'83 115hp no power under load

[Danny C]

Past Friday, on the way home with a full boat, lost power at almost full throttle. Engine would start up, idle fine, but has little power at WOT. You can hear the engine rpm increase and the boat try to increase power but would only run at slightly higher than idle speeds even at WOT. Had a similar issue lately and adjusting the idle screws on the carbs seemed to fix it. Took it out Saturday alone and spent over an hour adjusting those screws every which way. It seemed to help slightly but then two accelerations later it would be back to its issues. If I could get it onto plane I was usually good. Thought I had it fixed but the next day with a loaded boat no such luck. Idled out, anchored up, everyone else enjoyed the day while I could stop being bothered by my issues, then idled back in.

I plan to rebuild the fuel pump just in case (and its cheap and easy) to see if that helps. I am reluctantly thinking about tearing into the carbs as well. Also, I just remembered that I have had tape over the fuel vent for a year now that I put on to prevent water from splashing into it when beached. Had that issue last year. I would think that that is not helping my issues and probably creating unneeded load on the fuel pump, but have an fine like this for awhile (not completely cutting off vent).

Also, I recently had to repair and swap the carb linkages. Do these carbs need to be sync'd when this is done? Is this difficult to do? I believe I have a Merc manual somewhere but cannot find it for the life of me. May have to buy another to get the details of syncing.

Thanks

 

[Danny C]

Reading up on it more and am wondering if it could be a spun prop? seems like the symptoms match up...

 

[Chris1956]

It certainly sounds like a spun prop.

 

[444]

Is the engine revving up at full throttle and you're just not moving or is engine rpm staying low even at full throttle?

If it's the former, it could be a spun prop. I always keep a spare cheapie aluminum prop onboard along with a wrench.

I would do the link and sync. Any time you fiddle with these carbs you want to do the link and sync. It's not hard to do, but there are several steps. Buy another manual if you lost your old one. If that doesn't do it, I would start looking at fuel pump and carbs. These old merc fuel pump diaphragms don't seem to last many years IMO.

 

[Danny C]

Revs up at full throttle and little movement (like idle speeds). I would think that if the prop is slipping, it would go nowhere at WOT but maybe there is enough friction to give me some forward movement. I made the gamble to replace the hub without taking it to the lake to test if that is the actual issue because I am not able to get to the lake until this weekend. Just got it back today. Prop shop said the hub was bad, but I am not sure I believe him 100% as he basically said with my symptom, the only issue could be a bad hub when I am sure it could be other issues such as carbs ect. I hope he is correct, and this fixes it.

For the fun of it, I replaced all of my fuel lines and fuel filter (they were leaking a bit at a few connections) yesterday. I have a fuel pump rebuild kit in the car that I plan to throw on before this weekend. I tend to rebuild it at least once a year. Not necessarily because it needs it but it is easy and I am in the engine already.

I need to do a lync and sync but have not found the manual yet.

Been looking today at a cheap aluminum spare prop to keep in the boat. It would at least give me a little piece of mind.

 

[Danny C]

Prop rehubbed., rebuilt fuel pump, new fuel lines, out to the lake for a great day... nothing. Same issue. Got home replaced all plugs, did a compression check and 100 PSI on all cylinders. I do not have the manual but I am pretty sure that is much too low so I am not optimistic. Also found a bad solenoid. I have a spare that I am still trying to find and the shop does not open until Monday. So next steps for bad compression? Does that mean possible bad gasket or is it much worse?

 

[subliminaltrips]

so the rpms don't "tach out" under full throttle? like it just struggles to gain engine rpms? or does it rev as if it was in neutral just not going anywhere fast?

 

[Chris1956]

If the compression is even on all cylinders, it is fine, no matter what the total number is That motor does not have a head gasket, so that cannot be an issue.

What bad solenoid did you find? I still do not have enough info to narrow the issue to fuel or spark. What happens when you have a light load in the boat? Does she rev up? What prop pitch do you have and what kind of boat. What do the spark plugs look like?

 

[Danny C]

I do not have a tach so I cannot say for sure if it tachs out or not. It sounds like it is getting to full throttle but not going anywhere fast although I will say it sounds a bit different than normal. Maybe it is not getting to full throttle but it is at least getting to the higher rpms. I have noticed that I am burning through a bunch of gas while "idleing" at full throttle across the lake which also leads me to believe that the engine is running at the high rpms. It is hard to tell if I am at full throttle without a tach. I dropped off my passengers to mess with the carbs with a light load and it seams that the lighter load helps a bit as in I can get it to go slightly faster but still not anywhere near where I should be.

One thing I noticed yesterday was that If I had it in gear and throttled it by moving the carb linkage with my finger it bogged down immediately. I thought it was interesting that it had a different reaction then if I just hammered the throttle. I figure it should have the same effect. Don't know if that means anything.

When I pulled all of the plugs they all looked pretty clean except one (#5) was a bit oily. No char or soot. I decided to replace them anyway. With the new plugs all had good spark except #3 had no spark. I swapped plugs and wires from a different solenoid and still no spark. So I suspect a bad solenoid or wiring. I want to find the new one I have and replace or just try to swap the solenoid with a different on to see if the issue moves with it.

I have a 19 pitch prop running on a 115 merc on the back of an '83 checkmate enticer. Have run the same prop for years with no issue. Since I rehubbed the prop I am a bit worried it may be a worn coupler.

 

[Danny C]

This morning went to go change the bad solenoid. First switched with a different solenoid to make sure that was the issue and the problem followed the solenoid. Changed solenoid and issue (no spark) did not go away. Then found all plugs (2,4,6) off same distribution block not firing now. Suspected bad block. Ran motor to try to multimeter and all were firing now. Pulled plug and getting spark now. Hard to diagnose when issue keeps jumping around. Problem is I troubleshoot on the driveway and have to drive an hour to see if the issue is there or not.

If a distribution block is going in and out thus depriving 3 plugs from firing would motor still run? I am wondering if it is sparatically going in and out. It would make sense that I am running at half power. It does not sound like it is misfiring but I wonder if it is misfiring at such a timed fashion that it sounds like it is running smooth.

 

[Chris1956]

OK, when you say solenoid, you mean coil. When you say distribution block, you mean switchbox. That helps me a lot.

Get a new ign coil, and make sure the grounds are clean and tight. Start the motor on a flusher and put a timing light on each plug wire to make sure you have spark. A spark tester is a better idea. That motor will probably run on 3 cylinders, but likely not very well. It will run on 4 or 5 cylinders much better.

If your boat is 18 foot or less, she should perform pretty well with that motor. The CDIElectronics web site will have troubleshooting procedures for your ADI ignition system.

 

[Danny C]

Just got back from the lake. She performed great! Which is very bitter sweet. Since the last time out, I shot some carb cleaner straight in the carbs, replaced all plugs, and changed out a coil (and yes, Chris I meant coil and switchbox). The coil I changed ended up working when I checked it later. That is all I really did besides swapping wires and coils every which way during troubleshooting. My thoughts are still that the switchbox was not firing correctly. Whether or not it is the switchbox or power going to it or anything else is anyone's guess at this point.

The reason I say bitter sweet is one of the worse things for me is to fix an issue without knowing what it was in the first place. So frustrating and almost guaranteed to happen again in the future. At this point, while happy it is running, I am just waiting for the switchbox to go out again at the worst time (if that is what it was). I have a few big trips planned ahead and it doesn't sit well with me.

Thank you all for the help so far...