1985 Mercury 85hp (4 cyl) runs great for 20 minutes then boggs down

[Markslund]

Right from trailer and motor starts right up. Good stream of water coming from pee hole. Will run like a top at ¾ to full throttle for 20 to 30 minutes and then suddenly bog down so that you have to pull back on the throttle just to keep it running. It will die if you throttle it past a ¼ throttle. No weeds/obstructions. I can limp back to the landing but have to winch the boat back onto the trailer. I have taken the boat out at 6 times this summer with the same results every time.

After the issue occurs and with the motor still running (just poorly), the primer bulb seems firm (as it should), and squeezing it has little effect on motor improvement. Also, pushing in the electric choke (on ignition key), again while the motor is running poorly, makes the motor bog down worse.

Initial thoughts were a fuel supply issue. Ran some carb cleaner through and replaced the fuel filter and a fuel pump kit. Installed a new supply hose and primer bulb. Back to the lake. Exact same performance/issue (ran great for 20 minutes then bogged down). I opened the gas tank filler cap thinking I might be dealing with a tank venting issue. No change. Additionally, I disconnected the supply fuel line and connected to a separate portable gas tank with vent open and cap off. I purged the line and started, no improvement. Once again, crawled back to the landing and trailered home.

I then focused on the issue being electrical/ignition related...thinking something is heating up-then failing. Replaced stator and back to the lake. Same exact results as before (20 or so minutes, then…). Next, I replaced the trigger and the cdi switch assembly. Back to the lake. Same results…

Last items replaced where all four coils. Back to the lake. 25 minutes of running great and same old thing. Cooled down for 30 minutes, starts again, but still only able to crawl back across the lake to the landing. Please help, I'm running out of patients and $$

 

[Texasmark]

You hit the topics I would have recommended. I think you have isolated it to a spark problem. So the question becomes, what is in the ignition circuitry that hasn't been replaced that could be temperature sensitive and repetitive (exactly) in nature?

A bad ignition switch is a possibility if you have the OEM original control box. Switches are made of Phenolic (plastic-cloth-resin) with circuits etched thereupon with silver contacts and a brass wiper....usually....sometimes....I have encountered. Over time the Phenolic absorbs moisture and the silver contact "button" gets smeared by the wiper arm such that it makes a conductive trail between adjacent contacts.

The confusing part there is that the engine bogs rather than quits....which would indicate a short between ground and triggers within the switch, or a malfunctioning "kill switch". So, problem must not be control box related!

You didn't mention changing plugs. I've had plug failures at high RPMs that didn't show up at lower RPMs in auto engines. I usually ran Champion plugs at the time. Todays Mercury engines usually run NGK plugs. The www should have a cross reference chart for swapping if you are currently running Champions...or if NGK, a new set. Wouldn't hurt to spend a few bucks and try new plugs. Ebay may have your replacement....did for my applications.

 

[Markslund]

Texasmark, Thanks so much for your reply. I REALLY appreciate the help! I think the Ignition Switch that you are referencing is one of the parts that I did replace. I may not have the correct nomenclature. If it's the same...I heard it called The Switch Box, or CDI Box. The one I purchased was actually CDI brand. It is a rectangular molded-type unit that the trigger and stator feed into (see attached). I had found a wire coming off the trigger that had some insulation missing and thought it possibly was grounding out and thereby cutting out one of my cylinders. I read that it was wise to just replace the switch box along with the trigger if suspecting grounding out. But yeah...I replaced both plus the stator.

No, I did not replace the plugs. They seem new and I just didn't correlate the running fine for 20-30 minutes each time, then failing, to spark plugs. I guess I didn't know they could break down, so good to know! I also have not replaced the plug wires for that matter.

Someone recently mentioned that they thought my issue was the motor going into Limp Mode, which I know nothing about. They are thinking a sensor going off or defective. I would assume water temp? Although I do have a good stream coming out the pee hole. Anyway, I guess it's something to explore...I'm not sure where a sensor would be located. Thoughts?

Again, I really appreciate your help and any further ideas. -Mark

 

[Markslund]

Texasmark, Thanks so much for your reply. I REALLY appreciate the help! I think the Ignition Switch that you are referencing is one of the parts that I did replace. I may not have the correct nomenclature. If it's the same...I heard it called The Switch Box, or CDI Box. The one I purchased was actually CDI brand. It is a rectangular molded-type unit that the trigger and stator feed into (see attached). I had found a wire coming off the trigger that had some insulation missing and thought it possibly was grounding out and thereby cutting out one of my cylinders. I read that it was wise to just replace the switch box along with the trigger if suspecting grounding out. But yeah...I replaced both plus the stator.

