Idle RPM 3.0 OMC Cobra

[Sinko]

My 1990 3.0L OMC Cobra idles at approximatly 600RPM. It's rough (borderline stalls). What should it be? I also noticed a pully wobbling which could help with the shaking.
Sneak in one more question...it's pushing a Sunbird Corsica 175 which is slow out of the hole. Could this be the 17? prop or poor engine performance?

 

[Don S]

Re: Idle RPM 3.0 OMC Cobra

The prop has absolutely nothing to do with how the boat idles in neutral or in gear.
I would check for spark on all cylinderas and do a compression test?
When was the last time you had a full tuneup?

 

[Silvertip]

Re: Idle RPM 3.0 OMC Cobra

Ummm -- let's take these one at a time:

1) Almost stalls & runs rough: Solution: try a tune up
2) Vibration: Solution: Fix the wobbling pulley and then see #1 -- that's probably not normal!
3) Slow out of the hole: See numbers 1 and 2
4) could it be the 17 degree prop: No! Its a 17 inch "pitch" prop not 17 degrees. Again, see #1 and 2 and then provide wide open throttle RPM with an average load. Then and only then can you determine if the prop is correct.

 

[Sinko]

Re: Idle RPM 3.0 OMC Cobra

Got the point on tune up. Thanks for the coles notes on props. Admitting I'm new with this boat and new to inboards . I have only ran the boat once while test running it last weekend (purchased private).
Is 600 RPM idle normal?

 

[Don S]

Re: Idle RPM 3.0 OMC Cobra

Get an OEM service manual, not aftermarket. Then you will have some information about your engine and drive.
Yes, 600 is what the rpm should be. 550 to 600 to be exact. IN GEAR in the water. Not in neutral or in gear on the muffs.

 

[Sinko]

Re: Idle RPM 3.0 OMC Cobra

what about idle in neutral (in the water)?
Checked in with the last owner, the motor was tuned up. Checked the oil and it was clear. I have 2 year old gas in it which I'm hoping may be the cause of some of the shakey run.

 

[Coors]

Re: Idle RPM 3.0 OMC Cobra

what about idle in neutral (in the water)?
Checked in with the last owner, the motor was tuned up. Checked the oil and it was clear. I have 2 year old gas in it which I'm hoping may be the cause of some of the shakey run.

?????????????? maybe a problem

 

[Don S]

Re: Idle RPM 3.0 OMC Cobra

Checked in with the last owner, the motor was tuned up.

If the engine was winterized, the tuneup needs redone.
Do a compression test, tune it up, check the timing, and adjust the idle mixture and idle speed. You have changed fuel filters haven't you?