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

[Texasmark]

Moving right along. Marine carbs these days are real simple. The moving parts are the low speed adjustment screw, float and needle valve. The adjustments are the float level....usually with the carb inverted, the seam on the float is parallel with the top casting mating flange. The low speed needle valve is usually 1 1/2 turn out from a "soft seat". Final setup for idle is in the water, F gear 675 RPM +/- setting the timing by the knurled screw on the lever arm. Slow speed needle final adjustment is opened up just enough for the engine to take full throttle without bogging when doing the hole shot.

The high speed jet is usually buried in the bottom of the bowl and needs special attention to ensure that it is clean when finished, as are the venturi tubes. I had an engine with an intermittent fuel problem and in cleaning the carbs I was just doing the last of the air blowing out and accidentally dislodged a black speck of deteriorated fuel line in the high speed jet cavity. Problem solved.

Soaking with a good carb cleaner, lots of 100 PSI air blowing things out, a wire down the venturi tube and in any hole you can find....carefully....one strand of a piece of lamp cord wire works great,.,..is hard enough to clean out varnish but soft enough to not hurt anything. Any plates on the carb, remove and clean...I found specks of deteriorated fuel line under a top plate on one engine also. When you replaced your fuel filter, did you happen to cut the old one open and investigate the contents?

You may need a manual to do the link and sync...setting up the carb linkage with the spark advance of the stator linkage.

 

[Faztbullet]

Need to replace floats in carbs..you got one that is getting heavy and flooding cylinders. This is common on the older floats for this model

 

[Markslund]

Moving right along. Marine carbs these days are real simple. The moving parts are the low speed adjustment screw, float and needle valve. The adjustments are the float level....usually with the carb inverted, the seam on the float is parallel with the top casting mating flange. The low speed needle valve is usually 1 1/2 turn out from a "soft seat". Final setup for idle is in the water, F gear 675 RPM +/- setting the timing by the knurled screw on the lever arm. Slow speed needle final adjustment is opened up just enough for the engine to take full throttle without bogging when doing the hole shot.

The high speed jet is usually buried in the bottom of the bowl and needs special attention to ensure that it is clean when finished, as are the venturi tubes. I had an engine with an intermittent fuel problem and in cleaning the carbs I was just doing the last of the air blowing out and accidentally dislodged a black speck of deteriorated fuel line in the high speed jet cavity. Problem solved.

Soaking with a good carb cleaner, lots of 100 PSI air blowing things out, a wire down the venturi tube and in any hole you can find....carefully....one strand of a piece of lamp cord wire works great,.,..is hard enough to clean out varnish but soft enough to not hurt anything. Any plates on the carb, remove and clean...I found specks of deteriorated fuel line under a top plate on one engine also. When you replaced your fuel filter, did you happen to cut the old one open and investigate the contents?

You may need a manual to do the link and sync...setting up the carb linkage with the spark advance of the stator linkage.

Thanks a lot. I was feeling a bit intimidated about working on the the carbs...

 

[Markslund]

Need to replace floats in carbs..you got one that is getting heavy and flooding cylinders. This is common on the older floats for this model

Thanks! I will be gigging into the carbs soon. It's new territory for me

 

[Markslund]

Thanks! I will be gigging into the carbs soon. It's new territory for me

BTW: I’m trying to think through why or how one of the floats is getting heavy and how the motor would be running great for the first 15-20 minutes before it starts to bog.

 

[Faztbullet]

Floats start absorbing fuel and changes float level, this causes cylinder/s to run rich from flooding.

 

[Markslund]

Hmm! Maybe it will make more sense when I get to the bowl and see the float. Thanks!

 

[Texasmark]

Hmm! Maybe it will make more sense when I get to the bowl and see the float. Thanks!

Test them. Get a container of fuel and set the float in it with you propping up the pivotal end like happens in the carb. If plastic, it ought to float like a cork on your fishing rod...right on top of the fuel since objects displace their weight in a liquid and a plastic float is near weightless in comparison to its surface area.

 

[Markslund]

I pulled out the carbs and removed the floats. They are not plastic, more like foam. I also tried your float test. They do float and didn’t seem to get saturated. Although, they don’t necessarily sit on top of the fuel. I’ll replace them anyway. I guess I was hoping one of them would be obviously bad. I cleaned the carbs thoroughly. They didn’t seem too terrible. Have to pick up some gaskets and floats. But honestly, I’m not so sure my next trip to the lake will be any different.

