Fast idle issue.

[Ketchup]

1998 Yamaha 50 Four Stroke. First time out today. Started first crank of the key. Ran good across the lake, then when i pulled it into neutral, it idle'd up to about 3000 rpm on its own. When we would fish down the shoreline for a bit, and fire it up to move again, it would idle normal for a bit, then jump to that 3000 rpm on its own again. I would have to shut it off, as not to jam it in gear at that rpm. As soon as it fired, i would click it in forward. It ran really good while in gear, with the load of the boat, but when you click it in neutral, it would high idle after a few seconds.

I took the cowl off while it was running, and pushed on the throttle rod for the carbs, and got it to idle down, but as soon as i let go, back up it went.

Vaccum leak possible? If so how many vaccum lines, places that draw vaccum are there?

[fishinwrench]

It is not a vaccum leak. Since you were able to push the throttle cam back and get the RPM's back down it sounds like the throttle linkage is sticky or out of sync.

Or....If someone "untrained" has messed with the linkage they may have not set the throttle stop properly and now you have a bent throttle shaft or damaged throttle plate.

[Ketchup]

Thanks FW"

could it be the ISC valve? Or the ECU?

[fishinwrench]

I assumed it was a carb motor (not EFI) since you said you "pushed on the throttle rod for the carbs".

There is no ECU on a carbureted motor.

If it is a EFI motor equipped with an ECU then forget what I said above. It is likely some static in the TPS (throttle position sensor) which acts like a "volume knob" on a radio.

[Ketchup]

See, im showing my lack of outboard knowledge. Yes its a carbed motor.

The idle speed control valve, from what ive read, could possibly be it, but not reading how to check that. If im reading it right, this valve controls the air intake, thus controls the rpm's either low or high.

The only maintenance done has been done by a yamaha mech. at the shop. Tinkering with the rods that connect the carbs today while on the water, they seem to operate smoothly. At the very bottom, there looked to be an idle screw. That is what i pushed on to get the idle down. It was hard to push in, and as soon as i let go, idle jumped back up,

Could something in a jet cause this?

[fishinwrench]

Increased RPM is caused either by advancing the ignition timing, or by opening the carb throttle valve(s).

Nothing else can cause your engine to speed up.

I haven't looked it up specifically on your 1998 50 (carb 4S) but I don't think it has an idle control valve. Again, you appear to be studying stuff relevant to EFI engines.

[Ketchup]

Thanks FW,

My knowledge of mechanics is automobile only, so my comments on maybe a vaccum related issue relate to that. On a vehicle, if you have a vaccum leak, it will change the rpm's, depending on how severe the leak. Thats a carb motor.

My outboard is doing the same type of thing. Start it, let it idle for a bit, and the rpm's go from 800 to 2000 or more on its own. If i kill the engine, and fish for a bit, and start it back, same deal.

Here are a few pictures I just took. The picture of my finger pointing to that screw shows what i pushed, to get it to idle down, but after about 5 seconds, it rose in rpm, even pushing that screw/rod. That is attached to 2 other rods, one on the right, and another on the left. I looked the motor over good this morning, and see nothing abnormal. But,mive been known to overlook things. I am a guy.

Just stumped, without vaccum, that rpm can rise without touching throttle. Anything i can look at, test,replace, etc. without getting into a nickle and dime type of waste?