1989 70 hp mercury bogs under load after 5-10 min of running

[blakebre]

Hi all, thanks in advance for any good input! As stated, I?m having issues with my oil injected 3 cyl 2-stroke (S/N oc100101). Engine starts great, always idles fine, then runs fine for 5-10 min then starts to hesitate and recover, then eventually bogs and will not exceed ~2k rpm even at WOT. Problem first began after installing a CMC tilt/trim unit. Have always suspected ignition, but ruled out timing, fuel lines, fuel filters, fuel pump, bulb, carb cleanliness, carb tuning, bad fuel, fuel vent, air starvation, etc. Compression is 100 psi, low-ish but consistent on all 3 and has been stabile for 100+ hrs. Spark plugs are new but indicate a rich condition and / or poor spark. Like I said, I suspect ignition, but I don?t want to just start replacing parts? Oh, and stator / switch box test ok resistance-wise, but I don?t feel those tests tell the whole story. Stator is visually ok, but is the 8778A16 type with a bad reputation. Suggestions?
Thanks, Blake

 

[trendsetter240]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Problem first began after installing a CMC tilt/trim unit.

Have you checked that the engine still has a strong tell tale pee stream when running? With your new trim/tilt are you running the motor higher up in the water?

It sounds to me like it is running too hot.

 

[CharlieB]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Often a motor that slows and will not pull a load over 2K has a weak high speed stator winding,

HOWEVER< since there is NO RETURN on electrical parts you need to test and PROVE THE FAULT before thinking about ordering any electrical part.

Take along a timing light and a DVA Meter. When the condition first begins check to see if the timing light fails to flash , goes out at the bog, works at lower speeds, then before the motor can cool DVA test the stator following the detailed directions in the CDI Electronics Ignition Troubleshooting Guide for your motor.


This should be the link to the guide
CDI Electronics Practical Outboard Ignition Troubleshooting

Verify your work and repost before spending a bunch of money, you don't want to buy anything spendy that you may not need.

Fill out your profile with location.

I may have two or three new CDI stator on the shelf in the barn. Maybe the right one.

 

[quicktach]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Don't know which trim/tilt unit you have, but completely disconnect it and see if the problem goes away. Put motor to original level in water too.

 

[blakebre]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Have you checked that the engine still has a strong tell tale pee stream when running? With your new trim/tilt are you running the motor higher up in the water?

It sounds to me like it is running too hot.

I appreciate the reply. I feel I've ruled that out though: Tell tale is perfect, impellor and thermostat are new, poppet looked fine. Additionally I felt the heads / block immediately after poor running and everything felt right... Thanks!

...Oh, and the engine was too low the first time out actually, I have since brought it up and it's about right and definitely picking up water at plane.

 

[blakebre]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Often a motor that slows and will not pull a load over 2K has a weak high speed stator winding,

HOWEVER< since there is NO RETURN on electrical parts you need to test and PROVE THE FAULT before thinking about ordering any electrical part.

Take along a timing light and a DVA Meter. When the condition first begins check to see if the timing light fails to flash, goes out at the bog, works at lower speeds, then before the motor can cool DVA test the stator following the detailed directions in the CDI Electronics Ignition Troubleshooting Guide for your motor.


This should be the link to the guide
CDI Electronics Practical Outboard Ignition Troubleshooting

Verify your work and repost before spending a bunch of money, you don't want to buy anything spendy that you may not need.

Fill out your profile with location.

I may have two or three new CDI stator on the shelf in the barn. Maybe the right one.

Hey thanks for the input. I don't have a DVA though. By the time I get one, pay to tow my boat back and forth to the harbor, pay to launch it, I'm over half-way to a stator (not to mention lost time, which is precious to me with my work schedule). Granted, I'd have the tool, but I hope never to need it, haha. With a new stator / rectifier, I'd have more confidence in the OB when I'm way offshore. And if it turns out not to be the problem it'll be ok by me to have it in the spare parts kit on the boat (not that changing a stator offshore sounds fun, but beats a LONG tow or a 5 hr, 5 kt max speed trip back on my kicker). So I guess with your though aligning with mine regarding the high speed coil, I'm at peace with getting a new stator AND rectifier (which CDI's tech., Mr. McGowan's suggests to always change with the stator). Unless, of course, you smack me up side the head in a new post

To anyone who's wondering, the CDI tech guy got back to me very quickly and was very helpful.

