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1998 Yamaha 75HP not making full power [warning newbie]

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snp1973

New member
Greetings,

I've found myself caring for my parent's 20' pontoon with a Yamaha 75 2 stroke. It ran fine last year; I've put 4 hours on it this season and it's not getting up to full power. 3900 or so RPM's and 15 knots when the norm has been around 5200 and 21 knots. I fly jets for a living, not a piston guy let alone knowledgeable about marine engines but I checked the plugs... they seem ok. The fuel filter was clean, super clean. Engine only has 122 hours on it. My gut is that it is something simple... shop can't get out there for three weeks but said to change the plugs anyhow. Looking for anything I can do/check that is reasonable, keeping in mind I know nuthin'. :)

It's the C75T - I mix the oil/gas at 1:50... used a fresh tank of non-ethanol 89 octane.
 
Last edited:
Are you mixing the fuel at 1-50 or 50-1.
Difference is 50-1 = 50 parts fuel to 1 part oil which is what it should be.
1-50 is 1 part fuel to 50 parts oil.

It sounds like you are running on 2 cylinders easiest test is a cylinder drop test. Take a plastic pliers and while it is running pull the spark plug wires off 1 at a time and see if the idle changes. The plug that has no effect on the idle with the wire off is the one that doesn't work. Or you can get a spark checker and make sure each plug wire jumps an air gap 7/16"
 
I say another fuel flow issue. Old ethanol fuel has gummed up as the retirees don't use it enough, won't spend the $ on 100% gas if its available and won't pay to add fuel treatment at every fuel fill all the time. Check my response to another posted tonight. Welcome to the joys of this crap 10%ethanol gas we've had forced on us.
 
Thanks for for the input. It's 50:1 :)

If a cylinder is offline we are perhaps looking at replacing spark plug wires/plug boot?

flyingscott said:
Are you mixing the fuel at 1-50 or 50-1.
Difference is 50-1 = 50 parts fuel to 1 part oil which is what it should be.
1-50 is 1 part fuel to 50 parts oil.

It sounds like you are running on 2 cylinders easiest test is a cylinder drop test. Take a plastic pliers and while it is running pull the spark plug wires off 1 at a time and see if the idle changes. The plug that has no effect on the idle with the wire off is the one that doesn't work. Or you can get a spark checker and make sure each plug wire jumps an air gap 7/16"
Click to expand...
 
Ok so it's been a long process, getting on the service call schedule for a marina during the boating season. It was in fact running on 2 of the 3 cylinders. The shop cleaned the carburetors and installed a new fuel pump. It's running perfectly, now. I learned a few things in the process and am thankful for the input I received here.
 
snp1973 said:
Ok so it's been a long process, getting on the service call schedule for a marina during the boating season. It was in fact running on 2 of the 3 cylinders. The shop cleaned the carburetors and installed a new fuel pump. It's running perfectly, now. I learned a few things in the process and am thankful for the input I received here.
Click to expand...
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Glad it's running but not glad to hear you lost so much boating time waiting for Yamaha Mafia Service and that they sold you a new fuel pump you probably didn't need if I were to guess. At this stage, who knows? I won't even ask what the bill was as I know it was a big bill but, then again, it's your parent's boat, you don't have time to wrench on it yourself as you make your $ flying jets and under those circumstances it's best to just throw some money at it, get it fixed, and know mom and dad can take the boat out and not worry about it if they chose. The Yamaha Service Mafia has to make a living too (although we do our best here to see to it they don't eat steak for dinner every night as most of us don't). Now go have fun with family and friends between legs!
 
Run boat more often to avoid fuel issues, Add fuel treatments on every fillup. Follow storing instructions after season is over. Running fuel out of system at end of year might help to avoid same issue next year. there will be pros &cons on this subject.
 
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