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Korky in MN - Mercruiser miss

K

Korky

New member
1986 (approx) Four Winns
Mercruiser MCM 140 #A676736 (from engine placard), 4 cylinder
3.0L (assumed)

PROBLEM
After cruising for a short time, the engine develops a “hardmiss” which is persistent once the engine is hot. The tach confirms RPMfluctuation. Must throttle back and can only idle after the miss hasdeveloped. Once the engine is turned off and has cooled, then it’spossible to cruise again for a short time before the engine warms up and the“miss” returns.

Ignition problem? Fuel problem?

The following were changed May, 2017:
34235 breaker
33662 condenser
18-5418 rotor
18-5385 cap
RV9YC plugs

Ignition?
Replace the ignition coil and spark plug wires?

Fuel?
Replace fuel filter in fuel pump? Replace any otherfilters or screens in the fuel line and carburetor? Gaskets?

I don't have a lot of high tech testingequipment. Can you help me out with an educated guess on this problem?
 
Excellent. New plugs installed. Problem continued. Re-installed old plugs. Problem continued.
Dont think the plugs are suspect.

I plan to change the ignition coil and plug wires next.

Thoughts?
 
What color are the tips? They can tell if its a lean condition, or a too rich condition, or just right condition.
Blister white could mean water ingestion . If color is correct then suspect the wires or the coil
 
Bt Doctur said:
What color are the tips? They can tell if its a lean condition, or a too rich condition, or just right condition.
Blister white could mean water ingestion . If color is correct then suspect the wires or the coil
Click to expand...
THANK YOU. That's kinda what I figured to do next.
 
Also I would remove distributor cap and see if shaft moves side to side.

One issue of this design (also the inline six cylinder) is the distributor shaft bushings wear egg shaped due to distributor being on angle.

Also confirm mechanical advance mechanism under point plate is moving freely and fully. Not all rusted or frozen.
 
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