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Mercury 2 stroke 115 'tower of power' prop/hub problem

D

DrLivingston

New member
After several outings this season on my mercury 115 (1986 model) it was running fine, coming up on plane, carrying 4 passengers etc, it suddenly at sustained throttle position, started losing power. I would have to keep bumping the throttle to keep up, next thing I know I'm limping home at just above idle. Now thinking it was a fuel issues I replaced everything and cleaned carbs. Fuel filter, inline filters, plugs etc etc. Took it out and I have the throttle pinned and its barely moving the boat and the motor's revs are going up and down never reaching above maybe 3k.

A gentleman motors past and says it sounds and looks to be spun hub. Now I am able to do repairs myself, can assume how to remove the prop by basic parts diagrams as it seems to be much like a hub and bearing set on a car or trailer. But on the model I have being a 1986 did Mercury have in place the plastic hubs that were the fail safe before damaging any lower unit parts that I hear about? Is there an upgraded unit I should put in its place? If you guys need any prop info I can take a photo of what Im running. I cannot seem to find any videos or replacement threads online for this year span motor. Thanks in advance guys!
 
guyjg said:
A good prop rebuild shop can replace the rubber hub in the prop.
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I pulled the prop ready for a local shop tomorrow. I noticed the spline shaft is pressed into the prop which seems to be a one piece unit. Prop/spline housing. Not the updated plastic hub that is sold separately. Is there a sign to tell if that spine is spun inside its housing without pulling it apart? I will have it replaced anyway I am just trying to see the inner workings of that pressed spline and what could of stripped inside where i cannot see without removing that spline shaft. Its more curiosity than anything at this point as I will have it replaced anyway. Like I said the prop and motor get moving about 400 rpm above idle. But its under load at higher throttle position where the rpm's can't build and very sporadic up and down motor noise and lack of speed happen. So seems to be hub, correct?

Also side note: Moving at the fastest speed the boat will go (maybe 10-15mph) and I cut the steering to circle around if the prop catches a pocket of air the rpm's suddenly peak and drop as soon as it's back under load. If that helps the diagnosis.
 
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A slipping prop w/allow max RPM and little boat speed...your RPM is oscillating up/down...prop may be ok but get it checked anyway.
 
guyjg said:
A slipping prop w/allow max RPM and little boat speed...your RPM is oscillating up/down...prop may be ok but get it checked anyway.
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If at full throttle it is bogged down, builds no power, emits a sound of up and down RPM's that a passerby thought it sounded like a spinning prop and doesn't reach past 3k. Where may I look? And to mention just because it is reading approx 3k rpm it is NO WHERE NEAR full running power at what 3k was a few weeks ago. Its a miserable choppy 3k. Fuel pump? Anything electronic on this motor perhaps?
 
You can answer the question yourself easily. The splined inner part of the hub connects to the outer aluminum (probably) part of the hub via a pressed in rubber friction fit plug. If the rubber shock absorbing plug can no longer maintain adequate connection between the two surfaces the splined part and alum outer shell can turn to different positions.

Remove the prop and mark a straight line on the rear of the prop from the brass splined part straight out to the outer shell. Run the engine long enough to know it's not working. Get the prop off and look at it. If your lines are no longer aligned you have a spun hub and as Guy said, a prop shop can rebuild the prop for you, or price it out and it may be cheaper to just buy a new prop. Probably sell them on site or google boat propellers. Figure about $100 for a stock alum prop.

Mark
 
if you are running an aluminum prop you need a spare anyway...i would(and have in the past) have that one fixed if you buy a new one and that is the problem..
 
papyson said:
if you are running an aluminum prop you need a spare anyway...i would(and have in the past) have that one fixed if you buy a new one and that is the problem..
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Thanks for the replies. I will give that test tonight after work. Does someone mind explaining if the hub is spun and can't grab the prop to spin it like mentioned above shouldn't the motor be reaching much higher RPM's and not be limited to just 3k before falling on it's face if it's just a matter of the prop spinning. Because if I can imagine the spline and shaft would be move no matter what if that's the case.

I was reading some posts on switch box failures and one of the troubleshooting problems is 'missing at higher RPM's' or 'a bank of cylinders not firing' I have not tested that yet however because I just found that thread last night. Also a tilt/kill switch issues? Perhaps any of those would cause very rough running and no acceleration?

This is just for my understanding so thanks for the replies. Learn once, it'll last me forever.
 
check the prop first...if it is as shot as it would be to cause this problem it is in effect letting the motor rev with no load...this is not good for a 2cycle engine....funny things happen when you do that..and if its goes on long enough some not so funny expensive things happen...
 
