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3.0 idle adjustment

J

John4003

Regular Contributor
Hey experts. What is your method of idle speed and mixture adjustment on 2001 3.0? The manual is very vague so this is what I did. I turned the speed screw all the way out and then back in two turns. The mix screw I turned out about 3 turns and then back in until the engine sped up and then backed it out a little. I had to turn the speed screw almost all the way back out though to get the idle at about 800.
 
Typically start with idle speed screw adjusted so it idles about 600-650 in fwd in the water. Note that throttle cable has adjustment on it so make sure throttle is returning to closed and is against idle speed screw. Mixture screw usually start 1&1/2 turns out. Turn it in (lean) until it starts to stumble, then back it out (rich) until it starts to stumble. Best idle is between the two typically. After doing mixture may have to reset idle speed again. Also should set ignition base timing if not done yet.
 
I would begin by confirming that the BASE ignition advance is correct. The low speed fuel/air mixture adjustments will have no effect on this once set.

If the engine idle speed is too high, adjust the idle stop screw.
Do what Dieter suggests regarding the throttle cable travel.

Using a manifold pressure gauge (vacuum gauge) adjust the low speed mixture screw.
You will be adjusting the mixture as to obtain the highest vacuum reading.

You may need to simultaneously adjust the idle stop screw as to maintain the correct idle RPM.

Keep in mind that the GM 3.0L engine is rather prone to what's referred to as "run-on" or "dieseling" upon shut down.
This is commonly caused by excessive cylinder temperatures and a lean low speed circuit fuel/air mixture.
The cure is to;

..... allow the engine to idle for a minute or two prior to shut down.
..... fatten up the low speed mixture.


.
 
Idle adjustment in a car is easy--the motor rocks on the mounts when the adjustment is far off. Not so in a boat, with its stiff mounts. I've had good luck using a vacuum gage to get an indication of best idle.

Jeff
 
RicardoMarine said:
Keep in mind that the GM 3.0L engine is rather prone to what's referred to as "run-on" or "dieseling" upon shut down.
This is commonly caused by excessive cylinder temperatures and a lean low speed circuit fuel/air mixture.
The cure is to;

..... allow the engine to idle for a minute or two prior to shut down.
..... fatten up the low speed mixture.


.
Click to expand...

And Mr Wizards know this how??????????????

I have never seen this myself............Must be a west coast thing.

When "run on" occurs it due to high idle setting or to much initial advance or both.
 
Actually, on this unique engine (with the carb sitting atop the exhaust manifold!) it's a really good idea to let it idle for a few minutes after a hard run and even pull the doghouse open after shutdown. (EFI would make these motors perfect!)

That said, mine never "dieseled" after shutdown so this one must be idling too high and/ or too lean.

Jeff
 
kghost said:
And Mr Wizards know this how??????????????

I have never seen this myself............Must be a west coast thing.

When "run on" occurs it due to high idle setting or to much initial advance or both.
Click to expand...

not trying to take a side as OP wasn’t asking about dieseling.but I did see this myself with my 1990 3.0 which my parents and then I had for 20 years. One weird observation I had was the end cylinders always had black and sooty plugs two inner cylinders were a nice ash grey.

With any 3.0 you are leaning on it pretty hard when towing several skiers back to back or tubing with the kids, basically at 3800 or 4K for an hour or so except to pick up skiers. I would get dieseling after long hard runs (with lower pitch prop for hole shot) letting it cool after wards by idling definitely helped. It was definitely worse when I dropped the float level a smidge (which leaned out the end cylinders) so I brought it back up.

Best things I found was making sure to stay up with impeller maintenance and decarbonizing the cylinders had very good luck with GM top engine cleaner also good exults with sea foam, both dumped down carb at fast idle and stalling it. Let it soak in cylinders for 20 min, when you start it back up I got a ton of black smoke. A couple of cleanings and This definitely reduced the dieseling on mine. I suspect this was from carbon build up getting hot after beating on it A little bit.

im sure other causes are possible but just my observations.
 
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Just to add.... Best way to set idle speed on a shake and bake 3.0ltr after mixture is adjusted.... Have someone at the helm, untie the boat from the dock, put it in gear and adjust the idle while the boat is moving.... Got this information a bunch of years ago from Phil Sims, Toronto Mercruiser Tech Account Manager. Works flawless.

As for the dieseling run on issue, there is a service bulletin for this issue... If I remember the fix was to fatten up the fuel mixture at idle and let the engine run a few minutes to cool the combustion chambers......
 
Better to adjust the idle with the boat tied to the dock, in gear. It's easy to be tricked with it moving.

Jeff
 
Agreed that lots of carbon build up in the heads will cause after running, but in a hard working boat engine? It's possible, I guess.

Jeff
 
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