Rough / Bouncing Idle 3.0 Mercruiser

[int76]

Ok so I just purchased a boat (im pretty new to the boating thing but plenty mechanically inclined) and I cant get this thing to idle right. Its a 94 Glaston with a 3.0 Mercruiser Alpha One engine. The guy I bought it from had it in his shop all winter so it was in good shape and started right up first try once we pulled it out. At the time I just heard it run a few seconds, checked for leaks and made sure everything worked (trim, steering etc). So now I get it home and start getting it ready to take out. I put the muffs on and go to let it run for a bit in the driveway and the idle bounces around alot. If I mess with the idle adjust screw and get it below 1000 rpms it instantly dies, it wont idle unless its at atleast 1200 rpms, but even then it doesnt idle smoothly it kinda bogs and bounces around. Adjusting it up to 1800 rpms it runs fine but shouldnt it be able to idle around 800 RPM??? I have no clue about carbs and chokes etc thats all a little old for me so im stuck and dont know where/how to start. Any info would greatly be appreciated. Thanks - Josh

 

[PitchFork]

Re: Rough / Bouncing Idle 3.0 Mercruiser

Full tuneup may be in order. It was the first thing I did when I bought my 1994 boat three years ago. I would not put the drive in gear at 1200 RPMs. Be sure to check the timing just go over the whole engine becuase you do not know what is missing or the wrong part installed. Be sure to download the right maunals from the Adult section. I have worn out my PDF reader.

 

[Spinnaker]

Re: Rough / Bouncing Idle 3.0 Mercruiser

I think 800rpm might be a bit slow. Check your timing. I know these 3.0's are shaky engines to start with. What you describe sounds like timing though. Either way she's an easy fix for experienced.

 

[Bondo]

Re: Rough / Bouncing Idle 3.0 Mercruiser

Ayuh,... It should be idling at 650/ 700rpms, in gear, in the water...
Any faster than that,+ your outdrive will eat itself up shifting...

Other than the Tune Up suggested,...
It sounds to me like the Carb needs a Rebuild,... The Idle circuit is Plugged...

 

[Black as]

Re: Rough / Bouncing Idle 3.0 Mercruiser

Ok so I just purchased a boat (im pretty new to the boating thing but plenty mechanically inclined) and I cant get this thing to idle right. Its a 94 Glaston with a 3.0 Mercruiser Alpha One engine. The guy I bought it from had it in his shop all winter so it was in good shape and started right up first try once we pulled it out. At the time I just heard it run a few seconds, checked for leaks and made sure everything worked (trim, steering etc). So now I get it home and start getting it ready to take out. I put the muffs on and go to let it run for a bit in the driveway and the idle bounces around alot. If I mess with the idle adjust screw and get it below 1000 rpms it instantly dies, it wont idle unless its at atleast 1200 rpms, but even then it doesnt idle smoothly it kinda bogs and bounces around. Adjusting it up to 1800 rpms it runs fine but shouldnt it be able to idle around 800 RPM??? I have no clue about carbs and chokes etc thats all a little old for me so im stuck and dont know where/how to start. Any info would greatly be appreciated. Thanks - Josh

Dont stress about the idle or try and adjust the idle at this stage, your motor needs a good tune up and the rest will fall into place. Start with replacing plugs and leads etc,
Go to the adult section and downlaod the workshop manual and it shows how to tune an engine.

 

[int76]

Re: Rough / Bouncing Idle 3.0 Mercruiser

What should the idle be at approximately out of the water in neutral on the muffs?

 

[T-Bone Coug]

Re: Rough / Bouncing Idle 3.0 Mercruiser

Every boat will be slightly different, but with mine it idles at ~775 out of the water, 650 in the water. Adjustments made out of the water are only meant to get it close to spec. All fine tuning must be done in the water.

 

[searay3]

Re: Rough / Bouncing Idle 3.0 Mercruiser

IMHO start with a compression test to see what you have to work with. If it checks out ok, check the condition of the oil, although it may very well look good if replaced by the seller. Also check the condition of the outdrive lube. Problems there can cause engine strain. Then start spending the money on the tuneup parts.

 

[fat fanny]

Re: Rough / Bouncing Idle 3.0 Mercruiser

If it has the est iignition after the full tune up 1st then check to ensure your timming is correct and will advance correctly. Remember full tune up plugs,wires,filters and fluid then check all electrical connections for cleanliness and tighten and lube while @ it. The only $ money I would spend @ 1st would be on tune up parts and if issue still occurs trouble shoot per manual (FREE) up in the adults only sticky section read it and act on what you have read pretty straight forward. Good luck and post back!

 

[int76]

Re: Rough / Bouncing Idle 3.0 Mercruiser

Well this is where im at so far.....I changed the plugs with some "cross referenced" ones from AutoZone and turns out they were WAY off, I never knew spark plugs could make that huge of difference. Well I got them changed out with the recommended MR43t gapped at .035(the old champions were gapped at .040 when I pulled them). I checked all electrical connections and plug wires and all seems to be good, havent checked timing yet but will soon. Well its running better and had my buddy who worked up at the local bike/boat dealer come check out the carb and the idel air/fuel mix screw was bent (couldnt imagine how) so I have one of those on order from mercruiser and should be here next week. Hopefully once changing that everything will be all good. I read in the service manual that the idle air/fuel screw is to be tightened down all the way then backed off 1 1/4 turn, does this mean tighten until the spring is fully compressed then back off??? thanks for all the advise....

 

[Bondo]

Re: Rough / Bouncing Idle 3.0 Mercruiser

I read in the service manual that the idle air/fuel screw is to be tightened down all the way then backed off 1 1/4 turn, does this mean tighten until the spring is fully compressed then back off???

Ayuh,... Not necessarily,... It means Lightly Seated...

 

[T-Bone Coug]

Re: Rough / Bouncing Idle 3.0 Mercruiser

Keep in mind that 1-1/4 turn is only to get you close. Then you have to fine tune it in the water.