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1998 Merc 4 stroke 50hp problem

G

goa

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"I have read this forum lots a

"I have read this forum lots and lots and learned tons. Thank everyone for that. Problem....1998 merc 50 4 stroke, above 2000 rpms awsome. Below 2000 rpms horrible. Idles rough (only in gear), so bad sometimes it shakes the boat. May go through a a period of a few seconds where the problem goes away. In neutral it is perfect. Carbs pulled and cleaned, carbs sync'ed 3 times. Plugs ok. Seeems like when I pull the bottom plug wire (cyl. 4) no effect on idle. Any ideas? All this work was done by a reputable local shop but they cant seem to get it right. Even thought it was the auto start switch (which was bad) and bypassed it to work like a choke on every other outboard on the planet. Didnt help."
 
Put a timing light on this thi

Put a timing light on this thing while it is running.Observe the flash of the light. Check all cylinders.Compression test has been done?
 
this is going to turn out to b

this is going to turn out to be something out of the ordinary. This motor has passed every conceivable test. It has alot of pretty good wrenches stuck.
 
I had the valve lash checked a

I had the valve lash checked and it was within specs. They said no adjustment was needed. That was done in order to track down a problem it was having with a sneeze at low rpm.
 
"sneezing and rough idle=vacuu

"sneezing and rough idle=vacuum leak, take a plumbers torch and wave it around the engine top to bottom with the engine idling. If it revs up then you found the internal leak. PS torch not lit
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"never heard of that one, but

"never heard of that one, but ill try it. Thanks everyone for the ideas. Please keep them coming. this problem is starting to get annoying."
 
"ok....i tried the propane tor

"ok....i tried the propane torch and didnt affect the motor idle at all. cant really tell you that i performed this operation right though. i just turned on the gas and ran it all over the running motor paying attention to where i was and whether the motor changed at all. in my opinion it didnt. Im sorry to keep going on this thread, but i would appreciate any more ideas. i hate to spend 5 grand on another motor when this is obviously going to be something stupid that no one can find. Please keep the ideas coming if there are anymore. Please dig deep. Is it possible that even though the carbs are clean and synced that they are bad? Or one is bad? Possible that the low speed jet isnt working on carb 4 for some reason other than a clog? Another thing that may help is that when you mess with the idle air screw on carb 4 it has no effect on the motor?"
 
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