

Bob:<br /><br />I'm afraid you may be underpropped....<br /><br />I was surprised to find that WOT RPM for the 115 is 4750 to 5250.<br /><br />Comments from anyone?<br /><br />Joe

 

[quantumleap]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

You definately don't want to over-rev this motor as they don't take kindly to it.<br /><br />As for UU's comment on the aluminum prop, I found that switching to a ss prop amplified the rough idle effect of this motor and caused increased vibriation, but I still tend to use my ss most of the time. I've read that these motors don't handle stainless as well as the v-6's due to the increased weight of the prop. The instructions that came with my prop even stated that on some of the mid sized motors, increased vibration may occur. Just thought I'd pass on this info.

 

[Boatin Bob]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

Thanks for the info...didn't realize that the recommended WOT rpm's were that low, guess I don't want to go to a 17 then, almost sounds like going to a 21 will drop me right in the middle of the range, not going to do much for my hole shot though, might just as well stay with the 19 and explore the carb conversion in more detail....thanks for everyone's input!

 

[Bobby O]

Re: 2003 Merc 115hp 2cyl to 4cyl feature

It goes to show, you can have effiency or power, not both. The one with the electronics is close. New Carbs alone will not do it.