Mercury 75 HP

[LaqueRatt]

Some of you know already, but I bought a boat with a good-sized Mercury on it. Never owned a Merc, slowly getting acquainted with my new friend. Think it's a 1985 Model ELPT s/n: M30886 Looks pretty clean under the hood.

Found out when I went to get my newly acquired Crestliner Muskie that it hasn't been started since last year. Guy has been honest with me and the motor did kick a few times so rolled the dice. Finally got it started on Friday. It ran for about 10 secs and died. Did that over and over until I tried to start it at half throttle. Roared to life, sounds strong, but doesn't want to idle. Warmed it up and it still won't idle. I'm thinking dirty carbs, so I'll be pulling them off this week to inspect and clean. If anyone has any thoughts, feel free.

 

[LaqueRatt]

Messed around with it for a couple hours yesterday. Not knowing exactly how to pull the carbs, decided to shoot some carb spray where I could. Took out what I think are the low speed adjusters and also the drain plugs in the front. Sprayed the crap out of everything, ran carb brushes through what I could. Let it run for awhile. I got a lot of SeaFoam in it, so had fingers crossed that maybe I could avoid tearing the carbs down. Yeah I know. Wishful thinking. Or as my old man used to say: Wish in one hand and s**t in the other and see which hand fills up first!

She starts up easily and runs well at 1/2 throttle and beyond. Tried some minor adjusting, didn't do anything. Cleaned the plugs. Seems a bit smoother. Odd thing is, it kinda idles now, but with the stick quite a bit forward. If I put in the middle which I *think* is the shift position(?) it dies. Linkage out of whack maybe?

So this is my problem. I know zilch about this motor, but have pulled the carbs on several single carb OMC motors. Do I pull them as a pair? Looks like some plastic pieces that connect them together that I'd really hate to break. I sure could use a manual, a tutorial, or a youtube on R&Ring the carbs and how to sync them when finished, but I can't find hardly anything for this motor. Anybody able to steer me in the right direction? I think the deal is Merc only made 4 cyl 75s for 3 years, 84-86. Everything I'm finding is for the newer 3 cyl/3 carb 75.

 

[MrGiggles]

Idle speed is not dependent on the throttle linkage. The idle speed screw is on the throttle/timing arm near where the cables attach. Make sure you get the right screw, the timing stop screw is also right there and you don't want to mess with that one.

There is an adjustment screw on that plastic coupler between the carbs, that is your sync adjustment. If you're just removing and reinstalling, probably shouldn't have to mess with it but you should check it anyway. Just look down the carb throats at idle/part/full throttle and see if the butterflies are opened equally.

It appears that those are idle air bleed style adjusters, so they will work opposite, CW is rich, CCW leaner. They meter air instead of fuel. Will it idle with choke or partial choke?

Do you have a timing light? It's probably worth going through and checking the timing numbers before running it hard.

 

[LaqueRatt]

Thanks that's quite helpful. Think am about ready to make the plunge. Don't suppose you could post a pic of idle speed screw? I need to find that thing.

 

[LaqueRatt]

Got the carbs pulled without any trouble. Just hope can get them back on as easily. Some sort of cam thing at the top slipped right out, not sure if it goes on just one way or not. Guess I'll find out.

Main concern right now is a wire that's attached to the bar on the right side. Looks like a ground to me, but grounded to plastic??? The other end has an eye that appears to not have been attached to anything. Looks like should go under a screw maybe, but don't see any place good to attach it offhand. Seems like it was running with it unattached so should I just leave it that way?

 

[LaqueRatt]

Got the carbs R&Red without too much trouble. Running much better. If anything it may be idling too high now. How do I set the idle down? Think I'm going to need a pic. I looked on the port side and see some screws that look like they could be adjusted, but not going to mess with stuff until know what I'm doing.

Tried to dial in the carbs and she's a lot happier, but still not sure if they're set just right. I read these carbs are backwards, Clockwise on the adjusters RICHEN it up. Started with them both at 1 1/2 turns out and found it ran better with them both 1 turn out. Am I doing it right?

Odd thing was the top carb was cleaner than the bottom carb. Like some meathead only cleaned the one that was easy to get to!

 

[MrGiggles]

In this diagram I believe it is the lower screw #6, the throttle arm should rest against it with the control in the idle position. Screws #10 and #13 are your base and WOT timing adjustments. The upper #6 screw should be the throttle stop for syncing the throttle and timing advance. Check the little rubber nubs on the screws, they can wear through and cause the timing to over advance, bad news!

This is on the arm where the throttle cable is attached, port side. A service manual would likely be a great asset to you.

https://www.crowleymarine.com/mercu.../throttle-lever-and-spark-advance-lever?ref=9
If you're running it on muffs it will idle a little faster than it would if the prop were submerged. Generally the idle speed and mix are to be set with the engine idling in gear, submerged.

Many times the lowest carb will be the nastiest, fuel tends to drain into it and settle there the longest.

 

[LaqueRatt]

Very cool of you to post that MrG! Don't know if ever would have found that idle setting on my own. Your other tips are also quite helpful.

 

[LaqueRatt]

Tried adjusting the idle stop. Doesn't seem to really be sitting on the stop, so only a little drop in revs. Could it be the throttle cables need adjusting? I see there are a lot of threads on them things. Going to try her out on the lake tomorrow. Hopefully the revs drop some. I can hear the gears engage......makes my skin crawl.

 

[MrGiggles]

Tried adjusting the idle stop. Doesn't seem to really be sitting on the stop, so only a little drop in revs. Could it be the throttle cables need adjusting? I see there are a lot of threads on them things. Going to try her out on the lake tomorrow. Hopefully the revs drop some. I can hear the gears engage......makes my skin crawl.

Yes sir, the cable is probably holding it. Hope it runs good out there.

 

[LaqueRatt]

So the muffs fell off while I was busy messing with the carb. Not sure how long she ran without water, but now it'll only pee after I rev it for awhile. So guess I need to replace the impeller. Or do I need to replace the entire pump housing? A quick search seems to indicate Merc impellers are all the same maybe? Or at least a lot of them seem to interchange. Is aftermarket OK for these or should I stick with Mercury?