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40 Mariner rough Idle

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Rick Woeste

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" My 1983 40 Mariner has a rou

" My 1983 40 Mariner has a rough idle. It starts fine, runs good at fast idle and top speed. But at idle it appears to be starving for gas. Took it to a shop-- the guy replaced the fuel pump. Still a rough idle. He said its probably has low crank case preassure. Needs to have a seal replaced-- which would entail a rebuild- or a low preassure fuel pump can be added to prime the motor. I'm not sure what to do-- are there tests that can be performed for the crankase preassure? I had the engine checked before I bought and it has good compression 130 in each cylinder. Any ideas would be appreciated. Thanks Rick "
 
"Rick,

It sounds like you m

"Rick,

It sounds like you may have a carb problem. First make sure both throttle plates are closed when the throttle is at low idle, and that the idle mixture screws are equally adjusted. Screw them in until they seat lightly, and note the number of turns until seated. Back them out to the original position, which should be 1-1 1/2 turns. If these things are okay, I would advise you to remove the carbs and inspect the low speed jets and the entire low speed system.

While you have the carbs off take a look at the reed valves. If they are not seating properly you will lose crankcase pressure at idle.

"
 
Tony: Thanks I'll give th

Tony: Thanks I'll give that a try.
 
Tony: Do you know where I can

Tony: Do you know where I can get a service manual for this engine? I need diagrams etc. before I tear into the carbs. Rick
 
"Rick,

You could probably v

"Rick,

You could probably view the parts manual at the Mercury website if you have the serial number for the engine. You could obtain a service manual that would cover all models from any of the online parts dealers. The generic type of manual would be perfectly alright for a carb rebuild.

"
 
where are the reed valves located? would they be between the carbs and the inlet manifold of the engine block and are they on all 2 strokes as i havent seen them on my 25hp tohatsu when taking the carb off?
 
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