Mercury 80 HP outboard

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Kissdude

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I was wondering if anyone could tell me why in my driveway my motor runs nice in forward and reverse but when I try to leave the dock it will die in either forward or reverse. I have a quick silver metal can for the test in driveway but changed it to a new looking one that I had. Do these lose suction or something? Thanks in advance!
 
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Chris1956

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No, the difference is backpressure on the exhaust. In the driveway there is no backpressure. In the water there is some backpressure.

What is your idle speed? Have you made any changes or repairs to timing or carbs?
 
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Kissdude

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I haven’t made any changes and I don’t know what idle speed is, been trying to find it in the manual.
 
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racerone

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Open low speed mixture screws 1/8th turn at a time.----Test / repeat as needed.----Often makes the bogging go away.
 
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Chris1956

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I will assume that is a Merc from the late 70s or early 80s. They really liked a precise 50::1 gas::eek:il mix. When tuned properly, they will idle OK on that. However, it does not take much in tuning or fuel mix to make a lousy idle.
My advice is to mix a fresh 50::1 fuel source and run it on the water. Motor warm, in set the idle mixture screws to 1-1/2 turns open. She should run OK. Now try to accelerate. If she bogs or stalls open the top carb mixture screw 1/8 turn. Try to accelerate. Again, if she bogs open the bottom mixture screw 1/8 turn. Repeat the sequence until she rund OK at idle and good at high speed.
 
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Kissdude

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Yeah its a 1979 80 hp. will I be able to do these settings with motor in a big bucket instead of muffs? The only change that I made was take an empty newer fuel can with me and , filled it with fresh gas on the trip out but added oil when it was full. It ran really good in each gear before the trip using the older can.
 
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Chris1956

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The correct way is to set the carbs to 1-1/2 open, and run it on the water. Make sure the mixture is exactly 50::1 and tune the carb idle mixture to run as best as possible while allowing smooth acceleration.
 
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Kissdude

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So I turn the screw on front of carb correct? And when I do the 1-1/2 turns, I close one at a time and back out while running?
 
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airshot

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According to all the service manuals I have ever used, all Mfgrs recommend making final low speed adjustments and idle speed adjustments with motor warmed up and underway in the water ! Any other method will not give you the best idle performance. My 40 hp Mercy will idle all day at 550 rpm after proper adjustment.
 
jimmbo

jimmbo

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You have to disconnect the Throttle Cable when making the Idle Speed Adjustment(in Gear). Once it is correct, the Length of the Throttle Cable will need to be adjusted so there is a slight Preload(Pressure against the Idle Stop on the Throttle Linkage) on it. Too little Preload and the Idle Speed may Vary, Too much and it may be difficult to shift Into and Out of Gear
 
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Kissdude

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sorry but I'm not following your last post. I'm a little confused now on what to do. The motor starts good and will idle for a long time(in driveway with muffs) and shift into forward and reverse. when I took it out last and had it in the water shifted into reverse and it died. did the same thing in forward so now I don't know what adjustment I have to do and don't want to make it worse lol.
 
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racerone

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These carburetors are often set too lean.-----They idle smooth on a hose.-----That is NOT the correct setting when motor is in the water and needs to produce more power to spin the prop.-----Open the mixture screws 1/8th at a time and test.-----Repeat as needed.-----Often the bogging goes away.-----Easy for you to test and learn.-----No beer tokens spent trying this procedure!---And perhaps you need to adjust the idle stop screw to speed up low RPM as others suggested.
 
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Kissdude

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Idle stop screw hey! I don't recall reading that one but I will try and adjust that one then. Thanks
 
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Kissdude

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Yeah, I see that but didn't understand it because when I put it in gear it dies. how do I get it to stay running with cables disconnected when it dies?
 
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Kissdude

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jimmbo said:
You have to disconnect the Throttle Cable when making the Idle Speed Adjustment(in Gear). Once it is correct, the Length of the Throttle Cable will need to be adjusted so there is a slight Preload(Pressure against the Idle Stop on the Throttle Linkage) on it. Too little Preload and the Idle Speed may Vary, Too much and it may be difficult to shift Into and Out of Gear
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Can I do this adjustment in a test tank? on the water sounds kind of hard to get back there.
 
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