Idle adjust on 24 hour 4.3 with old fuel

[ricohman]

I finally got on the water for the first time this year. My 2011 boat has a 4.3 TKS. This is a Starcraft Islander with a massive fuel tank. I know the gas in there is nearly 2 years old. Boat has 24 hours on it.
Anyway, the idle in nuetral is low. About 550-600 according to the tach (which may be inaccurate). When its idling in nuetral I can hear some kind of rattling from both sides of the engine. I am an auto mechanic and I've never heard this type of thing before from a 4.3 so I was puzzled.
Seeing as I have 3/4 tank of crap gas I cruised around for many hours to burn some of this fuel off. I ran it for 3 hours straight and it runs great except for the idle thing. It will run just fine in gear at idle.
If I leave it in gear with the throttle back to minimum it runs fine with no noise as the idle is slightly faster (I am going to put my tach on this). I have had a quick look at the carb and the idle screw it out in the open. The noise seems to be related to the low rpm. All gauges read normal.
My question is this. Can I adjust this screw? This is hooked to the cable and that also has adjustment. I realize this may be related to the fuel or it just might be the carb needs adjustment. In my experience I have found engines usually speed up at idle once they are run in.
I am buying a factory manual. And I am going to do my best to minimize the amount of fuel left in the big tank. Last summer my dad got sick, and later died. I had no time for any fall fishing, but the fuel was treated. Its been a hell of a year.

 

[Fun Times]

Hi there, if you suspect bad fuel then a good test would be to run the engine on good gas out of a separate fuel tank to see if you get your normal idle speed back.

One thought for the noise you hear could be from your exhaust flappers located inside the exhaust Y pipe due to too low of an RPM letting them slap the Y pipe more than usual. Item #7,http://www.mercruiserparts.com/Show....png&inbr=12833&bnbr=230&bdesc=Exhaust+System

If you do adjust the throttle speed screw it is best and recommended that you remove the throttle cable from the carb stud, adjust the idle speeds and readjust the throttle cable barrel as needed. This step is to help avoid putting to much tension on all the moving components. You need some tension but not a lot.

Though your legally not supposed to, if you plan on adjusting the idle mixture then you would need to buy the mixture adjusting tool 91-866201. Google search the part number.

If you search online service manual #41, you'll be able to find it. Posting the manual violates new forum rules.

Hope this helps, good luck.

 

[ricohman]

Hi there, if you suspect bad fuel then a good test would be to run the engine on good gas out of a separate fuel tank to see if you get your normal idle speed back.
One thought for the noise you hear could be from your exhaust flappers located inside the exhaust Y pipe due to too low of an RPM letting them slap the Y pipe more than usual. Item #7,http://www.mercruiserparts.com/Show....png&inbr=12833&bnbr=230&bdesc=Exhaust+System
If you do adjust the throttle speed screw it is best and recommended that you remove the throttle cable from the carb stud, adjust the idle speeds and readjust the throttle cable barrel as needed. This step is to help avoid putting to much tension on all the moving components. You need some tension but not a lot.
Though your legally not supposed to, if you plan on adjusting the idle mixture then you would need to buy the mixture adjusting tool 91-866201. Google search the part number.
If you search online service manual #41, you'll be able to find it. Posting the manual violates new forum rules.
Hope this helps, good luck.

I never thought about the shutters. I don't know what they are made of but it could explain the rattling from both sides. If I idle it up 100rpm the motor is quiet. I'm not too concerned about it now! Thanks!

 

[NHGuy]

My OE metal shutters did that because I burned off the silicon covering that would have quieted them. Don't put the idle too high, that causes dieseling and hard shifting that can damage stuff. Just deal with the rattling by raising the engine speed in neutral and throttling back before you shift.

 

[ricohman]

I hope these shutters aren't burned out at 24 hours. The motor has never been run without water from muffs or the lake. I'm not sure what you mean by increasing the idle in nuetral as its locked in position. Of course I can push the center button and increase the rpm if thats what you mean.
At idle the motor is shaking and seems on the verge of stalling. I know that the rpm dips to 400 or so on the tach. I've got to run the tank out before buying new shutters.
After my run it did settle down and the rpm was at least 100 rpm higher. I've had some bikes that ran like this on old ethanol fuel.

 

[NHGuy]

I think the right idle is 725-750 in neutral, it will drop to 675 in gear. And you're right, just keep the button pushed and idle it up a touch if you don't want to hear the rattle. If you are like most folks you won't leave the bad flappers in for too long.

 

[ricohman]

I turned the idle screw up about 1/6th of a turn. The rpm was 650-700 but still a bit rough. I put in a bottle of ethanol treatment and 50l of premium fuel with no ethanol.
After an hour of cruising around on the lake the idle is much smoother and the rattle is gone. Runs great.
I think I'm going to store the boat with premium fuel. I can't stand ethanol and the problems it causes.