Update RE 115hp inline 6 1984 blue band RPM issues

Brent H

Recruit
Joined
Nov 27, 2006
Messages
4
Thanks for all those who posted suggestions to my last post a couple of days ago.

Took the boat back out, and it ran for about 20 mins at WOT (sitting on 4500rpm). It then died back down to 3000rpm.
I did a quick idle mixture adjustment, as best as i could while trying not got get run over by other boats, and that seemed to make a huge difference to low end performance. It would die out after about 10 secs at WOT, but would run below say 3500rpm all day long. Opened it back up to WOT and noticed that if i pumped the primer bulb, it would pick back up to WOT immediately. It took a bit of managing to get this right. If pumped too much, the engine would die immediately, and sounded like it was flooding. If pumped every 3 secs or so, it would stay around the WOT RPM mark. After replacing most of the fuel line components, I'm pretty sure its the fuel pump.
Off to the local marine dealer to get some new plugs ($9.50 each) and a rebuild kit for the pump ($60). Have ordered a rebuild kit for the carbs, which was quoted just under $100. I'm sure if Mercury o/boards were made in Australia, parts would be alot cheaper!
Do you think its worthwhile to pull the carbs and clean them out? In the Seloc Manual, they call out sizes in the main jets according to whether the engine is above or below a certain serial number. From what i can see, Pre 79 or 80 model 115hp inline 6's had a slightly smaller main jet and a slightly bigger idle jet. Post 79 or 80 model 115hp inline 6's had a slightly larger main and slightly smaller idle jet. I cannot see any difference between the two engine setups (eg carbs). Does anyone know why a larger main jet was used for the same engine setup? Has anyone dropped a size on the main jet with favourable results?
 

emckelvy

Commander
Joined
Jan 16, 2004
Messages
2,506
Re: Update RE 115hp inline 6 1984 blue band RPM issues

The diff in jets is more likely due to a change in the way they rated HP, from crankshaft to propshaft. The later models rated 90 HP (at the prop), for example, were roughly the equivalent of an older crankshaft-hp-rated 115. Later 115's were power-ported and probably close in output to a power-ported, crank-hp-rated 1400.

The carb jets reflect more accurately the airflow characteristics of each particular engine and this has to do mostly with reed blocks; intake/exhaust port size; and whether or not the block/pistons are power-ported type or not.

Your problem sounds more like leaking float needles or other issues with the carbs.

Rebuilding the fuel pump can't hurt, also you probably don't need the expensive 'carb rebuild' kits. Cheaper are the "carb packing" kits which have the gaskets, little washers, etc you'll need to take the carbs apart, clean them, and reassemble.

I'd also order new float needle/seat assy's and if when you take the carbs apart you don't have white plastic floats, get those new too. These are the upgraded floats which replace the old "foam" type.

Those parts shouldn't cost you near as much as the full carb rebuild kits, and 9 times out of 10 that's all you need to fix the carbs up good as new.

You can look up parts at the www.mercurypartsexpress.com site or www.dougrussell.com

BTW, if you can get Autolite plugs Down Under, the surface gap 2852's usually work well in this application and are considerably cheaper. Most auto parts stores can supply marine spark plugs way cheaper than the $tealer. NGK BUHW and Champion L76V are also used in your motor.

http://www.angelfire.com/extreme4/skidoolympique/SPARK_PLUG_CROSS_REFERENCE.htm

HTH........ed
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Re: Update RE 115hp inline 6 1984 blue band RPM issues

First of all the 1984 mercs were brown striped. Blue was used from 1972 thru 1983 in various variations.
Merc switched from crank rating to prop rating the inline sixes in 1982. Supposidly the 140 became the 115, and the 115 became the 90. The specs still showed the same operating ranges for the engines before and after the rating change 4500 - 5000 for the 90s and 5000 - 5500 for the 115s. The 140 was 5300 to 5800.

Reed blocks on the late 90hp were different than the reed blocks on the late 115s, which were differnt than the 140s.

Here are the jettings up to and including 1985
http://pic70.picturetrail.com/VOL1807/7600256/14391838/221107899.jpg
 
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