 

[Dave1027]

Could it be the fuel tank vent is plugged? When it bogs down try squeezing the primer bulb. Is the primer bulb collapsing?

 

[Markslund]

Could it be the fuel tank vent is plugged? When it bogs down try squeezing the primer bulb. Is the primer bulb collapsing?

Thanks. You probably didn’t read my original post. Tank vent was one of the first things to eliminate. But again. Thanks!

 

[racerone]

Look for issues with water leaking into one of the cylinders.-----Have you changed out the water pump impeller recently ?-------Checked fuel pump diaphragm and the check valves in the fuel pump ?

 

[Markslund]

Look for issues with water leaking into one of the cylinders.-----Have you changed out the water pump impeller recently ?-------Checked fuel pump diaphragm and the check valves in the fuel pump ?

Yes, on the impeller and the fuel pump. Not sure what you mean about water leaking into the cylinders?

 

[drenzer]

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!

Just had similar problem w/ 1968 1100EL 6 cyl 3 carbs. hadn't used it for 2 years even though I ran carbs dry and fogged carbs. Filters loaded with crap, love the ethanol in todays fuel. Changed filters and rebuilt carbs, runs like new

 

[drenzer]

Just had similar problem w/ 1968 1100EL 6 cyl 3 carbs. hadn't used it for 2 years even though I ran carbs dry and fogged carbs. Filters loaded with crap, love the ethanol in todays fuel. Changed filters and rebuilt carbs, runs like new

 

[Markslund]

I'm still trying to resolve my plaguing (bogs down after 20 minutes...every time issue). Since my last time in the water last Fall, the carbs were rebuilt and adjusted. I also found a leak around the fuel pump diaphragm. Not sure if this will resolve my issue or not. But while I'm waiting for the Minnesota lakes to thaw, I was wanting to replace my water pump impeller and the thermostat. The only problem is I cant find anywhere in the service manual where the thermostat is located. DOES ANYONE KNOW WHERE THE THERMOSTAT IS ON A 1985 75hp MERC? I did find a Temperature Switch Assembly #87-96829A1. I'm wondering if this is used instead of a traditional thermostat?? I recently spoke with a mechanic who stated that my issue sounds exactly like the Limp Mode is being activated due to overheating. This was also brought up previously within this iBoats thread, but someone commented that Limp Mode was not engineered on this motor in 1985. Well, I'm 2nd guessing that now since I found a Temperature Switch on the parts breakdown diagram. It makes perfect sense for my symptoms of bogging down EVERY STINKING TIME after running for 20 minutes. Perhaps the fuel leak I found was causing an overheat? Anyway, I'm trying to eliminate any possible thermal issue before getting it back into the water when the ice melts. Once again, ANY COMMENYS OR SUGGESTIONS ARE GREATLY APPRECIATED! -Mark

 

[racerone]

Simple----I do not believe there is a thermostat.----I do not believe there is a limp mode.----Test the overheat buzzer on your motor.-----Your fuel pump runs off the crankcase compression PRESSURE.-----So you should post cylinder compression test as it may offer a clue.----Post your numbers.----And fuel leak does not cause an overheat !!----Once again I say some simple trouble shooting needs to be done.

 

[Markslund]

Simple----I do not believe there is a thermostat.----I do not believe there is a limp mode.----Test the overheat buzzer on your motor.-----Your fuel pump runs off the crankcase compression PRESSURE.-----So you should post cylinder compression test as it may offer a clue.----Post your numbers.----And fuel leak does not cause an overheat !!----Once again I say some simple trouble shooting needs to be done.

Thanks for your input racerone. I do not have an overheat buzzer. Cylinder pressure is 140 on all 4 cylinders. I think I have tried simple troubleshooting...but I'm certainly open for something I have not tried.

 

[racerone]

Are the decals a brown / copper color ?-----I do not see and 85 HP listed for 1985.-----So what HP is it and what is the serial #.-------Are you sure you do not have an overheat buzzer ??----I am going to be blunt as usual.-----What electrical trouble shooting led you to replacing all the ignition components ?

 

[Mark Grant]

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 $$

Hi.Your issue is electrical I believe and NOT fuel.That's why bulb squeeze doesn't help.What model is your engine- Model #. You have a part (not familiar with mercs-that is heating up and then breaking down(like a bad coil on older performance engines). Weak spark, can't handle the fuel load.Here's a link to look up your specific engine and parts

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