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Mariner 25 weak Idle

N

newk

New member
"Hi there.
My Manual start 19

"Hi there.
My Manual start 1985 is giving me some headaches.
I have to set the idle fast to keep it running, which presents the big 'clunk' when going into gear.
It runs fine and strong through low, medium and full power, near idle is the problem.

At Idle the lower cyl seems weaker, noticed by pulling each plug lead off while running. With the fast idle setting it will keep running on the upper cyl alone but not the lower. Installed new plugs... no change.

I have not tested compression but it seems good and even when pulling over.

While playing around, I noticed as the throttle was advanced there was no rpm increase during the ignition advance phase. Once the cam on the carb was contacted and the carb began to open there were no problems, she was away.

My initial feeling was the lower cyl coil, however the 'no advance response' cast some doubt. Would the advance issue be in the stator or the CDI unit?
Don't want to waste cash bolting on new bits until I eventually find the problem!

Any help appreciated."
 
"If the carb is running rich,

"If the carb is running rich, the lower cylinder will always run poorer.

Tie the boat down, put it in gear, and readjust the idle mixture. You might have to manually give it some gas to keep her from stalling before you get it right. Take your time, turning the screw a 1/4 turn at a time and WAITING 10 seconds to let it stabilize.

Jeff"
 
"Jeff.
I did a compression te

"Jeff.
I did a compression test.
Top Cyl was 110 psi and lower was 103 after 5 pulls on the starter for each.
I moved the idle mixture about 1/4 anti-clockwise and have a much better idle. I was able to lower the idle speed as it was set artificially high to keep it running.
That was done on the trailer with muffs, so will do as you suggested next time I put it in the water.
Thanks for your help.
Cheers"
 
"Way to go, David.

Those c

"Way to go, David.

Those comp readings are a bit low for that motor BUT the tester you used might be the reason. All testers are not the same. The giant V-8 tester I use has so much volumn inside it always reads lower than a dedicated OB tester. Perhaps yours is similar?

Let us know how the 'final' adjustment comes out.

Jeff"
 
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