If your motor does not want to rev over 3000 RPM it is not a problem with the prop.-----------Now you must be carefull in running a 2 stroke motor with a lack of power.---------If a carburetor were plugged then 2 cylinders would not be getting fuel.----Also they would not be getting oil.------Start with a compression test.---Then check to make sure you have spark on all 6 cylinders.
 
racerone said:
If your motor does not want to rev over 3000 RPM it is not a problem with the prop.-----------Now you must be carefull in running a 2 stroke motor with a lack of power.---------If a carburetor were plugged then 2 cylinders would not be getting fuel.----Also they would not be getting oil.------Start with a compression test.---Then check to make sure you have spark on all 6 cylinders.
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Last week I took apart the carbs and soaked and sprayed all 3. Replaced worn gaskets and checked floats. reassembled and then same issue. So I am off work now and waiting on a friend to be a second set of hands and this is my plan of attack:

I have previously checked compression when I purchased the boat just two months ago. 110-120 on all 6.
I am pulling each plug individually while he bumps the motor and will test spark-
If it is a certain bank of cyl or consecutive(1,2,3) non-fire it should narrow down to stator or switch box:
If there is no issue in spark I move to the following

Water test lightly to replicate problem and then run a DVA check on switch box at higher RPM for cyl. 3 and 6 to narrow down stator issue or switch box. This is per this PDF troubleshooting guide:
http://www.cdielectronics.com/wp-content/uploads/2013/01/114-7778.pdf

Anything I may be missing to narrow it down?

I marked the hub and prop to see if there is a variance when it is tested
 
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DrLivingston said:
Ok here is an update. In driveway I am getting spark on all cyl.
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Better yet here is a video. Recap: Cleaned carbs by taking off, soaking and spraying, floats etc. Pulled all plugs and getting spark sitting in driveway. New fuel filter inline, new water separator filter. Prop has not spun hub. I marked the hib and bolt and it is in same spot after this test.

**If it is worth mentioning while running in driveway if i remove plug wires 3 OR 4 indivually it affects the motor the most and almost bogs down to die. The other plugs if I pull the wires it just misses a tad. If that's worth anything for info. Not sure the cause of that even if it has nothing to do with my issue.


So I am down to electrical or fuel pump...unless you have other suggestions?

Here is a video of it a bump above idle in gear running fine and not missing:
 
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Obvious ignition problem--you can see the whole motor jerking as it catches and misses.

Is that a distributor Merc, or an ADI model?

Jeff
 
That pdf I included up top has a 'high speed miss' troubleshoot guide to see if the rectifier or stator. I was confused what wires to test (red and blue going to stator on cyl 3 and 6-per instructions) and how to disconnect the rectifier to see if that is the issue.
 
The rectifier and that part of the stator just charges the battery. You can remove one of the two yellow wires and tape it off for testing. (The tach won't work.)

I suspect you have a bad switch box. There's two of them on there, and they are cheap on eBay. First, make sure ALL of the little black insulators under each wire's connection are in place and TIGHT. Mine skipped like crazy when a few of them weren't.

Jeff
 
fastjeff said:
The rectifier and that part of the stator just charges the battery. You can remove one of the two yellow wires and tape it off for testing. (The tach won't work.)

I suspect you have a bad switch box. There's two of them on there, and they are cheap on eBay. First, make sure ALL of the little black insulators under each wire's connection are in place and TIGHT. Mine skipped like crazy when a few of them weren't.

Jeff
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Oddly enough I just went to start the boat at the ramp to do a drop in test by pulling off the rectifier per the PDF troubleshooting guide for 'high speed miss' and low and behold a few cranks and the battery drained to its last turn. Then couldn't even move the flywheel. So I'm hopefully using this as a tool to narrow down what I have going on. I checked all plugs and they had spark so if I read right it not always is the switch box. However how they perform under load I didn't get to test after this dead battery. So I will charge the battery, then pull the rectifier and see if the problem is there. If it goes away that's it (per the troubleshoot). If it is still there is said to test ohm's to the red and blue wire on 3 and 6 cyl from stator and see if there is a a steady climb or a jump. If its not steady voltage the stator is usually the case. Not sure how to test the switch boxes, however I am aware of their location and that there are two of them. I'll tell you what I'm learning a crash course in all I need to know in older electronics on this thing.

**took advice from previous poster and this is the following update:

All connections were tight HOWEVER the screws holding the top switch box in place were kind of 'loose', enough to have the box move a bit. One of the screws on the bottom left had a ground wire attached that ran to the top screw with other grounds and hot wires. Not sure if that would supersede the grounding from other points to malfunction the system but could be. Not sure if that is any correlation to charging system?....

Also another side note the hot wire going to the 6 cyl from the coil was corroded bad at the coil pack side. Only one for some reason. Cleaned and put back on.

The screwdriver is pointing to the ground wire that was in between a loosened screw holding switch box in.



This is a pic of the corroded wire.
 
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I see two wires (lower photo) without the insulated caps in place or missing. Not good! We're talking 300 volts here, and that will jump quite a distance to cause trouble.

Jeff
 
fastjeff said:
I see two wires (lower photo) without the insulated caps in place or missing. Not good! We're talking 300 volts here, and that will jump quite a distance to cause trouble.

Jeff
Click to expand...

