Dropping Idle - Evinrude 48 SPL

OldTinBoater

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Aug 6, 2020
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I have a 1990 48 HP 2-stroke that struggles to keep idle throughout the day. I’ll tuned it to the right idle RPM using the Service Manual, and all is well for the first half day. Then towards the end of day the idle slowly goes lower and lower to the point of cutting out when at neutral or forward/reverse without throttle. I can turn the idle speed screw to raise it back up again, but later in the day it’ll need even more adjustment. It runs fine with some throttle and WOT seems strong. It’s done this for a few trips now. I do tilt up the motor when I’m beached. I’ve completely disassembled the carbs and gave a good cleaning. All seemed well in there. I didn’t use a rebuild kit and I may do that next. I refuel with fresh gas.

Any ideas? Reeds?
 

ct1762@gmail.com

Petty Officer 1st Class
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Jan 17, 2019
Messages
231
it sounds like it's either a fuel pump or running hot/lean. i'd check head temp and make sure the warning horn/temp switch is working. when you push in the primer at idle, what happens? does it pick up RPM or die off? if the low speed needle is making it respond, its probably not dirty carbs/idle circuit but would definitely go through them. only takes an hour.
 

racerone

Supreme Mariner
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Dec 28, 2013
Messages
23,130
I do not believe this is a carburetor or reed issue.-----I would start the trouble shooting with a compression test.----Post your numbers.------running with 50:1 mix ?-----Water pump checked ?----Overheat alarm checked ?----Flywheel key checked ?
 

OldTinBoater

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Aug 6, 2020
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I’ll check this weekend and report back:

- compression (previous owner said it was good but its been a while and I haven’t checked)
- RPM when choke enabled
- any fuel spit back from intake

I changed the water pump fairly recently. The tell-tale is always flowing strong and the water feels about the right temp (not too hot, not cold). I always use 50:1 premix with Quicksilver Outboard 2-stroke oil.

Not sure why the flywheel key might be an issue? I’ve never messed with that. I cleaned the carbs some time ago and it still had the bad behavior. If I’m pulling apart the carbs again, I’m just going to order/install a rebuild kit.
 

ct1762@gmail.com

Petty Officer 1st Class
Joined
Jan 17, 2019
Messages
231
I’ll check this weekend and report back:

- compression (previous owner said it was good but its been a while and I haven’t checked)
- RPM when choke enabled
- any fuel spit back from intake

I changed the water pump fairly recently. The tell-tale is always flowing strong and the water feels about the right temp (not too hot, not cold). I always use 50:1 premix with Quicksilver Outboard 2-stroke oil.

Not sure why the flywheel key might be an issue? I’ve never messed with that. I cleaned the carbs some time ago and it still had the bad behavior. If I’m pulling apart the carbs again, I’m just going to order/install a rebuild kit.
i guess it's possible the emulsion tube in one of the carbs is partially blocked. the primer will just squirt a shot of fuel in with a quick bump of the key so its not a choke. just a little tap is all thats needed to see if it's lean or rich or just right.
 

juno pierrat

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Dec 14, 2013
Messages
311
when you get the rebuild kits, would suggest removing the core plug on top of carb , make sure small holes under it are open, part of idle circuit
 

OldTinBoater

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Aug 6, 2020
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5
A few things to report back:

- Compression was about 130/135 psi. I believe that's good.
- The RPMs drop when the primer is bumped, almost dies. I think that's expected behavior.
- I noticed some fuel on upper carb. The drain plug was loose and slow dripping. Tightened that up.
- Removed airbox and no fuel spit back. Good.
- I played with the air/fuel mix screws. Upper carb about right, the lower carb gave a big RPM bump when I back it out 1/2 turn, then RPM dropped again. I turned it back in 1/4 turn and it's at a good high idle. Not sure what happen there, but one guess might have been a partial blockage at that needle that was dislodged when I backed out the screw - don't really know. I was able to put the main idle screw back to the original settings, so I think I may have "fixed" the issue.
- I ordered a rebuild kit and will do a cleaning and install when it arrives. Might as well fix what ain't broken.
- The. core plugs on top of carbs look sealed. How is it removed?
 

juno pierrat

Petty Officer 1st Class
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Dec 14, 2013
Messages
311
put hole into plug, carefully, drill hole or tap small hole with screwdriver and hammer(not to deep) , then pry out. to install, place new plug in place, take ratchet extension(3/8 or 1/4) and small hammer and tap domed plug till caved in a little, careful not to damage emulsion tube during process
 
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