Re: Yamaha 25hp, Idle issue, carb rebuilt, whats next?
There is an adjustable screw with a spring on it that adjusts idle speed. It's likely on the middle carb. It's easy enough to just adjust the idle speed, but don't do it! First, you have to try to fix the problem before just circumventing it.
Besides, there is a recommended number of RPM's for idle in your manual, and artificially increasing it without addressing the underlying problem is foolish. I think your problem is a clogged idle passage in the carbs. I know you already, yada yada yada. What you need to do is take the carbs apart and look for the tiny pin holes in the top of the carb throat. There is usually one for idle speed, which is on the inside of the butterfly plate when the plate is fully closed. This is where fuel enters the motor at idle speed. As you open the butterfly plate, you will expose a series of pinholes, perhaps 3 or 4 or them. Through these holes is how the motor gets fuel as it transitions from idle speed to high speed. The copper tube at the bottom of the carb throat is the high speed jet, and this is how the motor gets fuel at higher speeds.
Now that you know where fuel must flow from in order to achieve normal operation at idle speed, what you need to do it take the carbs apart. Take the top cover off and follow the route that fuel takes to get to that pin hole. If you trace it all the way, it will pass by an idle screw, and will lead back to the float bowl. It's ok if you can't tell exactly where the "circuit" goes, because some of it is hidden inside the carb. All you need to do is know that the passage from the carb bowl to the idle's pin hole is clear. If there is a little rubber bumper below the carb next to the high speed jet, remove it. It exposes another jet. You can cover the high speed jet from inside the carb throat with your finger while spraying carb cleaner into either the high speed jet or the other jet from underneath. Carb cleaner sprayed in either one of these should come out the idle pin hole at the top of the carb throat. Careful, most of the carb cleaner will shoot out of a vent on the top of the carb. Doesn't matter. What's important is that fuel is coming out of the idle's pin hole. Once you achieve this, your motor should stay running at idle. If this sounds too involved, you could take the easy way out and remove all plastic and rubber parts to the carbs and soak them in Techron for a couple days. This will clear out all of the jets and passages. After removing them from the bath, then spray carb cleaner through all the orifices you can find.
I think it's a better approach to learn the circuitry and attack it with carb cleaner, but if it looks too complicated, the bath is the idiot proof method. Try the circuitry approach first. If it doesn't work, then bathe the carbs.
Once you reassemble the motor, it should run fine in idle. If it doesn't, you can adjust the idle speed screw, but only if you know the idle RPM's from the manual, and only if you have a tachometer. Otherwise it's not a good idea to play with it.