I took those caps off for purpose of the photo to show the bottom wire is corroded and the middle wire is what is 'should' look like. I cleaned the bottom wire and put the black caps back on
 
Side note on your comment about loose screws and wires under them. The wires are ground wires and the SB probably grounds some internal electronics to it's case and the case to the engine block aka battery- via these screws. Clean and bright and tight. Bad grounds can cause some weird problems.

Corroded screw with green wire needs to be cleaned up. If the inner surfaces, where current has to pass, is corroded also that could add some circuit resistance which could keep the circuit from performing as designed.

Don't know anything about this SB. Just basic circuit theory comments.

Mark
 
fastjeff said:
Keep in mind that the stator produces about 300 volts at WOT. That'll jump a long way to ground.

Jeff
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Ok well changed rectifier got a new sierra for 20 bucks and my buddy works parts counter so I can return if not the problem. It's NOT the problem. I cleaned the corroded wire and tightened everything up. Exact same issue. Getting frustrating BUT hope I have it narrowed down to a bad stator as around 2k rpm is the intermittent miss. So I am running a timing lite tomorrow at boat ramp under load to see what my spark looks like to narrow if a bad switch box or stator is to blame.



Anyone thinkbit may be a fuel pump issue at all? Air in lines perhaps? Just narrowing down best way possible as this issue can't be found on any other forums. :/
 
fastjeff said:
NOT a fuel issue; the skip is far too rapid for fuel problems (they cause sloooow surges).

Jeff
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Well here is where I'm at. I bought an inductive timing light. Bought a new 1000 MCCA batery Replaced the rectifier to no avail. Thinking high speed portion of stator is bad. Took down to ramp, cranked her up and got her to replicate the problem and jacked at around 3k hit every wire with light, it was at ten pm so the light was as clear as day....steady stream...on all 6. Something told me to grab the fuel bulb, I could squeeze it with minimal effort and when it was depressed I could feel the fuel coming in thru it in spurts. Now the bulb is rock hard at idle ( where it runs fine). So air in the lines? Bad bulb? Bad fuel pump?

So what I am thinking to start with is get an external gas tank and run a line either right from it to the fuel pump and run to narrow if an air leak is from engine back or interior engine ( fuel pump, carbs) But should I buy a new primer bulb and run a new line from the external tank, to new bulb to engine or bypass bulb and go from external tank to fuel pump and work forwards or backwards then to save buying a bulb for no reason if thats not something I 'need' to buy.
 
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kimcrwbr1 said:
You say you get spark but are you using a spark gap tester? Just looking at the plugs is useless in your diagnostic. All the wires should jump a gap of at least 9/16 inch brite blue crisp snap? Another field test to see if it is spark just hook your timing light up and lay it on the deck test all the wires and see if it is spark related when you give it gas. Another possibility is your plug wires and boots are old and arcing to the green coil wires. If you dont know how old the plug wires are replace them and rout the green wire so they are not close to the plug wires. Running the engine at night will tell you if your high tension leads are leaking you will see the light show. I wouldnt recommend messing with those wires when the engine is running. Clean all the lug connectors with a small wire brush and route all wires away from the plug wires.
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I gave it the timing light test tonight at night. Didn't see any arcing but wasnt looking that direction anyways. I ran it to mock the 'misfire' and all wires ran a steady stream of timing light. One thing to mention is when I was driveway testing spark plugs just by bumping the motor (which didn't mock full load test of course) the 4th cyl plug was completely black. Still had arc but was much more darker than the rest. I suppose a fresh set of plugs and wires wouldn't hurt in the scheme of things anyways, I do not know the age of either.
 
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It is a common misconception that the bulb should be hard as the motors is running--it will be soft.

Since pumping it improved the operation, then you're not building enough fuel pressure to prevent lean out (which is dangerous to pistons!) Here's what I'd do: Rig up a fuel pressure gage to the lines after the pump. You're looking for about 5 psi. If it can't make it, look for air leaks on the suction side and, if there isn't any, start a fuel pump rebuild (diaphragm and check valves).

I know it's a pain to rig up that pressure gage, but it eliminates a ton of variables. I Tee them in just after the fuel pump--it's easier that way.

Jeff
 
While the engine is running the bulb should be soft, not hard like when priming, and not sucked flat. If you have the OEM gray fuel line you might want to replace that and the bulb with OEM bulb. At 10 years I had the mentioned line degenerate filling the fuel system with tan colored pieces of gunk. Other than a clogged fuel distribution system, if you have fuel delivery problems, and obviously the engine uses more at the higher rpms, if you squeeze the bulb and get improvement time to look at fuel pump, air leaks, no vent to tank and things like that.

Agree with others on the spark problem, but on my engine (90 3 cyl), if the timing light fires when within 1" of the plug wire, even on muffs, I have a good fire to that cylinder. I have had bad CDI (Capacitor Discharge Ignition) modules over the years, some temperature sensitive and the light caught them every time. The CDI is just trigger, high voltage spark coil and all in one little module, a later ignition version from what you have. It's quicker and easier for me rather than fooling with the gap jumping and all.

I certainly wouldn't be chasing this problem with unknown quality plugs and wires.

My 2c,
Mark
 
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