Rochester 4 barrel-5.7L struggling to idle-gctid804217

[Guest]

Bought my 2255 bayliner 5.7 mercruiser a year ago. Never had any issues with the idle last year (once it died idling coming into a dock). This year I can't seem to get the idle correct. I set the idle screw to 750rpms & it will die if I put it in gear. I turn the idle up to where I can actually put it in gear without it dying & its between 1100 & 1500 rpms. ( a little nerve racking when docking & putting it in gear). When i do set the idle to 750 or so it seems to bounce around 100 rpms or so. It has a Rochester 4 barrel. Thinking a vacuum leak or a carb rebuild/cleaning, any ideas? Thx


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[canoel]

First of all did you run fuel stabilizer in the tank , and allow the engine run time with the stabilizer in the tank? If this was not done, then most likely the carb will need to be removed and cleaned/rebuilt.

Second, when was the last full tune up done?

It could very well be a vacuum leak, be most of the time this will cause a high idle.

[Guest]

Yes....I always run Sta-bil marine 360 per manufacture dosage. When I filled up this spring I added 2 red bottles of Heet to help dissolve any water in fuel. I also replaced my fuel/water separator with a new quicksilver filter 2 weeks ago just b4 going out. The only vacuum line I see is a short piece for a diaphragm on the carb. I checked the rotor & cap & visually it looked great. I have not done a tune up but it looks like tune up parts aren't that old. Thinking the carb needs a good cleaning/rebuild. I asked the previous owner , " is there anything you would do to the boat for maintenance?" His reply was "well maybe a new carb". Which makes me wonder about issues he had with it.

[canoel]

Yes, the idle circuits in the rochesters are prone to clogging. You can get a rebuild kit and do it yourself, or order a rebuilt one.

These guys are usually pretty good, and usually have one ready to go on the shelf. http://flyingfishcarburetors.com/

[Chief_Alen]

http://www.crankshaftcoalition.com/wiki/How_to_rebuild_a_Rochester_Quadrajet_4MV_carburetor

Idle circuit clogged up.

Engine might need a tune up .

Missing on 1 or more cylinders the engine will never idle correctly. In other words low compression on 1 or more cylinders could be a reason.

Vacuum leak . Vacuum test ...

http://www.secondchancegarage.com/public/186.cfm

[Download_Complete]

Maybe it's due for a tuneup?

[2850Bounty]

Before you pull the carburetor off, try to clean out the low speed metering circuits using a good carburetor spray cleaner. IMO, the spray cleaners containing MEK work best.

Aim the straw into the low speed metering air orifices. This will direct the chemical into the passages that may be restricted.

[Ruffryder]

& 1500RPM is way too much for that Alpha dog clutch to engage!

Sounds like your lower shift cable is binding or adjusted improperly. ..

[Guest]

I had my Rochester carb rebuilt at Carbs Unlimited in Auburn, Wa. Slapped it back on and now idles better than it ever has since I purchased the boat (feb 2016). Thanks for all the comments & suggestions!

[canoel]

Glad to hear you got it repaired, and are back on the water!