Fact or fiction about my Merc 80?

[pyro225]

Hi all for once a question about my other outboard which hasn't had so much attention of late, it's a mid 80s grey band Merc 80.
I bought it off someone who used it on the river for use mainly on the river. They claimedit had a 15p prop on it but as the river limits are as low as 4mph in some parts (you could be on at that speed for 4 hours making a wrong turn) he said the plugs kept fouling so he invested in a 10p prop... Is this right? Should they foul if it's just going v slow? Reason I ask is with that prop its mpg is about a third down and would like to put my 19p prop on but if I have a slow day don't want to rip through plugs every time..
What do you think?

Thanks

 

[merc850]

You have to have a tach to start propping trials I don't know the exact max RPM of that motor but I would guess it's 5000-5500 so you choose a prop that lets the motor turn about 5300 max. at full throttle. The low speed will take care of itself, my 850 will idle down to 700 rpm and that's about 3mph - I could troll if I were a fisher guy.
The manual says to avoid long periods of slow speed probably to avoid carbon buildup in the exhaust.
Plug fouling is caused by poor oil/fuel mix and/or wrong heat range for plugs, what is your plug Nr.?

 

[pyro225]

hmmm not sure off my head ill have a little look - thanks

 

[Texasmark]

Agree with Mr. 850 which I owned back in 1973 time frame.....best engine I ever had. What I used to do was to interrupt the trolling periodically and run for a few minutes at WOT to clean things out. It was obvious that the plugs were loading up as initially on the run-up I would have a cloud of smoke out the back.

Changing to a synthetic blend, like the Pennzoil marine WM sells around here, maybe there too, works real well at reducing smoke. Assume you are running 1 pt. of oil to 6 gal gas (50:1) and surface gap plugs.

I think a 10" pitch engine is overkill and a PIA for having to deal with it when wanting to do other things with your boat on the outing. Depending on your rpms the 19 may be a little too much, but then it may be just right. Rpms at WOT tell the story with the normal boat load and the engine tilt/trim set for best mph.

Mark

 

[Chris1956]

Two cycle motors can idle very well, if they are tuned properly. I have a 150HPV6 motor that will idle all day, and then run at top speed without any issue.

You might makes sure the floats are good, and set properly. Make sure the idle mixture is set as lean as possible, but still allows acceleration.

 

[Texasmark]

Two cycle motors can idle very well, if they are tuned properly. I have a 150HPV6 motor that will idle all day, and then run at top speed without any issue.

You might makes sure the floats are good, and set properly. Make sure the idle mixture is set as lean as possible, but still allows acceleration.[/QUOTE

Chris that's amazing, all day and not even a puff of smoke on the hole shot! But it's yours and you if anyone should know.

 

[Chris1956]

Mark, that was on my Johnny 150HPV6 motor. It runs amazingly well. The carb on that Merc 80 may not be as good, but there is nothing inherent in 2 cycle technology to prevent a good idle. I have found that with a fresh 50::1 mixture and a good carb adjustment, those old Mercs will idle fine. Because of the nature of the Merc carbs, I would recommend a lean idle mixture to achieve good idle, yet still allow good acceleration.

 

[Scott Danforth]

not sure about the 80, however the 50 and 60's from the mid 80's would idle all day long trolling for walleye without fouling. guessing you have either the wrong plugs or a dirty carb issue.

 

[Texasmark]

Mark, that was on my Johnny 150HPV6 motor. It runs amazingly well. The carb on that Merc 80 may not be as good, but there is nothing inherent in 2 cycle technology to prevent a good idle. I have found that with a fresh 50::1 mixture and a good carb adjustment, those old Mercs will idle fine. Because of the nature of the Merc carbs, I would recommend a lean idle mixture to achieve good idle, yet still allow good acceleration.

Ok, whatever you say. I remember the days where Merc in white paint, came out with their fisherman which was angled forward about 10 degrees and the main reason was to keep unburned fuel from puddling in the cylinders for those who trolled a lot. Must be something to it for Merc of all mfgrs. to adapt a whole line of fishing motors to keep engines running right with a lot of trolling.

On the old OMC cross flow, I had a '72 125 Johnny what was a superb engine. Didn't troll with it but just a reliable powerhouse that wasn't a looper but had most all the other amenities learned back when the 3 cyl Rude Triumph 55 came out which made it much better on gas than the earlier basic cross flows.

Mark

 

[emckelvy]

A little late to this thread, but a grey-band 80hp is likely to be a Big-Bore 3-cyl, since the last 80hp 4-cyl produced in the 80's was the 1983 model, and that didn't have grey striping.

So, I'm assuming it's a 3-cylinder, which does have a thermostat (unlike the older 4-cyl models which did tend to run on the cool side).

Barring any internal wear issues that would keep the motor from idling well, a likely problem is malfunctioning thermostat, not letting the motor get up to normal operating temperature. Won't idle well and likely will foul plugs.

So, that's something to check. A 143 deg F 'stat is installed, so if it's working properly the block should feel nice 'n toasty once it's warmed up.

Make sure you have the correct plugs in there, factory-issue are NGK BUHW-2 surface gaps. Standard-electrode plugs might run a bit warmer at idle than a surface-gap plug, but you'd have to consult your owner's manual for any recommendations for that. If you were to use the incorrect-heat-range standard-gap plug in there, you stand a chance of detonation and smoking your pistons.

Each carb has an idle-mix screw, at the top of the carb on the Port side. Adjust each screw for best idle quality while still having a good hole-shot.

That's all I can think of off-hand; barring ignition problems or flooding carbs, the Big Three should be quite happy to troll along for hours. My buddy rebuild a 90hp Triple and it was a really nice motor; plenty of power, quiet, and good on gas. It didn't have the silky-smooth idle of a Tower of Power, but it did OK.

If you do have a 4-cyl, that's another bag of worms, and we'll need to talk!

HTH..........ed