Straight 6 Merc 115hp Revs Under Load Problem

[Tallburgo]

Help!! Three weeks ago I had a great day in the water planing at 45mph off my Merc 115hp, but yesterday (the next time I was on the water for a spot of fishing) was whole different story. Engine fired up sweetly, but it wouldn't rev above about 1000rpm and ended up limping around the bay at less than 10mph. On occassion if I full throttled it, I'd get to 1100rpm and the engine would cut out. But when the engine was out of the water or was in neutral it revved fine (it would have done above 2000rpm if I'd let it). I check the obvious things (enough fuel, oil and no kinks in the fuel line).

Any suggestions where I should start? The troubleshooting section in my maintenance book is suggesting 100s of possible things...

Thanks all
Simon

 

[emckelvy]

Re: Straight 6 Merc 115hp Revs Under Load Problem

Simon, the first thing that comes to mind is a failure of the stator. This would only be applicable if you have a later-model motor with ADI (Alternator-Driven Ignition). In other words, distributorless with (2) ignition modules and (6) ign coils on the Stbd side of the motor. It definitely would help to post year or at least Ser # of your motor.

Do a keyword search on 'stator' to find tons of previous posts on the failure of stator windings. I suspect the Hi-Speed windings in your stator are bad, this'll allow the motor to start & idle but will fall on its face when attempting to throttle-up in gear in the water.

The stator Hi and Low-speed windings are easily resistance-checked with a meter. If you don't have one, check your local Sears or Radio Shack. You should be able to find one good enough for these readings for as little as $9.95 on sale.

If you have a distributor model, I'd check for good spark on all cylinders. Drop the distributor cap down and inspect/clean.

Fuel-related problems can cause similar symptoms. Check/clean your fuel filter(s) if your motor has them. Remove the brass main jet plug at the bottom-front of one of your carbs and see if any water drains out. With all the ethanol in fuel nowadays, it's always possible to get a bad tank of gas with a ton of water in it.

If you find you have issues with the fuel, change tanks then flush fuel thru each carb (with its main jet plug removed) until the discharge runs clear.

Be sure to disconnect your battery before doing this, and have an appropriate container to catch the discharge.

Here' a few links which may help, in looking up parts for your motor and also troubleshooting the problem:

http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2

http://www.boatpartstore.com/mercreadings.asp

http://outboardparts.com/mercury/troubleshooting/troubleshooting10.htm

http://dolphinmarineservice.homestead.com/ign5.html

HTH.............ed

 

[Tallburgo]

Re: Straight 6 Merc 115hp Revs Under Load Problem

Many thanks for your help.

The motor is 1988, serial number B358890 and model number 1-115452B.

I've had a look today and it seems to be distributor-less so will check the resistance tomorrow. I'll also check the carbs. When I was out on the water, my feeling was that it was fuel related but I'm a rookie so could eaily be completely wrong!

Cheers
Simon

 

[Tallburgo]

Re: Straight 6 Merc 115hp Revs Under Load Problem

OK. I think you've pointed me to my problem. I disconnected all the stator and trigger assembly connections and got the following resistances of the following wires (the range in brackets are the ranges within which my manual says they should test within):-

Stator Test
Blue and red stator leads = 0 (5400-6200 Ohms)
Blue/white and red/white stator leads = 0 (5400-6200 Ohms)
Red and Engine ground = 0 (125-175 Ohms)
Red/white and Engine ground = 127 (125-175 Ohms)

Trigger Assembly Test
Brown without yellow sleeve and White with yellow sleeve = 1207 (1100-1400 Ohms)
White without yellow sleeve and violet with yellow sleeve = 1229 (1100-1400 Ohms)
Violet without yellow sleeve and brown with yellow sleeve = 1207 (1100-1400 Ohms)

Do the readings therefore make sense as to the problem I have experienced and is the only way of resolving this is by replacing the Stator? Why did the stator go in the first place and is there anything I can do to stop this from happening again?

Many thanks
Simon

 

[emckelvy]

Re: Straight 6 Merc 115hp Revs Under Load Problem

The original stators were prone to failure due to poor design and issues with the original vendors who manufactured them.

Replacement stators are much more reliable. I recommend the CDI brand replacements, have never had one of them go bad.

I don't know if you can get one of these thru iboats, you can check their store.

This is the place I use:

http://www.boatfix.com/searchcdi.asp

You're also gonna need a proper flywheel puller, one which screws into the center hole of the flywheel. Jaw-type pullers will only damage the flywheel.

Mercury sells a puller, part # 91- 849154T1, retail is $92.75

Here's another one:

http://www.jaysmithracing.com/shopwork.htm (you'll have to scroll down the page)

The JSRE puller is likely one of the best out there, especially if you're working on these all the time. I have one of these and it works great.

For occasional use, there are less expensive ones that will do the job:

http://www.amazon.com/gp/product/B0..._m=ATVPDKIKX0DER&pf_rd_r=1NXG5PZBDVCFDABYBHJ9

http://motors.shop.ebay.com/i.html?_nkw=mercury+flywheel+puller&_armrs=1&_from=&_ipg=&_newu=1&_sop=1

Be sure to torque the flywheel nut per specs when you're done.

HTH & G'luck with the repairs..........ed