I'm in Orange County, CA btw (will update), where's your barn?

 

[blakebre]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Don't know which trim/tilt unit you have, but completely disconnect it and see if the problem goes away. Put motor to original level in water too.

Those were my initial thoughts exactly... Done, and Done. Thank you!

 

[CharlieB]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Mid-Missouri

If you have are are willing to get a cheap Volt/Ohm Meter you can build your own DVA Adapter very cheaply.

See Simple, easy to make DVA adapter for standard Multimeter

Spark test cranking will verify the low speed windings of the stator, but if the high speed windings are failing then the ignition will not provide sufficient voltage to produce a quality spark under load at RPM and fail to accelerate over 2000 - 2500 thus DVA testing both low and high speed circuits.

The CDI Troubleshooting Guide is THE 'Go To' reference for solving most every ignition question/problem.

Because of the expense, and NO RETURN on electrical parts it is very good practice to test and PROVE any fault so as to be sure of the repair and self confidence that it is safe to go out on the water.

 

[blakebre]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Mid-Missouri

If you have are are willing to get a cheap Volt/Ohm Meter you can build your own DVA Adapter very cheaply.

See Simple, easy to make DVA adapter for standard Multimeter

Spark test cranking will verify the low speed windings of the stator, but if the high speed windings are failing then the ignition will not provide sufficient voltage to produce a quality spark under load at RPM and fail to accelerate over 2000 - 2500 thus DVA testing both low and high speed circuits.

The CDI Troubleshooting Guide is THE 'Go To' reference for solving most every ignition question/problem.

Because of the expense, and NO RETURN on electrical parts it is very good practice to test and PROVE any fault so as to be sure of the repair and self confidence that it is safe to go out on the water.

I had seen a DVA schematic like that recently. Seems like the laggy jumpy nature of my basic DMM would make it mediocre at best... Do you have any practical experience with one?

As for the CDI troubleshooting guide, I do agree it is very well done as I have now been through it far more than I'd like at this point, haha. From it I have only the stator high speed winding and rectifier as suspects ignition-wise (other issues seem to be localized to one cylinder). The 3 consistently wet jet-black spark plugs really lead me away from carb or coil related issues (pulled them yesterday after being out of town for a week since the last time I ran it) and that's the end of my suspect list, as the fuel is new and has stabil in it, and runs like a top in my auxiliary off the same tank. Am I missing anything? If not I'm thinking: replace the rectifier and stator, as I wouldn't replace one without the other anyway since a broken rectifier is probably caused by a bad stator or voltage spike of some sort, and I wouldn't do a stator without also doing a rectifier per CDI instruction (which also makes good sense to me based on their relationship). Thoughts?

And again thanks for your time, I really appreciate it!

 

[CharlieB]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

The DVA circuit accomplishes two things

The cap holds a voltage long enough to slow the change so as to get a more stable reading

The resistance provides a load so as the charge coil does actual work, a broken winding would fail to generate any loaded voltage.

Generally DVA tests are done on dis-connected circuit so as not to have any other ignition component bias a reading.

If your DMM has a peak hold feature you 'may' get a 'fair' reading if high speed voltage by leaving all coils connected,

HOWEVER, the motor must be loaded, not in Neutral, and the prop must be under load, in the water, not free air, so the intake charge is real worldload, otherwise the cyl compression will not be there to cause the spark voltage rise.

So while running in the water watch the high speed voltage rise with RPM, if the stator is bad the voltage will only rise to a point and hang, never reaching near value in the CDI chart.

So you see the DVA test, cranking in the shop, are far easier, accurate, and repeatable.

 

[CharlieB]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Just looked at the CDI Guide again and see the CDM section fails to include the 'Fails to Accelerate' section that is critical to your problem.

Back up in the book a few pages, follow the directions to test the red/blue stator wires to see cranking voltage, then running and watch how voltage rises with loaded RPM.

Low speed coils voltage will rise fine, high speed coils will rise but not near as much as loaded RPM as again, without the actual cyl pressure the arc will occur at a much lower voltage and you can't tell if the stator is capable of generating 'full' power' necessary to get to WOT RPM. (about 400V).

 

[quicktach]

Re: 1989 70 hp mercury bogs under load after 5-10 min of running

Did you check the trigger coil and it's linkages ? Is it moving OK over it's range ? Check to see that the wiring from the trigger coil to the CDI isn't preventing the trigger coil plate from moving smoothly over it's full range.