No, I did not replace the plugs. They seem new and I just didn't correlate the running fine for 20-30 minutes each time, then failing, to spark plugs. I guess I didn't know they could break down, so good to know! I also have not replaced the plug wires for that matter.

Someone recently mentioned that they thought my issue was the motor going into Limp Mode, which I know nothing about. They are thinking a sensor going off or defective. I would assume water temp? Although I do have a good stream coming out the pee hole. Anyway, I guess it's something to explore...I'm not sure where a sensor would be located. Thoughts?

Again, I really appreciate your help and any further ideas. -Mark

BTW: Its a 75hp motor, not a 85 as my post title reads...Can't find an EDIT option.

 

[Markslund]

Turns out that Limp Mode is not a design feature with my 1985 Merc. Does anyone have any other suggestions for me? Thanks!

 

[The Force power]

Have you changed the plugs yet?

 

[Markslund]

Yes, I changed the plugs. Same results. Thanks!

 

[racerone]

Put timing light on plugs.-----Observe the flashing light when motor bogs.------May help you determine if it is ignition related.

 

[Markslund]

Thanks. I’ll have to look into getting a timing light.

 

[Texasmark]

Thanks. I’ll have to look into getting a timing light.

Get one that attaches to the battery for power, vs one that plugs into an AC wall socket. Get a helper to drive while you sit by the engine and monitor the spark. When connecting the high voltage clip, observe that the direction arrow on the clip, points toward the plug as indicated. If the spark is equal on all cylinders, time to go after fuel.
If you can find a Starting Fluid aerosol with lube fine. Otherwise Sea Foam aerosol which contains a light petroleum oil (for 2 stroke lube requirements) has been used on forum boaters for this purpose. Access the carb venturis (remove the plastic cover) and when it bogs shoot a squirt of fluid in both carbs. If you get an immediate response go after a fuel problem.

 

[Markslund]

Thanks for the troubleshooting steps! One question concerning the immediate response that you mentioned when spraying into carbs. Any type of response would indicate a fuel issue?would a normal response be that nothing happens?
I had a mechanic tell me that when it bogs to push in my electric choke at my ignition key. And that if it revs up, then I have a fuel issue. If it sounds worse, it is as it should be. Thanks again!

 

[racerone]

Very correct.-----If carburetors are working normally then extra fuel should slow the motor down.

 

[Markslund]

Ok. If that is the case-it slows down-is there any further need to do a spray shot into the carbs test?

 

[racerone]

Inspect all wiring on this motor.----And test run with another tank and hose.------Test run with the timing light.-----In my opinion it should be easy to find the issue !!

 

[The Force power]

What condition is the impeller? is it maybe overheating??
What fuel-mixture are you running it on (50 to 1 I hope)

 

[Markslund]

Thanks racerone. I have tried another tank and fuel. And don’t have the timing light yet. I’ll pick one up soon, if I am confident I don’t still have a fuel issue. My wiring looks good.
Texasmark: Do you feel it is still necessary to shoot sea foam into carbs if my motor is slowing down when I press the electric choke?
Thanks guys!

 

[The Force power]

Do you feel it is still necessary to shoot sea foam into carbs if my motor is slowing down when I press the electric choke?

You will have very little effect when spraying into a carb with the choke butter-flies closed.

Did you replace the Stator/ trigger with new or "good used" ?
What is your max RPMs ?

 

[Markslund]

Yes, new. New stator, trigger, CDI Switch Unit, coils, and spark plugs.

 

[The Force power]

Yes, new. New stator, trigger, CDI Switch Unit, coils, and spark plugs.

When you had the flywheel off did you see if there were any magnets missing ? (on flywheel)
But you will have to get a timing-light to see if she's firing on all in all modes

 

[Markslund]

Well, I believe it is a fuel issue. I took it out again today. When it bogged down again after roughly 15-20 minutes of running great at 3/4 throttle, I **** it down and removed all cowling to include the cover that exposes the two carb venturis. I then started it back up thinking I would spray some sea foam aerosol into venturis. To my surprise it started running great again, as if the cover off was making the difference?? But then it did bog down again after another 5 minutes. While still running I sprayed some sea foam into the venturis, it made the motor surge. I did this a few times with the same results. Funny thing is, pressing the electronic choke still makes it worse.
Another question: I was running off a separate portable tank and I went through about a gallon and a half running at 3/4 throttle for 20-25 minutes. Is that normal?
I’ guessing I will be pulling out the two carburetors next. This will also be something new for me. I’ve messed with small engine carbs in the past, but never marine. More advice is greatly appreciated.
Thanks again!