Rough Idle - Mercruiser 350 with 2-brl. carb

[djmika]

Hello,

I'm new to the forum and looking for some assistance. I have a 1969 Donzi 18 Classic 2+3. It has a newer engine/outdrive installed around 2004. It isn't a current model in the Merc line, but probably the previous generation. I think it's 260HP. I think it has a 2 brl. carb which I believe to be a Rochester.

At idle it runs a little rough. When cold it it gets pretty choppy when out of gear and will sometimes die when in gear. I have a feeling the idle might be set a little low. I think it sits somewhere between 450-500 RPM in gear and below 600 when out of gear.

It smooths out some when it's warmed up but not a ton. The boat is light and it has a 19 pictch prop which allow me to cruise around 25 mph at about 2100 rpm (this is a estimate as I don't have a speedo). After sitting at around 2000-2500 RPM for some time then backing down to idle again it continues to run rough. Possibly rougher.

I called my local boat shop yesterday seeking information. Not surprisingly they told me it sounds like it needs a carb rebuild but that they can't do it for a few weeks. I am taking the boat for a long weekend starting next Saturday and I was looking for a quicker fix.

It's been a while since I've done any engine work but in my mind it seems like it's running a little rich at idle. I could also up the RPM at idle as well.

Off idle the engine runs strong. No hesitation, no hiccups, nothing. It might need a carb rebuild but I want to explore my options before I do that.

Sorry for the long post. I'm looking forward to hearing from anyone willing to help.

Thanks,

Micah

 

[Bondo]

Re: Rough Idle - Mercruiser 350 with Q-jet carb

Re: Rough Idle - Mercruiser 350 with Q-jet carb

I called my local boat shop yesterday seeking information. Not surprisingly they told me it sounds like it needs a carb rebuild but that they can't do it for a few weeks. I am taking the boat for a long weekend starting next Saturday and I was looking for a quicker fix.

Ayuh,.... Welcome Aboard,......
I'd give that Rottenchester the "Float Test" right there at the dock....

Edlebrock sells a Carb called the Performer series,..
There's a 650cfm model that'll make your 350 run like a Dream,.....
Usually right out of the Box....

 

[djmika]

Re: Rough Idle - Mercruiser 350 with Q-jet carb

Re: Rough Idle - Mercruiser 350 with Q-jet carb

Float test?

Like the one where I check to see if it floats?

I did some research before posting and found some good pointers on tuning carbs. I will most likely give them a shot.

if not, is the Edebbrock the best way to go? I imagine that the boat shop will charge me a pretty penny to rebuild the carb. I would think that it would be close to the cost of buying a Edelbrock Marine. Does the Edelbrock offer a performance upgrade? Any difference in economy? How about reliability?

Also, how difficult is the swap? Does the Edelbrock come with everything necessary?

While advice on a new carb is appreciated I would like to explore the options of tuning the existing one first.

Thanks for the reply.

 

[djmika]

Re: Rough Idle - Mercruiser 350 with 2-brl. carb

Update - I'm 99% sure it's a 2 brl. carb although I don't have the boat here to check. I did find the manual for the 98-01 Mercruiser 350 and I believe that's what I have.

Thanks again for the help.

 

[djmika]

Re: Rough Idle - Mercruiser 350 with 2-brl. carb

I bought the boat in March. It was inspected including removal of the drive before i bought it. According to the guy who looked at it everything was good.

I've since found that to no be completely accurate. Along with the idle problem, the throddle has a little problem as well. Sometimes when taking it out of gear it "clunks" a few times along the way. It's never done any worse than that so I assume this is an adjustment.

Everything in the boat is relitively new so I don't think I have any major issues... That's why I'm here, looking for pointers on making the adjustments it needs.

But to answer the question directly, No. I have not had any work done to the boat since I have owned it.