Surging idle (1996 Evinrude Vindicator 200hp E200STLEDM)

[m_g]

Hi all,
I have been troubleshooting this engine for a while now (rough idle, stall in gear). Whilst doing a decarb today, I noticed that the engine would start up and idle (in neutral) at appx 2000 RPM on earmuffs (this has been normal since the mechanic adjusted it, it idles appx 1000 in the water). Anyhow, after appx 10 seconds, it will gradually rise in RPMs, getting up to 3800 RPM, holding for appx 10 seconds, then drop to 2000, slowly rise to 3800 RPM etc. It never stalls, or even sounds like stalling.

What could be going on here? Could this be an issue with the fast start function? Maybe a vacuum leak? Need to sort this quick because running the engine at these speeds in my driveway will not impress the neighbours!!!

History: new plugs, new fuel pump (VRO disconnected), polish fuel in tank, new primer bulb/filter/hoses, all cylinders within 15% PSI of each other

Any help much appreciated.

Thanks

[AndrewM-Fishing]

Classic sign of a vacuum leak but first disconnect the quick start function to eliminate a bad power pack, temp switch or start solenoid first.

[ChampioNman]

Don't over look your recirculation valves. You should test them as they are a one way check valve and you have 6 on your motor.

[m_g]

ok thank you, I will check these and report back!