Good afternoon,
I've had this Mariner 75HP (grey, ugly, Mercury) for a number of years and I've never been able to make it run really well. The primary thing that bugs me is it just doesn't run well at mid-throttle, say 3,000 - 4,000 RPM, which just happens to be where I like to cruse, lower then 2,500 and the boat really isn't planned out. What it does is just randomly miss in that range, you can even see the tach wiggle. Sometimes it will loose RPM too, you will be cursing along at 3,000 and then you will feel it start to miss and the RPM will drop to 2,800. Above 4,000 it goes like mad and when the boats trimmed out it sits just a hair under 5,500 RPM which is just about perfect. It also seems to want to miss more when I have a boat load of people.
The other issues I have is I can adjust the carbs so I get a good start and idle but crappy acceleration or I can richen then up a tad and get good acceleration but the engine is hard starting and just doesn't idle well.
This week it wouldn't start at all for me. I pulled the plugs and #1-#3 looked pretty normal but #4 was completely washed, no dark spot at all. I threw in a used set of buhw NGKs that I had before I realized it was suppose to have buhw-2. It started after that and ran but still had the mid-throttle issues.
Here's what I know:
I've checked the compression a few times and all cylinders have tested good consistantly. I don't recall the numbers but I'm pretty sure they were all over 150PSI and with in a PSI or two of each other.
I've rebuilt the carbs, I'm usually pretty good with a carbs and these were pretty simple. The only thing I ever questioned my self on was float level. The floats had funny bottoms on them so getting a precise measurement was kind of a crap shoot. I've also rebuilt the pump and updated the cover to the new metal one and threw that old leaky plastic job away.
It has all new bleed restrictors.
Every electrical ignition component has been changed out other then the coils. I got the engine because of mouse damage, the stator and trigger are CDI brand, the switch is a used OE Merc and it has new plug wires too. The timing was reset to service manual spec.
If I'm will to compromise on acceleration then it's a really good starting and idling engine, well as good as an old 2-Stroke can be and it runs strong at 4k and over. I don't think it's a mechanical problem. I'm planning on throwing an new set of buhw-2 in first thing since that's cheap and who knows what shape the plugs are that are in there. I may tinker with the idle circuit a bit more since that's not all that hard to do. Other then that I'm stumped and open to any ideas.
Sorry for being wordy, I just wanted to makes sure I got all the details in.
Thanks,
K
I've had this Mariner 75HP (grey, ugly, Mercury) for a number of years and I've never been able to make it run really well. The primary thing that bugs me is it just doesn't run well at mid-throttle, say 3,000 - 4,000 RPM, which just happens to be where I like to cruse, lower then 2,500 and the boat really isn't planned out. What it does is just randomly miss in that range, you can even see the tach wiggle. Sometimes it will loose RPM too, you will be cursing along at 3,000 and then you will feel it start to miss and the RPM will drop to 2,800. Above 4,000 it goes like mad and when the boats trimmed out it sits just a hair under 5,500 RPM which is just about perfect. It also seems to want to miss more when I have a boat load of people.
The other issues I have is I can adjust the carbs so I get a good start and idle but crappy acceleration or I can richen then up a tad and get good acceleration but the engine is hard starting and just doesn't idle well.
This week it wouldn't start at all for me. I pulled the plugs and #1-#3 looked pretty normal but #4 was completely washed, no dark spot at all. I threw in a used set of buhw NGKs that I had before I realized it was suppose to have buhw-2. It started after that and ran but still had the mid-throttle issues.
Here's what I know:
I've checked the compression a few times and all cylinders have tested good consistantly. I don't recall the numbers but I'm pretty sure they were all over 150PSI and with in a PSI or two of each other.
I've rebuilt the carbs, I'm usually pretty good with a carbs and these were pretty simple. The only thing I ever questioned my self on was float level. The floats had funny bottoms on them so getting a precise measurement was kind of a crap shoot. I've also rebuilt the pump and updated the cover to the new metal one and threw that old leaky plastic job away.
It has all new bleed restrictors.
Every electrical ignition component has been changed out other then the coils. I got the engine because of mouse damage, the stator and trigger are CDI brand, the switch is a used OE Merc and it has new plug wires too. The timing was reset to service manual spec.
If I'm will to compromise on acceleration then it's a really good starting and idling engine, well as good as an old 2-Stroke can be and it runs strong at 4k and over. I don't think it's a mechanical problem. I'm planning on throwing an new set of buhw-2 in first thing since that's cheap and who knows what shape the plugs are that are in there. I may tinker with the idle circuit a bit more since that's not all that hard to do. Other then that I'm stumped and open to any ideas.
Sorry for being wordy, I just wanted to makes sure I got all the details in.
Thanks,
K