1978 Mercury 85hp outboard

[tootall204]

I just got a boat with a mercury 1978 85hp outboard. The engine runs fine when hooked up to the hose it also runs well when reving at full with the hose. But when the boat is in the lake/ocean it stalls when it goes above 1900 rpm..

Does anyone have any ideas what may cause this problem?

Thanks in advance

 

[Mi duckdown]

Re: 1978 Mercury 85hp outboard

First. Check compression on all cylinders than spark. Over reving is a bad idea on the hose. no resistance at prop.

 

[Gomer50]

Re: 1978 Mercury 85hp outboard

Whatever you do DO NOT RUN YOUR MOTOR ABOVE 1000/1200 RPM ON THE HOSE,,,This will shorten your boating experience there is no load on that motor when running on muffs.Can cause engine run away twisted push rods etc.As for not being able to get above 1900 RPM, as the gentleman above mentioned check compression,correct spark plugs(good spark plugs) a good decarb is in order,check carbs,check fuel lines all the back to your fuel tank,fuel pump and if that is all good check your max timing.(Decarb) Seafoam Deep Creep spray in your carbs,remove spark plugs spray some in the cylinders let sit for 15 minutes. Mix some pre-mix 50:1 gas into a seperate container 1 gallon add 4 to 5 ounces of seafoam and run it empty now this will cause alot of smoke but it will clean out your engine,See if this helps...

 

[tootall204]

Re: 1978 Mercury 85hp outboard

Thank you both for your fast reply's and the help. I will try what you recomended and let you know how it turned out.

Thanks again

 

[chum1]

Re: 1978 Mercury 85hp outboard

could be a number of problems, need more detail,was it sitting for years? could be spark, could be compression, could be fuel. you may want to read through some of the previous post. it doesnt take much to destroy an engine in the driveway, also ground plugs when checking for spark or you may damage the coils, i have found there is a big learning curve on these mercs ( have a 77' 85hp) they are very simple only after you know what to do.

PS stay away from mickey moused salt water mercs unless you want to become a machinist, metallurgist and helicoil installer expert.

 

[tootall204]

Re: 1978 Mercury 85hp outboard

I just picked up the boat and motor Yesterday and knew when buying it, it had this problem.

I hooked up the hose again today to see how it sounded and sounded fine. Idiols nicely.



The guy i bought it from used it in the ocean about a dozen times. He said he cleaned the motor after each use ... But could that be the problem..Maybe it doesn't like salt water???

I am taking the boat up to a local lake tomorrow to see what it does. I will keep it close to shore just incase it cuts out

BTW i have owned a few boat before.. I'm not a total newbie. Just never had a boat with problems.

Thanks for all your help

 

[Mi duckdown]

Re: 1978 Mercury 85hp outboard

Salt water should not be the problem. Unless corroded wires.
COMPRESSION CHECK FIRST.
Driving around is not going to fix anything.

 

[chum1]

Re: 1978 Mercury 85hp outboard

compression is a good starting point but i would suspect a rotten/corroded wire issue, your engine could be firing on two cylinders at the higher rpms. The wires and shielding for this vintage engine apparently came from the same bad vendor and will go bad on new parts just sitting on the shelf, (dont know what they made the shielding out of ...catgut with food coloring?)

I am in the process now with waiting for a new cdi trigger assembly to arrive then install, found one on ebay for $58.00 shipped. Tested wires per cdi instructions and found an open in two of the four trigger wires.

bought this engine in pieces and it has been slow going to get it back to its original reliable state.

 

[tootall204]

Re: 1978 Mercury 85hp outboard

Salt water should not be the problem. Unless corroded wires.
COMPRESSION CHECK FIRST.
Driving around is not going to fix anything.

Ya i didn't think it would but i can see for myself what accually happen when it goes over 1900 RPM

 

[tootall204]

Re: 1978 Mercury 85hp outboard

So I took the boat to the lake and yep it stalled out at 1900 rpm's

So i checked the compression and all 4 are fine

I took the Carb cap off the motor and played with the throttle..

The butterfly on the carb only moved when i put the throttle all the way down.. Of course with the motor off. But It stayed perfectly still untill the trottle was all the way down

Would this cause the motor to stall at higher RPM's?

 

[roncoop75]

Re: 1978 Mercury 85hp outboard

Ya, your butterfly's should be all the way open at WOT. If you're saying yours just begin to move at that point... THERE'S YOUR PROBLEM!

I see some linkage adjustments in your future.

 

[SeaKaye12]

Re: 1978 Mercury 85hp outboard

The issue would be in the linkage if the butterflys are not open **FULLY** when your throttle lever is pushed all the way down.

You don't see a lot of movement in the butterflys as you gradually advance the lever; because it's spark advance that is causing a lot of the RPM increase.

But; at the point when the lever is all the way down, those butterflys need to be fully open.

I'm surprised that no one here has also mentioned the possibility of bad stator wiring or a bad switchbox. In reading these forums it comes up a lot as a deterrent to full throttle operation.

Chuck

 

[tootall204]

Re: 1978 Mercury 85hp outboard

The issue would be in the linkage if the butterflys are not open **FULLY** when your throttle lever is pushed all the way down.

You don't see a lot of movement in the butterflys as you gradually advance the lever; because it's spark advance that is causing a lot of the RPM increase.

But; at the point when the lever is all the way down, those butterflys need to be fully open.

I'm surprised that no one here has also mentioned the possibility of bad stator wiring or a bad switchbox. In reading these forums it comes up a lot as a deterrent to full throttle operation.

Chuck

I am only getting movement (butterfly) when the lever is all the way down.

The Stator was replaced this year

I was looking at where the lever moves on the side of the motor, when it's full it hits the stop spark

Should that be ajusted or is that normal at full throttle

 

[SeaKaye12]

Re: 1978 Mercury 85hp outboard

Well; sounds like you need to do a lync-n-sync. You can download a manual for your engine from the "Sticky" post on this forum.

When you're done with that, the butterflys will be open fully by the time your throttle lever is pushed all the way forward.

Chuck

 

[SeaKaye12]

Re: 1978 Mercury 85hp outboard

Read this too; from Clams....

http://forums.iboats.com/showthread.php?t=168855

 

[tootall204]

Re: 1978 Mercury 85hp outboard

I Just wanted to say thanks to all that replied to this thread.

The motor is now running great.. Thank you

 

[Coastal Rick]

Re: 1978 Mercury 85hp outboard

Can you tell us what changed so that it is running better?

 

[tootall204]

Re: 1978 Mercury 85hp outboard

Can you tell us what changed so that it is running better?

It was just some minor linkage adjustment. A friend of mine who knows more about boat motors came with me today out to the lake.. He was driving the boat at the time and was thottling forward when the motor was starting to die so he hit the chock button beside the throttle and it let him get to higher RPMs around 5000. After that he went back to the motor and turned a screw on one of the linkage and it runs fine now. No stalling

I was so excited when it happened i forgot to ask him which screw he turned out. I will ask and get back to ya

Thanks again