motor will not exceed 2k rpm under load

[mtjbrown]

Hi guys and gals!! I have a new to me 1997 Merc 75 outboard that I have purchased used and have on an 18' pontoon. Cranks ok and idles ok, but will not power past 2000 rpm under a load. i was told that my prop was too big, 21 pitch so I purchased a 15 pitch and tried it out today. Sad to report no change!!! any ideas?? I had an iphone on board to try to come up with speed numbers but couldnt make it work. May have hit 8-10 mph???

 

[oldman570]

Re: motor will not exceed 2k rpm under load

Using a timming light, check to be sure all the plugs get fire. If so replace the plugs with ones that are recommended by the manufacture. If no fire going to the plug then you could have several things that could cause trouble.
Oldman570

 

[cultured_farmboy]

Re: motor will not exceed 2k rpm under load

Had a 115 Merc do the same thing. Two things to check besides fueling issues. High speed stator could be bad thus not supplying enough power to create a spark. Second is the power pack powering the coils. My engine had a bad stator and one of the packs was bad. Stator is easy to check with an ohm meter. A good merc repair shop should be willing to give you the instructions to test. Power Pack needs to be done with a special piece of equipment. Shop in my area charged $100 for the test.

Replacement aftermarket stator cost me $225 CDN and the replacement pack was $125 for an aftermarket.

 

[Mi duckdown]

Re: motor will not exceed 2k rpm under load

the First thing i would do, Is check compresion on the cylinders. .02

 

[mtjbrown]

Re: motor will not exceed 2k rpm under load

Time to report back in! First , thanks for the replies!! Compression is at 120 lbs on all 3 cylinders. But when I checked the firing using my timing light, the middle cylinder is not firing at all> I switched plugs between the middle cyl and one that is firing, NOTHING, NADA ZIP!!! is this a coil pack or where do I need to go now?? Thanks for the help so far!!!!

 

[leach0369]

Re: motor will not exceed 2k rpm under load

sounds like the same problem I was having with my 135.. My problem was a bad switch box.. You have to have a DVA tester to test it from what I have heard I just took mine to the shop cost around $125 for a new box.

 

[oldman570]

Re: motor will not exceed 2k rpm under load

Test the coils by switching the wires from the switchbox to the coils and spin it over while watching for fire on bad coil. Make sure you have the gas disconected and water going to the motor with muffs on it. If the coil now fires then you will need to test the stator and trigger. I have had to replace the switchbox on more than one of these motors, as they seem to be the weak link in the electecial system. Only use new switchboxes as you could end up with a used one that is bad also.
Never disconect any electrical while the motor is running and be watchful when hooking up a battrey jump or charger as if wrong polarity will blow the electrical componates. Never turn the motor over with any plugs disconnected as it will blow the switchbox.
Oldman570

 

[mtjbrown]

Re: motor will not exceed 2k rpm under load

Test the coils by switching the wires from the switchbox to the coils and spin it over while watching for fire on bad coil. Make sure you have the gas disconected and water going to the motor with muffs on it. If the coil now fires then you will need to test the stator and trigger. I have had to replace the switchbox on more than one of these motors, as they seem to be the weak link in the electecial system. Only use new switchboxes as you could end up with a used one that is bad also.
Never disconect any electrical while the motor is running and be watchful when hooking up a battrey jump or charger as if wrong polarity will blow the electrical componates. Never turn the motor over with any plugs disconnected as it will blow the switchbox.
Oldman570

Oldman570, thanks but I want to be clear what wires you are talking about?? The 2 wires to each coil, swap to another coil??

 

[circle hook]

Re: motor will not exceed 2k rpm under load

also make sure that your not running to hot or the safety will kick in and not allow you to over rev around 2,000 rpm also check & make sure that you have a good water stream

 

[oldman570]

Re: motor will not exceed 2k rpm under load

the coils all go to ground on the - side, switch the wires from the switchbox to the + post on the coils. if the plug that was not firing fires now, the coil will be good. Switch the wires back ASAP as the trouble will probbly be the switchbox. A test of the stator and trigger should be done first before spending $$$ on any electrical parts, as once they go out the door you cannot return them, and no sence buying something you do not need an cannot return.
Oldman570

 

[mtjbrown]

Re: motor will not exceed 2k rpm under load

the coils all go to ground on the - side, switch the wires from the switchbox to the + post on the coils. if the plug that was not firing fires now, the coil will be good. Switch the wires back ASAP as the trouble will probbly be the switchbox. A test of the stator and trigger should be done first before spending $$$ on any electrical parts, as once they go out the door you cannot return them, and no sence buying something you do not need an cannot return.
Oldman570

OK, tonight I switched the green/red wire between the coil that is not firing and an adjacent one. The no-spark moved with the wire. What now, anything left to check or is it the switch box?? Thanks for the help so far!!!

 

[oldman570]

Re: motor will not exceed 2k rpm under load

Now test the stator and trigger. if they test good the switchbox will be bad. test between the red and blue stator leads with a meter and reading with meter set RX 1000 reading should be 6.0-7.0. then test fromblock Grd. to the blue stator lead should read 90-140 with the meter set at RX1. now test the trigger with meter set at RX100 testbrown and Black/ Wht reading shoud be 7-10. now test between Wht and Violet leads, reading should be 7-10 with meter set RX100. All stator and triger leads has to be disconnected to do the test. My bet is the switchbox will be bad.
Oldman570

 

[mtjbrown]

Re: motor will not exceed 2k rpm under load

Oldman570, you were correct! I changed out the switchbox after checking evrything you said. Now all three cylinders are firing. Going to put it in the water tomorrow evening, keeping my fingers crossed!! Thanks so much for the help!!!