'84 Merc 115 HP idles ok, but no power

Sylvan_84

Recruit
Joined
Oct 19, 2010
Messages
1
Help - 115 Merc 6 cyl,
Just brought the boat back from the mechanic(at least the carbs are done) and still has the same problem - the boat will start and idle but can't get it over 20 mph and uses all the gas up. Plugs were changed twice. Wondering if its coils or timing. But went through a 1/2 tank of gas in a 10 mile ride.

Ken
 

mrallen007

Petty Officer 1st Class
Joined
Aug 19, 2010
Messages
234
Re: '84 Merc 115 HP idles ok, but no power

Dadgum! Thirsty feller... I'd be inclined to look for leaks for one. Check all the lines from the tank on into the motor. I've had power issues with mine too, but it never burned gas like that. I'd start with a simple visual inspection and go from there. After my last incident, I found the throttle linkage was busted and just hanging. I had idle but no power with the throttle open. A $6.50 bushing fixed that.

It's always worth checking the timing. Keeping up routine maintenance can keep you from tons of issues. Also, it may be worth flushing the fuel you have and starting fresh. I dumped all mine into a spare can, filled the tank with new fuel, and added marine grade Sta-Bil to counter the ethanol. I also installed a fuel water separator filter. My motor has run like a champ after all that. Hope this helps...
 

Forza

Cadet
Joined
Aug 13, 2009
Messages
6
Re: '84 Merc 115 HP idles ok, but no power

are you sure you have spark in all 6 cylinders?
 

Dave1027

Lieutenant Junior Grade
Joined
May 25, 2010
Messages
1,078
Re: '84 Merc 115 HP idles ok, but no power

It sounds like you have a sucky mechanic. What did you pay him for? Did you ask him to rebuild your carbs or did you ask to fix the motor?
 

blue90HP50

Seaman
Joined
Jun 9, 2009
Messages
61
Re: '84 Merc 115 HP idles ok, but no power

Suspect its the stator, high speed winding. Visit CDI website for troubleshooting guidelines. If you choose to change the stator yourself, please mark the timing pointer location before removing. You'll need a mercury flywheel puller (for quick removal). Blue
 

Firefox11

Cadet
Joined
Oct 22, 2010
Messages
18
Re: '84 Merc 115 HP idles ok, but no power

Wow, a stator would do that??? The stator is for your charging system, not your ignition. My motor ran great without a working stator for 7 years!

Seriously, that sounds like a case of some cylinders not firing. 2 strokes are thirsty anyway, but if two cylinders aren't working then you would have a SERIOUS lack of power (I.E.-20mph) not to mention a gas guzzler cuz 2 cylinders are throwing unspent gas through the exhaust and sucking full cylinders more of fuel. Not to mention the running cylinders are having to work EXTRA hard to get the boat to do 20mph.

A simple check for that exact problem is to remove the spark plug wires one at a time while the engine is running. When u remove a "Working" cylinder the motor will NOT like it and will protest. If u remove a wire and the motor changes but very little there is your problem child. When is the last time you changed your plugs??? How about Wires??? If all that was done, what is your compression numbers on the motor?? (Did that scare you?)
 

Cogburn

Seaman
Joined
Aug 24, 2010
Messages
51
Re: '84 Merc 115 HP idles ok, but no power

If the stator has bad windings it will act as described. Try unhooking both yellow wires coming from the stator to the rectifier and see if it'll go then. If it does it probably needs a stator.

The switchboxes may also cause a problem as can a trigger.

Look up CDI's troubleshooting on their website.
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: '84 Merc 115 HP idles ok, but no power

If your ignition uses the two switchboxes, each firing three cyl's, then a bad rectifier can cause problems. The previous poster advised to disconnect the rectifier and retest performance, his error is that if performance improves it is a bad rectifier, NOT stator.

However, this model ignition system the stator DOES have separate low speed and high speed circuits which are best tested using a DVA meter as well spelled out in the free Ignition Troubleshooting Guide available from CDIElectronics.com

A weak high speed circuit WILL limit WOT power and RPM

Because there is NO RETURN on electrical parts, it is very HIGHLY RECOMMENDED you properly test each component before making any part purchases.
 

swordfish25

Petty Officer 2nd Class
Joined
Mar 18, 2009
Messages
117
Re: '84 Merc 115 HP idles ok, but no power

I had one that had a bad fuelpump diaphram and sucked the fuel straight into the block. When he said it ran out of gas does he mean the carbs not the fuel tank ?
 

redjmp

Chief Petty Officer
Joined
Mar 16, 2005
Messages
536
Re: '84 Merc 115 HP idles ok, but no power

Ok you need to find out if all the plugs are firing so you need to run the motor under load for a few minutes and immediately pull the plugs.
Any that are cold and wet obviously are not firing.
Report back with your results.
 

arsenalpsu

Petty Officer 1st Class
Joined
Dec 1, 2008
Messages
290
Re: '84 Merc 115 HP idles ok, but no power

I don't agree with the last suggestion, he won't be able to tell the difference. It would be better to crank the engine with a plug out one at a time grounded and see if it sparks. But i'd recommend you go to the CDI website and find how to test your motors' electrical system. You'll need a tester and a DVA adaptor.

If you plan to do your own work on this motor do yourself a huge favor, go find an OEM manual for the old girl, look on e-may that's where i found mine.
 

bernieb

Petty Officer 1st Class
Joined
Feb 9, 2003
Messages
209
Re: '84 Merc 115 HP idles ok, but no power

1. do a compression test on all cylinders.

2. do the sea foam gas additive to clean out the built up carbon.

3. put the fuel supply line in a vacuum using a hand pump and gauge, see if it holds.

4. if you don't know how old fuel pumps are, change them.
 

redjmp

Chief Petty Officer
Joined
Mar 16, 2005
Messages
536
Re: '84 Merc 115 HP idles ok, but no power

It would be better to crank the engine with a plug out one at a time grounded and see if it sparks.

It may spark in open air but if the ignition is weak it will not fire under cylinder pressure.

I think the OP has eyes and a sense of touch and can tell the difference between hot and cold, wet and dry?

I've also seen plugs that light up my handheld spark tester indicating spark is getting to the plug yet the plug was cold and wet.

Turned out that it was shorting to ground by following some dirt/carbon on the insulator/ plug boot.

The only reliable way to tell if the plug has actually been firing is to verify that it has caused combustion.

If it has it will be hot and dry. If not it will be cool and wet compared to one that was firing properly.
 
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