Weird rough running issue with OMC 5.7

Rudinski

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Joined
Jul 30, 2020
Messages
3
I have gone through internet for quite a bit and still cannot find a solution for the following:

Symptom: Engine sounds like running on seven / running rough every time after when started, regardless of being cold or hot. The problem clears when I rev the engine once close to 4000 RPM and after it it runs very smoothly in every condition. After longer drive with low rpms it might reoccur, but systematically occurs after every start. Also when the fault is "on" the boat will not plane and engine will backfire at 2500 rpm.

Troubleshooting/replaced parts this far:
-Rebuilt Quadrajet Carb, with electric choke, tuned idle air/fuel with vacuum, choke works as it should
-Distributor cap + cam
-Pointless ignition (Ignitor I)
-Coil
-Sparkplugs and wires + coil wire
-Base timing ok (one spring was missing in distributor, added another one and it runs good when the fault is cleared, will find the correct srping), distributor cleaned and the timing mechanism lubricated
-Overruled faulty tachometer possibility as someone had found his faulty and causing backfire
-Run into problem with a bad 50 amp fuse causing other type of misfiring, but repairing this didn't help with the original issue

The wierdest thing is that after revving the engine one time high enough it runs good as new.

Any ideas welcome.

Cheers!
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,306
your running lean, go back to the fuel system
 

Rudinski

Recruit
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Jul 30, 2020
Messages
3
The symptom is the same regardless whether the engine is cold or hot. I would assume too lean not being issue when on choke condition.
 

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
11,802
are you sure you are not getting water in a cyl, I had that when my head gaskets were starting to go bad. Pull the plugs before you start it, disable the ignition and crank it over. Watch if water shoots out of a cyl. Any milky oil?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,306
The symptom is the same regardless whether the engine is cold or hot. I would assume too lean not being issue when on choke condition.

your idle circuit and internal passages in the carb do not care where the choke is. the choke is an external flap designed to stop incoming air to richen up the mix. not control the flow of fuel like the internal passages in the carb.
 

Rudinski

Recruit
Joined
Jul 30, 2020
Messages
3
The small water leak would be plausible theory as it is not tied to fuel nor ignition. Oil is normal, so it must be directly through the head gasket to cylinder leak. I will try to check this next.
 

MRS

Commander
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Jul 10, 2005
Messages
2,555
He was just asking if he figured out his problem not starting a new thread? We need to be nice to new posters 🙂
 
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