evinrude running rough

[muddypete]

Greetings
Don't know if anyone can help with this
Have a 1973 25hp evinrude sportsman which runs very rough on idle
Have only had it 4-5 weeks usually starts fairly easily and have just had a service done . both cylinders show 135 compression
Petrol used is unleaded with a 50-1 oil ratio
when running at any speed it runs beautifully but on idle it is very rough with revs dropping and going back to normal occasionally will stop altogether but is easy to start again.
Notice plugs are are a bit oily Underneath the pull cord are the controls
left side is pull out choke right side is stop button
Now in the centre is a rotating switch at the top right it says stop
quarter past says lean at the bottom says something speed (maybe low speed ) and the rest is missing
The mechanic disconnected this saying that it is not needed
Is there a simple explanation for the missing or should I go to another mechanic for a tune up and hopefully resolve the problem?
Appreciate any guidance plus the name of a good mechanic in the redlands region

[Noelm]

on the old motors the carby mixture was a very manual process, you just varied the mixture (via the knob you described) to suit your particualr motor and state of tune/condition.

[FNQCairns]

Yes as above the mixture can be at the heart of it. Setting mixture for best running at idle can impact cold starts and visa versa.

On the mixture screw...missing it just potentially dumbs a person down.

Just give each mixture change 5 seconds to settle down before moving it again.

Suspect you now have an engine set up with mixture setting to accommodate all RPM/engine temperatures, great on paper but the real world is entirely different.

Another thing to look at is the cam position at the carby, this relates to throttle position and ignition timing at all revs. This needs to have a place to settle correctly and then work in unison all the way to full throttle...so take any slack oo overrun out of it...easy to do without the engine running.

But don't go changing the ignition timing proper.

[Huey]

Hi, good advise and some more things that you need to make sure of if you want that old girl to run her best. Make sure the thermo is there and working to allow the engine to run at correct temp at idle. As mentioned the mixture screw can be adjusted to vary fuel/air ratio and make sure the carby butterfly is completing shut before attempting any adjustments with the mixture screw.

Make sure you also are running J4C plugs gapped to .030.

Cheers,

Huey.

[NTMID8]

I dont know if its a similar setup but i have a 93 60hp 2strk Evinrude on my boat that has a similar problem. I took it to my local mechanic who diagnosed it as a jammed thermostat, meaning the engine runs cold at idle as it lets all the water in. I never bothered to rectify it as i dont troll or have the engine idling all that often. It ultimetly comes down to the tune which he stated is difficult to tune when its running cold, but as soon as i give it any throttle it purrs.

[cormorant]

Your downside of running it without a thermostat, or a jammed one is that you may end up with excess carbon build up as they don't run as clean at cooler temps. Dirtys plugs , stuck rings , more wear, more maintainance, less reliability in the long term.. I would use a very good quality oil as they seem to produce less carbon and deposits and smoke no matter what the temp of the motor.

Thermostat is a cheap fix and makes the motor run a lot smoother

Not replacing it may also hide a underlying problem such as blocked water passages , stuffed impeller, leak, etc that will lead to a seizure

[muddypete]

thanks for all the comments
Had a play with the mixture screw and is now running fine at idle
Plugs used are ngk B7S Manual says these ere the right ones (hopefully)
Mechanic didn't look at the thermostat so will see how things go next trip and will check that if any problems
Still not too sure if I should connect the mixture knob (needs circlip) or leave it off as mechanic advised
Does anyone know what the rest of the wording might be around the mixture knob?
won't be out fishing til late next week as her indoors has given me a list of jobs to do as long as your arm (maybe need to invite a few mates around for a beer ; Bring your own tools) Heh Heh