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Old 04 May 2015, 08:26   #11
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Well it might but it might also cause "fluffing" or even stalling as you raise the throttle just above idle.

One thing that can cause a high idle is an air leak which on your carb could be at the carb to engine face?? Did the guy use a new gasket set?

Also does your engine have the small "balancing" hoses between crankcase and just below the carb flange? Mine was perished and split in two places which could upset things.

See Mariner 15 2-stroke spitting fuel near carb.
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Old 04 May 2015, 13:40   #12
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matt no it probaly wont drop the revs, you can try it though
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Old 04 May 2015, 15:35   #13
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Originally Posted by The Gurnard View Post
I think there could be an access plug on the side just off the right of your photo..it should be under the plug..if its the same model Im thinking ?

Here is the complete service manual for the mercury 20-25hp engines..and I have opened it at the page covering the idle adjustment screw

Boatinfo - Mercury Service Manual for 20jet-20-25hp

hope that helps..there are some differences in models ..but its hopefully there

Or turn to page 102 which give a bit of detail on setting the idle mixture screw etc. Im no expert though..I just read the manual for the bits I do on my mariner 25Hp engine
Page 111 shows "adjusting idle speed" by adjusting the idle mixture?? I am confused as page 105 shows to adjust this screw here???!
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Old 04 May 2015, 15:37   #14
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Originally Posted by Fenlander View Post
Well it might but it might also cause "fluffing" or even stalling as you raise the throttle just above idle.

One thing that can cause a high idle is an air leak which on your carb could be at the carb to engine face?? Did the guy use a new gasket set?

Also does your engine have the small "balancing" hoses between crankcase and just below the carb flange? Mine was perished and split in two places which could upset things.

See Mariner 15 2-stroke spitting fuel near carb.
Ive checked out your thread and my carb doesn't have the barb on the flange but does have two pipes under the intake which I noticed didn't look in great condition but I couldn't see any splits so left them
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Old 04 May 2015, 16:02   #15
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page 105 shows to adjust this screw here???!


Ahh no.. that's to adjust the degree of fast idle when you turn the fast idle knob.

Looking at your last image I'm reminded that in taking the carb off you disconnect the fast idle link rod that goes to alter the timing... are you 100% sure that's gone back properly (could be an error at either end as to how it was refitted) and isn't forcing the idle speed up?
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Old 04 May 2015, 16:27   #16
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hmmm not sure, the rod has fair bit of slack in it but not sure how its supposed to be?
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Old 05 May 2015, 07:04   #17
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Just to throw a curveball in here - you aren't testing this on muffs are you?

The lack of exhaust back pressure results in a faster idle on muffs (and conversley if you set it perfectly on muffs it won't run in the water...)
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Old 05 May 2015, 07:10   #18
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Just to throw a curveball in here - you aren't testing this on muffs are you?

The lack of exhaust back pressure results in a faster idle on muffs (and conversley if you set it perfectly on muffs it won't run in the water...)
Yes I am running on muffs, and this is the first time run on muffs as ive previously started up (pre carb rebuild) in a bin full of water.

I think I will try it in a bin tonight and if its still high I will adjust the idle mixture slightly I know its 2 turns out whereas the manual suggests 1 1/4 out with 1/4 +/- from there.
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Old 05 May 2015, 08:06   #19
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Because I never use muffs I'd not thought to ask if you were.... good call 9D280!

Not only should you set up a carb idle in a bin (or on the water) but the idle mixture should be set in forward gear.

Matt forgive me if you know all this but....

Screwing the idle mixture screw in or out will slow the idle speed because that's how you determine where it should be set. The 1.5 turns out from lightly seated is a base setting to get the motor running then when warm and in gear you screw it out until it starts to slow/falter then in until the same happens noting the number of turns each time (in/out from the 1.5)... then unscrew to halfway between these two positions and that is where it should be best.

You will usually find the outboard picks up best from idle with a mixture that is a fraction rich (as long as it still idles smoothly) and you should never turn it in too lean.

That fast idle rod we were discussing has an odd "connection" at the flywheel end in that it should be able to pull the timing plate round when the fast idle knob is twisted but when the throttle is used to raise the revs the timing plate is able to move without moving the rod. The rod slides in the block it locates in at the flywheel end when you are using the throttle.

Easier to make sense when you are looking at it really.
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Old 10 May 2015, 16:00   #20
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ok guys, ran it up on the muffs and adjusted the idle mixture screw service manual says base mark is 1 1/4 turns out, (it was out 2 turns out) if I leaned it in to 1 1/4 the revs dropped to what it should be, but I am really nervous about running it too lean? From what ive read you should set the idle mixture to the highest rev point?
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