4.3 V6 Mercruiser fast idle

[natureschild88]

I have a 1990 Mercruiser 4.3L V6 (2 barrel Mercarb) that has a fast idle. Sorry, no serial number on hand. At start up it revs at 900 rpm and after running for a minute it goes up to 11 or 1200 rpm. I recently rebuilt the top half of the carb (accelerator pump, float, cleaned out ports and jets). Before I cleaned out the ports, half of them were clogged shut. I had the idle dialed in at 650 rpm after I rebuilt the top half of the carb.

The idle has slowly gotten faster and faster in the past month and a half (I've taken it out once a week). I tried adjusting the idle screw, but it doesn't change anything. I had to pull the screw all the way out, and the idle finally dropped to 500 rpm then died. I don't think there is a vacuum leak, as there are no hoses that go to the carb.

I can get it to idle at 700 rpm by covering the idle screw hole half way with my thumb, and it jumps back up to 1000 when i put the idle screw in without even starting the threads. I took apart the top half of the carb and cleaned out all the jets again. I shot a bunch of carb cleaner into the hole for the idle screw and it made no difference.

Any ideas as to what can be causing this?

 

[achris]

Re: 4.3 V6 Mercruiser fast idle

Before adjusting the idle speed, you need to remove the throttle cable... Once you have the idle speed correct, then adjust the barrel on the cable so the throttle cable puts a VERY slight amount of pressure on the carb (towards closing it).... refit the cable....

The only other thing that can cause the idle speed to increase is if the timing is advancing... Check the dissy isn't moving... (Grab it and try turning it with your hand. Engine off of course)...

Chris......

 

[NHGuy]

Re: 4.3 V6 Mercruiser fast idle

You also may have air leaking, Is the hole you are covering the idle mixture hole? The mix screw is turned down softly til it stops then backed out a set amount (you'll need to look it up)for your initial setting. This controls how much air & fuel mix at closed throttle settings. Run the motor til warm, adjust the idle SPEED screw on your throttle linkage to your 650 rpm. Set the idle MIX screw halfway between stumbling and bogging, and reset the SPEED screw after the MIX is right. Hopefully that makes sense.
Check that the screws that hold it to the intake are snug, and any that hold the carb together need to be tight too. The intake manifold needs to seal to the heads, check those bolts also.

There was so much crud in it when you first opened it up that I'd redo it when you can. There is very possibly more junk in some passages in the base.
I'd take the whole thing off. Disassemble it, dip all the parts that have orifices and passages. Clear them out, make sure air gets through where it should and not where it shouldn't. Replace the gaskets. Carefully set the float exactly to it's spec. Those little machinists rulers with a sliding T on them are great for setting float height. They cost $4 at Harbor Freight. Find every orifice, blow air backwards through any passages you find. Poke little bits of wire through them, they have to be clear of obstructions.

 

[natureschild88]

Re: 4.3 V6 Mercruiser fast idle

Thanks for the help guys. Checked the distributor first, felt tight. I did not actually check the timing but the motor sounds smooth, so I don't think its too retarded or advanced.

Didn't even think about the idle SPEED screw!! :facepalm:

Brought her back down to 600 rpm, set idle mix screw to a good sounding heart beat, now she's purring again.

With how many times I've taken apart the carb now, I must have knocked the speed screw around a bit here and there. For how much build up and crud I had coming from my tank I was worried I was gonna have to take the whole carb off and rebuild from the ground up (I've been trying to avoid buying the full gasket kit and only take it apart to the half way point where the gasket is reusable)

Ready to hit the bay and do some serious damage this evening now that I got my slow troll back :eagerness: :fish2: