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75HP Mariner having issues - Poor running/starting - How to tune/check oil mix?

BigTom209

New member
I have an older early 1990's 75HP Mariner boat motor and I'm looking for advice.



It's always been a super reliable motor, we have done minimal maintenance to it over the years and usually we just go out to the cabin in the summer, pop in a battery and hit the water and away we go. This last year, the lack of maintenance I think has finally kicked us in the ass.

It's having a hard time running. We can get the motor to fire up, idles kindof rough but it does idle, but putting it into gear often stalls it out. Sometimes if you rev it up and then as soon as it idles down put it in gear it'll catch and off you go. Anyway you cut it, not overly reliable.

I did some checking and it seemed like it was getting WAY to much oil, the plugs were old likely my dad was the last to replace them so probably 10+ years old so I put new plugs in and fired the motor up and shut it down probably within a minute and checked the plugs and they looked soaked in oil. I noticed there is a valve or pump on the oil which was adjustable by moving the threaded rod so I spent some time tuning and tweaking it, and basically giving it less oil (motor was on dry land with ear-muffs on providing water) and we got it to the point it would fire up nice, idle decent, and there was still some blue smoke and the plugs had a bit of oil on it so I wasn't worried about lack of oil, but taking it out on the lake it was still not right and having a rough time. And I didn't like restricting the oil past the "marks" on the valve cam.

So... it's got issues. I figure best thing to do this year is to start off by replacing the fuel filter and I ordered a fuel pump diaphram kit since that's cheap/easy and being almost 30 years old probably it's not working very well either. Once I get the fuel pump and filter replaced I'll put the oil valve back to where it was but I'd like to properly adjust it if possible, but I don't know how? I've heard/read somewhere to run pre-mix, and remove the oil line and run it to a measuring cup, then letting the motor idle for like 10min measure the amount of oil in the cup and tune that way?

I'd like some of you guys advice if I'm going down the right path or if there is other things I should be looking into. I work on vehicles and farm on the side so I'm very handy with pretty much anything mechanical, but oil injected 2 stroke boat motors... can't say I'd done much with them.
I've attached some pics, of the motor, the before/after on the oil cam, and just an overview.
 

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Reducing the amount of oil can lead to disaster for a motor!!----Start with a compression test on this motor.

Judging by the smoke in the exhaust and evenly oil soaked plugs and the fact this was being tested almost entirely off-water under basically no load, I'm not worried. When I rebuild the fuel pump and replace filter, then I'd be worried leaving it as-is which is why I'd like to make sure it's tuned properly.


I will take a compression tester and check next time I'm out, but judging by the fact once in gear it would run fairly normally and never had normal indications of a misfire, I doubt that will be an issue but it's worth a quick check.
 
You did say you took it out on the lake after " tuning and tweaking " on the oil pump !-----But good luck with the project.
 
You did say you took it out on the lake after " tuning and tweaking " on the oil pump !-----But good luck with the project.


Yes I did say that, I'll be more specific


Once we tuned down the oil "cam" position (I have no idea what it's actually called) on land with the earmuffs on, the motor was able to start and idle and go into gear and spin the prop. But when I would run it for a very short period of time, ie maybe a minute, and then check the plugs they were still oil soaked and there was still seemingly alot of smoke coming out of the exhaust so again I'm not worried about a lack of oil. We took the boat out and launched it and putted around a few hundred yards in the bay just to see if it had improved or not, it still would stall when putting in gear in the water. We never once went for a spirited drive because that'd be kindof silly with a motor I didn't trust I'd prefer to be within paddling range.

So, in my head I have an issue with the fuel/oil mix... I seem to have way to much oil the only way I could get the motor to even run half decent was to seemingly turn down the amount of oil it was getting and it still seemed to have to much. Now, the fuel filter is after the pump and therefore after the fuel/oil is already mixed... so I wouldn't think it's that, same kindof goes for pump itself it's obviously working (or at least somewhat) because the motor will run... but, the diaphrams kit for the pump and a new filter is like $50 so I'm going to replace it anyway. I may even go so far as to replace the fuel line entirely because if there is something plugged further down (ie in or around the fuel tank) maybe the pump is working, but instead of sucking in fuel it's sucking in excessive oil? Dunno.


So, now the point of this thread and the title "How to tune/check oil mix"
I can set the "cam" back to where it was previously, not hard that's why I took pictures. But I'd prefer to actually know how to set it as-per factory spec. I will still check the compression just because it's not a bad idea and takes a few minutes, but based on alot of motor mechanic experience I'm thinking it's not a problem... it may very well be which is why I'll check, but when it is idling the motor is not shaking, all the plugs are equally wet... there just doesn't seem like a tell tale sign of a bad cylinder
 
Okay Tom. The adjustment on the pump should never have to be changed, so we have another problem. You are going to find, like I did on my 115, that the pump needs replacement or overhaul. Set the new pump to specs and you will be in great shape. Don't forget to replace all lines and hoses while your into it. Racer is one of our top techs and his advice is golden.
EDIT: Racer beat me to the post. We are on the same page.
 
