Weird high idle issue Gen VI 7.4L carb

[mr300z87]

I am having a weird high idle issue when coming off cruising speeds and I am not sure what to look for. The boat is a 1990Sundancer 250 with Gen VI 7.4L/Edelbrock carb bought new in June of 2017thunderbolt V ignition with knock sensor. The boat runs awesome no issues getting on plane cruises at 27mph (gps) at 3400 rpms. Last night we when out for sunset cruise and were going to anchor in one of the coves when I back off the throttle to entered the cove at idle speed all was good until I put it into neutral to prepare the anchor and the idle was up around 1200 RPMs and would not return to normal. With the idle so high it was having issues going in out of gear so we decided to just head back to the marina. So we cruised at 3400 to the no wake, then 1400 RPMs almost all the way to the marine entrance. I then put he into neutral and RPMs were again at 1200 but this time after a few seconds the idle dropped to normal, this has happened in the past, idle was good and shifted perfect getting in to the slip. I have confirmed the throttle cable is not binding and the throttle is closed all the way but this has only been at the dock and not after cruising. I have not checked the timing recently but nothing has changed and as I mentioned she runs great so I do not suspect the base timing to be off. I was thinking could have some to do the knock sensor, sensor circuit or knock sensor module a timing to change and not returning to normal. I am hoping this to be a simple issue with a easy fix. I do have a complete spare TB V ignition set up including distributor and modules but I have never tested it. Another though I had was a vacuum leak but I would think the high idle would not return to normal if there was. If anybody has some ideas as what to look for it would be greatly appreciated. I hope to be headed to the boat later this morning to mess around with it a little. Thanks in advance.

Mike

 

[Bondo]

Ayuh,..... My guess is ya got some slop in the throttle cable,.....

Readjust it so it holds tension against the idle adjustment screw when in neutral/ idle,.....

 

[mr300z87]

Thanks for the quick response, I will double check that today.

 

[mr300z87]

So we went down to the boat today and messed around a bit. First I started her up and ran to temp so the choke was 100% off. I noticed if I rev'ed the engine in neutral the idle would stay at 1200-1300 RPMs for a few seconds then return to what I thought was a good idle speed maybe 850 RPMs. Then shut her down and took the throttle cable off to eliminate it being the cause. After re firing I could rev the engine by hand and it acted the same 1200 RPMs for a few then drop to 850. while siting it the bilge playing with the carb I notice on the engine cover the idle should be 650 RPMs and also that the throttle plate was just barely open, I adjusted the idle screw to lower the idle down closer to 650 and the throttle plates closed. Now when I rev the engine it is returning to 650+-. I then re adjusted the throttle cable making sure there is no slack, re-attached the cable and put her back together for a test run. She ran great no high idle issue and even shifted in and out of gear better. This was a simple fix of an issue caused probably by myself. I grew up in the 80s and have been working on cars and boat since then nut most of my cars have been fuel injected so my carburetor knowledge in limited even though I own a 1968 Plymouth. Have a great night.