IDLING ISSUE ON A 470 / 3.7 MERCRUISER

[RenkenII]

Last season I began to notice a skip on idle in my 1987 Renken 1750 Limited with the 470 / 3.7. This spring I re-wired the motor and improved grounds trying to solve this problem. Under load, there does not seem to a problem. It is very smooth and powerful. The skip is also not rhythmic. such as a ignition problem. Plugs, wires, coil and ignition (electronic) are all new last year. Is there a condition where water gets into a cylinder from the cooling system? Also - on the muffs, the port side exhaust has little water exiting and the starboard side has almost all of the water exiting. The outdrive was serviced this spring with new impeller installed. I think I have covered it? I'm in need of guidance for diagnosis. Also - I replaced exhaust flaps last year. Thank you

 

[Mercruiser420]

The riser and manifold can certainly leak. Pressure test them if your suspicious. What's your spark plugs look like? Is your ignition timing correct? Idle air/fuel mixture adjusted correctly? Hook up a vacuum gauge and see what its doing.

 

[stonyloam]

while it is running at idle, pull the plug wires one at a time to see if you have a weak cylinder ( no or little change in RPM when you pull the wire). Do a compression check, and while you are at it check the color of the plug insulators. A snow white insulator indicates a coolant leak into that cylinder (head gasket?).

 

[RenkenII]

Timing is set to about 10 degrees. Idle mixture is set correctly. I will start pulling plugs and wires. The good thing is it is not consuming anit-freeze. Thanks for the help. I'm sure I'll have more questions.

 

[RenkenII]

No Title

OK - found a couple of surprises - my compression ranges from 135 to 145 and the plugs are ALL equally bad. They clearly were not changed last season, but no washing. The riser was bone dry and the fluid was just below the level of the riser. If you look at picture #3 - on the left by the antifreeze port, it appears to have been leaking there. I will pressure test the riser and replace the spark plugs and retry ever thing. Am I missing anything? How about the amount of carbon on the plugs - too rich? Idle screw mixture and how should that be adjusted? Thanks again everyone.

 

[stonyloam]

Check your plug wires, might be your problem. Start it up in the dark, and mist a little water on the wires and look for sparks from the wires. Turn the idle screw all the way in till it stops, do not over tighten and keep track of the number of turns. Turn back out 1 1/2 turns. Start engine and turn screw out (slowly) until the RPM starts to drop off, then in until the RPM drops off again. Proper setting is between those two points.