Ok, was out at the cabin today checking on things and tinkered a bit.

Compression from top to bottom
108
104
109

This motor hasn't been turned over in probably 6+ months so I'm happy with those numbers considering the cylinders may even be a bit dry.

Removed the fuel pump and filter, and brought them home. Good news the pump is rebuilt and ready to go back in, bad news it was exceptionally clean and in good shape for it's age. The gaskets all were still supple, rubber seals all bend freely, pump housing itself was clean just a quick wipe down with a rag and it looked near new. Fuel filter I could easily blow thru so it doesn't appear plugged up either

So I was hoping to find maybe a little something off... but nope.
 
Also spoke with the mercury mechanic when I picked up the seal kit, he said they always just line up the notch on the cam to the other notch. I asked about if they ever remove the lines to measure the fluid after so many minutes etc and he said you can, but they never do.

And the original picture, the marks were "slightly" off from lining up, I wouldn't think enough to really cause much of an issue though.
 
To me threads like this are just more proof that lubrication in 2 stroke engines is not well understood !!


I'm really happy to hear you're starting to understand and catch on!

4 stroke engines, I've rebuilt probably a dozen from the block up... diesels, helped rebuild a semi and combine engine... 2 stroke engines, I took the theory of how they work in shop class back in the late 1990s, and besides that I mix 1 jerry can of pre-mix for my chainsaw once in a blue moon.


So if I'm doing something wrong, tell me. If my information is wrong, tell me. If you'd like me to do more testing or checking, tell me. All I have is the information what I'm seeing and what I've done up until now. The boat is also at my cabin and not in my driveway so I'd like to have a list of things to check/do and next time I'm out that way.
 
Have you checked to make sure the linkage rod for your spark advance didn't come off. Idle is controlled by timing on that motor. That is what happened to mine and it ran bad and would die before you could get it into gear. Smoked a lot too. Just a thought. Like someone else already said, cutting back the oil is a recipe for disaster.
 
Have you checked to make sure the linkage rod for your spark advance didn't come off. Idle is controlled by timing on that motor. That is what happened to mine and it ran bad and would die before you could get it into gear. Smoked a lot too. Just a thought. Like someone else already said, cutting back the oil is a recipe for disaster.


Which linkage rod are you talking about exactly? Is that the rod I think that goes under the flywheel to adjust the timing? just going off memory so that statement could be totally wrong

I did tinker with it a few weeks ago when I was at the cabin.

Rebuilt the fuel pump as previously mentioned, got it installed with new fuel filter, also drained all the old oil and replaced with new. While I was there I also removed the carbs and tore them down, same as the fuel pump inside shockingly clean everything inside looked damn near new... I’ve torn down a lot of carbs and for a nearly 30 year old motor with low maintenance done to it I was shocked, figured they’d be at least slightly gummed up but nope.

Still did my my best to check all jets and passages and cleaned with carb cleaner and blew air thru everything. Put them back on and everything back together.

Put the oil pump back to where it was before so that the marks line up so it’s back to where it was

Brought in new gas, primed everything up with the primer bulb


Same thing. It was really hard to start, wound over lots but eventually when it did fire up it seemed to idle better and with the motor just in the bucket of water it seemed to be able to go into gear but I couldn’t keep it in gear with just the way the bucket was. We hauled the boat to the launch and backed in the motor to try and see but couldn’t get it to start at the launch.

So gave up for the moment. Did a compression test after wed been mucking around with it and running it a little 120-119-120 so almost all exactly the same and improved since the spring when I checked it dry



So, I’m starting to lean towards timing or spark being my issue. I was going to check the timing, only to figure out the cover for the flywheel with the timing window is missing! My dad must of removed it since we bought this motor new so no reason it shouldn’t be there. Spark wise, there is spark but maybe it’s weak? Not sure. But I don’t think it’s a fuel issue... can’t really be a air issue... got compression... maybe somehow the timing is way off?? Or weak spark?? Dunno. Trying to find a timing cover now, found someone with a 90HP mariner which I think is the same and he said we could borrow the cover to try it out. But not sure what the timing is supposed to be set for
 
Yes, the one that goes up under the flywheel. It must be hooked up then! My advice is to take a step back get ahold of the service manual for that motor and check all the basics. You will need a spark gap tester, a dial indicator, timing light, and probably a DVA adapter if you own a digital volt/ohm meter. Another thing you should do since your motor has not had much service is to do a voltage drop test. Here is a link that might be helpful http://www.cdielectronics.com/troubleshootingguide/ Good luck.
 
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