Mercruiser 3.0 L Quick question about idle RPM

[eavega]

So after some adventures with a Pertronix ignition conversion, I got the motor running, and figured I would verify the idle timing (which was where the whole ignition conversion journey began). The manual states to set the idle timing in gear @ 700 RPM. Now, my motor on muffs will idle at about 1000 RPM. I recall on outboards that the "idle on muffs" RPM is one thing, the "idle in gear on the water" is another number due to the increase resistance of the prop in water. Does the same apply to this motor? Should I be looking to set the idle RPM (and the idle timing) while on the water with real resistance on the prop, or should I be setting the idle RPM to 700 on dry land on muffs?

For the record, my motor is running at 8* BTDC at 1000 RPM. Shifting into gear does not change the idle RPM at all. The manual and the motor specification decal says that it should be 6* BTDC at 700 RPM in gear. Motor is running smooth as silk.

Rgds

 

[Don S]

Re: Mercruiser 3.0 L Quick question about idle RPM

You need your timing at 6? BTDC at idle rpm of 650 to 700 rpm, not 1000. At that rpm you will be advancing the timing.

 

[eavega]

Re: Mercruiser 3.0 L Quick question about idle RPM

As always, thanks for the quick Reply, Don S. The real question I have is should I set the Idle RPM on land with the motor running on muffs, or should I set/verify the idle RPM in the water in gear under load? in either case I know that the timing needs to be set with the motor turning 700 RPM. I'm just worried that if I set the idle RPM at 700 RPM on muffs, then when I get it in the water under load at idle, the resistance on the prop will cause the RPMs to drop too low and stall out the motor.


Rgds

 

[Don S]

Re: Mercruiser 3.0 L Quick question about idle RPM

Sure, set the idle rpm to 650-700 on land so the timing is right at that rpm. Then adjust the rpm to 700 to 750 in neutral. if you go much above that, you will end up with dieseling problems.

 

[Bondo]

Re: Mercruiser 3.0 L Quick question about idle RPM

As always, thanks for the quick Reply, Don S. The real question I have is should I set the Idle RPM on land with the motor running on muffs, or should I set/verify the idle RPM in the water in gear under load? in either case I know that the timing needs to be set with the motor turning 700 RPM. I'm just worried that if I set the idle RPM at 700 RPM on muffs, then when I get it in the water under load at idle, the resistance on the prop will cause the RPMs to drop too low and stall out the motor.


Rgds

Ayuh,.... If that happens, just turn up the idle screw to get 650/ 700 rpms...

After it's in the water...

 

[fishrdan]

Re: Mercruiser 3.0 L Quick question about idle RPM

From experience with my 140 Mercruiser... Set timing at the proper idle RPM (700) out of water and lock it down, then bump up the carb's idle adjustment to 750-800RPM. Once the boat is in the water, the engine is warmed up, and it's idling in gear (not tied to the dock) adjust the carb's idle to 700RPM. Play with the idle speed and idle mixture screws to get best idle.

If you are lucky, the dry land 750-800RPM idle setting will drop down to the proper idle while in the water and you won't have to mess with things.

 

[eavega]

Re: Mercruiser 3.0 L Quick question about idle RPM

Okay, one further question about setting the idle RPM. The manual is not terribly forthcoming with images identifying parts when you need them. On the procedure to set the idle speed in the #10 manual, the step I have some confusion on reads "Disconnect throttle cable barrel from the anchor stud. Be sure not to lose spacer or anchor stud". Are they referring to the post where the throttle cable connects to the throttle cam on the carb, or the brass connector where the throttle cable runs through which is at the top of the motor towards the back. A picture or diagram would be awesome.

Also, when you re-connect the throttle cable, I would assume that you can't let the cable move the cam any, or else it will re-set the idle speed, correct?

Any help here would be appreciated. Diagrams or pictures would be especially helpful.

Rgds

 

[brnschoneck]

Re: Mercruiser 3.0 L Quick question about idle RPM

Hmmm good ??? I use to have this motor im thinking u wanna just disconnect the cable where it moves the throttle arm at the bottom back side not for sure but i think sooo !!!!i beleive its just a nut holing it on dont loose it ...I BELEIVE IT KINDA POPS IN LIKE A BICYLCE BRAKE CABLE MAKE SENSE ????

 

[eavega]

Re: Mercruiser 3.0 L Quick question about idle RPM

Hmmm good ??? I use to have this motor im thinking u wanna just disconnect the cable where it moves the throttle arm at the bottom back side not for sure but i think sooo !!!!i beleive its just a nut holing it on dont loose it ...I BELEIVE IT KINDA POPS IN LIKE A BICYLCE BRAKE CABLE MAKE SENSE ????

That is what I see as one of the two places where you can disconnect the cable, but that location doesn't fit in to what the step indicates, as there is no mention of the spring washer but it does mention a spacer, which according to the diagram is at the brass connector.

Rgds

 

[Don S]

Re: Mercruiser 3.0 L Quick question about idle RPM

Have a look at page 2A-8 of your #10 service manual.

 

[eavega]

Re: Mercruiser 3.0 L Quick question about idle RPM

And that, Don S, is what I was looking for! Thank you so much.

-Rgds

 

[Don S]

Re: Mercruiser 3.0 L Quick question about idle RPM

This might even help a little more.

View attachment #10 GM 4 Cyl 85-89 68.pdf

 

[achris]

Re: Mercruiser 3.0 L Quick question about idle RPM

Sure, set the idle rpm to 650-700 on land so the timing is right at that rpm. Then adjust the rpm to 700 to 750 in neutral. if you go much above that, you will end up with dieseling problems.

and a broken gearbox.

 

[eavega]

Re: Mercruiser 3.0 L Quick question about idle RPM

thank you for the additional info Don S. The notes will be followed.

rgds