Mercruiser 3.0L Rough Idle

[Daritzz]

Hello, I recently purchased my first boat with a 93 Mercruiser 3.0L with Alpha one stern drive. I had a mechanic inspect the engine and he recommended that i get a rebuild carburetor, which I did. I also had a tune up done at a local marina. The engine still idles very rough and shakes when below 1000rpm but seems to run smoother above 2000. I am going to check the timing and change the spark plugs again, as they were completely black in color.

Also, oil looks good and compression is 120 across the board. (that's what I was told)

Is there anything else I should take a look at this weekend that could potentially cause the engine to idle rough?

Thank you in advance for any comments. I am new to boating and have had a very rough start so your help would be greatly appreciated.

Thank you,
Greg

 

[solar7647]

Re: Mercruiser 3.0L Rough Idle

Try to adjust the idler needles, turn all the way in then out 1.5 turns start there and adjust them while the engine is running. Also double check your plug wires and if they are pluged into the distributor cap right. If you dont have the manual it is in the adult only section of the of this thread. Good Luck

 

[Bondo]

Re: Mercruiser 3.0L Rough Idle

I am going to check the timing and change the spark plugs again, as they were completely black in color.

Ayuh,... Fix the Carb,... It's running Way too Rich... That'll probably cure the Idle too...

 

[Daritzz]

Re: Mercruiser 3.0L Rough Idle

thank you for the advice, I got the manual and will follow the instructions carefully. I'll check back in with the results...

 

[Rocky_Road]

Re: Mercruiser 3.0L Rough Idle

thank you for the advice, I got the manual and will follow the instructions carefully. I'll check back in with the results...

If you do change the idle mixture screw...be sure not to torque down, when it bottoms out! This screw is VERY delicate, and will be trashed if turned too far once it is all the way in....

 

[Daritzz]

Re: Mercruiser 3.0L Rough Idle

so i worked on the boat this weekend with my neighbor and we noticed several problems with it, luckily all cost me nothing to fix. (except for the nice bottle of wine for the neighbor). First, the timinig was way off. I assumed the guys at the marina who did my tune up would take care of that. Secondly we noticed that the enigne was not getting hot no matter how long we ran it, stayed below 120 degrees at all times. We checked the thermostat and the valve it controls was completed open due to junk preventing it from closing at low temps. We cleaned the valve and flushed out junk from the cooling hoses. We adjusted the idle speed and idle mixture screw and it idled well and ran great at higher rpms.

On sunday I had trouble starting the engine when cold, took a while to catch on. We took in out on the water and ran well. When I got back home i ran the engine on a hose to flush out the seawater and again had trouble starting the engine and the idle was not smooth as before. I shut down the engine and checked the carb. The choke was stuck open all the way and i had to apply some force to let it loose. Would that explain the trouble i had with a cold start and the rough idle after taking it out of the water?

Thank you

 

[Rocky_Road]

Re: Mercruiser 3.0L Rough Idle

Short answer...absolutely!

 

[Daritzz]

Re: Mercruiser 3.0L Rough Idle

great, thank you... is there anything i could do to prevent it from getting stuck again?

 

[Fishermark]

Re: Mercruiser 3.0L Rough Idle

great, thank you... is there anything i could do to prevent it from getting stuck again?

Clean the shaft good with some carb cleaner.

You mentioned in your original post that you got a rebuilt carb. Does that mean you purchased a rebuilt carb, or did you do the rebuldling yourself, or did you have someone do it for you?

A lot of questions I know!

A sticky choke should have been taken care of in the rebuild.

Also - if the choke is stuck open, it would be difficult to start when cold, but would have nothing to do with it being difficult to start after it warms up - nor would it have anything to do with the engine running poorly after warming up.

 

[Daritzz]

Re: Mercruiser 3.0L Rough Idle

the carb was rebuilt professionally by a marine shop but considering the lack of care they took in adjusting the engine i'm not sure about the quality of the work. It does look nice though, like new. I'll try the carb cleaner and see if it happens again.

The engine started back up immediately (with a roar) when i ran it on the hose for a couple of minutes and then did a quick turn off and turn on. I only had a rough start in the morning when testing the engine before putting in the water, and then once i put in the water which was about 20 minutes later.

btw... i appreciate the questions......