Merc 75 4 cylinders, 2 carbs

[LaqueRatt]

Just bought a boat with this motor on it. Been sitting for a few months. Got it running and it seems like a strong runner. All cylinders are pretty even @150 lbs. Issue is it dies when I throttle it down. Actually is seems to idle halfway decent with the stick forward a bit, but not when the stick is straight up. Which it has to be to shift gears, right?

I guess since Merc only made this motor for 3 years, 84-86 there isn't much info on it. I'd like to pull the carbs and give them a thorough cleaning, but never worked on a two carb motor before and sure don't want to break the plastic linkage. Cannot find hardly anything about it online. Can somebody point me to a tutorial or a youtube vid? I could use a manual for sure, but am too cheap to pay some schmuck $40 for a digital download on something they probably don't even have copyright on. Has anybody worked on one like this before? Do I pull the carbs as a pair, linkage and all?

 

[BWR1953]

Yes, do be careful removing the plastic linkage. New ones are available, but they're $50!

 

[LaqueRatt]

I actually turned this one up, pretty much all I can find. Wish it showed how to pull them. Guess I can figure it out. Be nice to have a manual, but not even sure if that's an option. Ones I've looked at seem to be for newer motors or cover a bunch of sizes which may not be of much use?

Sure wish knew how to adj a multi-carb motor. Once I get the carbs cleaned then what? I don't even know what the ballpark setting is for the low speed jet.....if that's even what the screws on the upper right are for. It came with the top carb backed out 2 1/4, bottom was 1 1/2. That doesn't seem right. Tried other settings but didn't seem to make much difference.

 

[BWR1953]

It's worth it if you can find an actual Mercury manual. I got mine on ebay for something like fifty bucks. It's used, but everything's there and it comes in quite handy.

I can't comment on your particular engine, by my 50HP Merc also has dual carbs and the starting point is 1 1/4 turns out from lightly seated.

I'm sure others will chime in soon.

 

[racerone]

They made that motor for many , many years.----Just not with 75 HP on the decals !!

 

[racerone]

Marketed as a 75 / 80 / 85 HP for about 20 years I believe.

 

[brodmann]

I've had great success finding old outboard manuals at my public library. It's pretty cool finding among all of those books a section of greasy, oil soaked car and boat repair manuals mixed in with all the others. I have very frequently found a repair manual for what I was working on and just used their copier to print off the pages I needed. 10 cents a copy isn't a bad deal if you only need a few pages on a link and sync procedure. It's been a while, it might cost a lot more now.

 

[Texasmark]

I had the mid 1970's I think, 4 cylinder 85.....the decal was blue, silver and red and pointed down on the tail end....Mercury had a lower case "m" (which I thought was very in-appropriate...a proper noun with a lower case first letter). Another of my fine Mercury engines. At the ramp it always started right up.....while other folks were fiddling with theirs trying to get them running.
Idle is set so that in F gear at idle, the engine will remain running yet when its time to shift, the RPMs aren't so high as to cause clunking and undue stress on the drive train. Manual for similar engines is: F gear in the water, boat moving on its own, idle RPM at 650 give or take 50.

Go set that up and then see if you still have a problem.

 

[racerone]

That would have been a 1976 /77 model.----Great motors those inline 4 cylinder ones.

 

[LaqueRatt]

Thanks for all the helpful info. Never thought about trying the library. I'm definitely going to check into that. A manual would be nice, but I can't seem to dig up anything that I'm sure is the correct one. Seems most cover a range of engines, kinda like the old Chilton manuals for cars I'm guessing? Not sure that would be of much use. There is that one vid on youtube though where a guy tears em down, but doesn't actually show how to pull or adj them. Anyhow think am ready to make the plunge. The clock is ticking. Got 3 days in the 80s coming and then the bottom is due to drop out weather-wise. Sure would like to splash the new boat at least once this year!

 

[LaqueRatt]

Got the carbs off, cleaned, and reinstalled yesterday. Running much better. Starts easy, doesn't die. Idles a bit high though. I found the idle stop, but that didn't seem to drop the idle very much. I see where it looks like the cables are adjustable. Not sure want to mess with that or not. Not going to get too excited til I splash the Crestliner tomorrow and see if how it performs on the lake. I sure don't like to hear that gear clash though. My other motor, 25 HP 'rude is a lot smoother and idles a whole lot lower.

 

[airshot]

I have a 1992 40 hp four cylinder with two carbs, have not had any issues so far, but it idles excellent...nice and slow, can almost hear each cylinder fire. So...the potential is there for a nice slow idle, as mentuoned these inline 4's are great motors !!

 

[Chris1956]

The only real synch necessary is to make sure the carbs open at the same time. There are adjustments on the bottom carb where the plastic linkage attaches. Set the throttle so the top carb to ready to open, but still closed, and then loosen the linkage on the bottom carb and set it the same way.

Mix up some fresh 50::1 mix and adjust the carb idle mixtures so she will idle reasonably well, but still accelerate smoothly. If that doesn't fix the stalling issue, and she is turning less than 900RPM in neutral, you can turn up the idle speed a bit to make it idle better.

Those 4 cylinder motors were pretty easy to tune, vs the 6 cylinder, 3 carb ones.

 

[Willyclay]

Those 4 cylinder motors were pretty easy to tune, vs the 6 cylinder, 3 carb ones.

Everybody should own a ToP once in their boating-life!

 

[LaqueRatt]

Well the winds really kicked up with the temps, so the launch got aborted.

Current issue is the idle is too high and the idle stop doesn't seem to be the problem. I'm thinking the cables are. Anybody know how to adjust them? I see a bunch of threads showing so they must be adjustable. I didn't put a tach on it yet, but it is idling WAY higher than it should. I gotta fix this. Otherwise seems to be running pretty well. I checked the center rod and it seems to be set pretty much on the money, both carbs are opening together. As for adjusting all I did was crank the screws IN which I *think* richens the mix on this motor. From the initial setting of 1 1/2 turns out, I set them on 1 turn out and she definitely likes that setting better.

 

[LaqueRatt]

BTW, I checked the compression on this motor the other day and got 150 across the board! Kinda shocked actually. Not complaining, just never saw figures this high on any of the small OMCs I've messed with.

 

[Chris1956]

The idle speed is controlled by the idle stop screw, on the throttle arm. Of course, the throttle cable pushes against the idle stop screw. So turn the idle stop screw to lower the idle speed. It will push against the throttle cable. If the push is too much, turn the barrel nut on the throttle cable to back it off a bit.

 

[LaqueRatt]

Thank you sir, I'll see if can figure out how to do that. The cable just pops off and then it spins I reckon?

 

[Chris1956]

Most Mercs have a piece of plastic that flips to release the throttle cable. The idle stop set screw is usually on the bottom of the throttle arm. You usually back it off to lower the idle and adjust the barrel on the throttle cable to push the throttle closed.

 

[LaqueRatt]

Messed with that plastic clip, never actually got it loose. Also tugged on cables a bit. Then was able to idle it down, but something seems sticky. The idle is floating around. Also think may have killed my impeller. Those stupid muffs fell off and I didn't notice right away. Geez, I just can't get a break. Now looks like I may need an impeller since it's not peeing at low revs anymore. Ordered new set of muff, round ones, not square. Hopefully the impeller isn't cooked, but not too optimistic.