5.0 mercruiser won’t idle under 1100(ish) rpm’s

G

Gcotton07

Cadet
Joined
Apr 4, 2023
Messages
6
so this boat is new to me and after routine tune up stuff it still won’t idle under 1000 rpm. I’ve checked for vacuum leaks and currently have all of the vacuum lines capped off right now. Carb was freshly rebuilt. It starts smoothly without hesitation. Double checked idle screw and fuel air mixture screws. Any help is appreciated
 
Lou C

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
12,653
Tension adjustment on the throttle cable? These are usually adjustable.
 
alldodge

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
42,054
If ya got a vacuum line on the carb it's automotive, but put a vacuum gauge on it before replacing to see what the reading is
 
Lou C

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
12,653
Other signs of an auto carb are:
No J-tubes for the bowl vents (although Marine Quadrajets didn’t have the same J tubes like Holleys do)
No nipple to hook up the safety overflow yellow tygon hose if using a mechanical fuel pump (although again certain Holleys & Quick Fuel carbs don’t have these either). In those cases you’d have to use a flame arrestor with that fitting on it.
 
G

Gcotton07

Cadet
Joined
Apr 4, 2023
Messages
6
IMG_2222.jpegIMG_2228.jpegIMG_2354.jpeg
So the vacuum line is on the base plate attached to the carb, it is a marine carb from what I can tell by the part number. It was high before and after the rebuild. The throttle cable has been disconnected so I can eliminate that.
 
Lou C

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
12,653
I see you have the J-tube for the bowl vent,
did you verify float level, and also before installing the carb, set the idle screw so that the idle transfer slots are not over-exposed?
This appears to be an OMC 5.0 Ford, judging by the color, stickers etc and exhaust system.
What I would do is take your OMC model # (on that sticker on the valve cover or on the exhaust elbow) and look in an OEM parts manual for your actual model, at a pic of the carb/intake manifold. You can look on the BRP/Evinrude site, or others who sell OMC parts.
 

Attachments

  • Holley idle.png
    Holley idle.png
    655.9 KB · Views: 9
G

Gcotton07

Cadet
Joined
Apr 4, 2023
Messages
6
Lou C said:
I see you have the J-tube for the bowl vent,
did you verify float level, and also before installing the carb, set the idle screw so that the idle transfer slots are not over-exposed?
This appears to be an OMC 5.0 Ford, judging by the color, stickers etc and exhaust system.
What I would do is take your OMC model # (on that sticker on the valve cover or on the exhaust elbow) and look in an OEM parts manual for your actual model, at a pic of the carb/intake manifold. You can look on the BRP/Evinrude site, or others who sell OMC parts.
Click to expand...
Any idea on the recommended amount of transfer slot exposed ? I have seen anywhere between .0002-.0004 of an inch
 
Scott Danforth

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
49,538
Gcotton07 said:
Any idea on the recommended amount of transfer slot exposed ? I have seen anywhere between .0002-.0004 of an inch
Click to expand...
Throttle blades need to be in middle of slot. .020" or so. The slot should look square
 
Lou C

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
12,653
Sounds like your idle system is clogged; either the jets or the air bleeds. Or there is a float/needle valve issue.
 
Scott Danforth

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
49,538
there are like 6 parts to your carb. you will need both the bowl gasket and the metering gasket to pull the carb apart to clean the idle circuit.
 
D

dubs283

Vice Admiral
Joined
Jul 27, 2005
Messages
5,322
As long as the ignition/engine mechanical is sound an experienced marine mechanic would be able to remove that carburetor, dismantle, clean, install new parts, assemble, install, adjust and have the engine running well in about an hour or two. Probably one of if not the simplest carbs on a marine engine

Is the timing set properly? Firing order correct? Compression good?
 
You must log in or register to reply here.
Top