Rough Idle on 1995 90hp

[Bragger]

Hi - I am new to this forum & to being a boat owner. I just purchsed my first boat: a 17ft 1995 Aluma-weld bass boat with a 1995 90hp Yamaha engine. The engine is emaculate - hardly looks like it has been used.

I have taken the boat out a couple of times and am very pleased, the engine runs great at any speed except at idle. The longer it idles the rougher it runs until it eventually dies (only takes a couple minutes). I took a look at the spark plugs and they look new.

Any advise on where to start for fixing the rought idle? New plugs, checking compression, gas addative, cleaning the carbs, Should I be using the high octane gas???

Thanks

[boats.net]

The part about hardly looks like it has been used may be the problem, the carbs may be varnished up and need to be cleaned. If the sits with gas in carbs the gas will turn to varnesh and plug up jets and passages in carbs.

[Ken]

Bragger -
If the motor was allowed to set up for more than a month w/o being run and w/o procedure for winter storage per the owner's manual, then the carbs could very well be clogged with gum/varnish.
I would do a shock treatment with SeaFoam or RingFree - sometimes this works. If not, remove the jets and squirt some Berryman's Chemtool into the orifices. If this does not work, then need to remove carbs, teardown, clean, and install a carb kit.
You need a Yamaha manual to guide you along for the above - can order online at www.yamahapubs.com.
And, no, you do not need gas rated any higher than 87 octane.
Good luck [img]smile.gif[/img] ,
Ken K

[Bragger]

Thanks - where can I find the proceedure for the shock treatment?

[Ken]

Bragger -
There are several differnt ways to do a "shock" treatment on your motor using a strong detergent like SeaFoam or RingFree.
I believe RingFree has a procedure on the can.
I personally prefer to use SeaFoam, and here is my procedure - works for me:

Two strokes build up a lot of carbon internally and need to be de-carbonized by doing a "shock treatment" with a detergent like Seafoam.
I like to do the shock treatment once a year on my C115 - in late winter when I am going thru my list of "spring make-ready".
If you use a detergent like Yamaha Ring Free or Seafoam all the time in your fuel tank, then the carbon buildup is not as bad. Still, a lot of 2-stroke owners will de-carbonize once a year - you will get various opinions on this.
I "shock-treat" my C115 by running the following mixture thru it. This will smoke up the area pretty good, and leave a lot of carbon on the drive way:
16 oz can Seafoam
3/4 gal gas
3 oz oil (2 if oil injected)
Disconnect the fuel line at the input to the bulb and attach a three foot piece of fuel line to the bulb. Mix the above in a small container -
I use an empty 5 quart oil jug. Set the container on the deck of your boat and insert the fuel line and secure to make sure the end stays on the bottom of the container. Pump up bulb and start outboard with earmuffs and water hose, of course -run a little above idle for 15 minutes and turn off. Let sit for 15 minutes. Run again for 15 minutes. Let sit 15 minutes. Repeat until all mixture is consumed. Reattach main tank hose to bulb and run regular fuel/mixture thru motor for 10 minutes - you want to get the concentrated detergent out of the fuel system.
Now, clean up the drive way before your wife sees all that nasty carbon and makes you get rid of your boat.

The shock treatment will not only get rid of a lot of carbon buildup, but also clean out the fuel system - will dissolve a lot of gum/varnish in the carbs.
Good luck [img]smile.gif[/img] ,
Ken K

[Bragger]

Ken, I will follow your method and see if the idle gets better.

Thanks for your help!