Rough Idle always, at hot or cold start; smooths after rev.

[Klutchy14]

Hi iBoats,

Turning here for help, since I haven't been able to figure this out through other channels (gearhead friends, calls with local shops), and don't want to turn to bringing this in to a shop yet. I've searched through the forums for a similar problem but haven't been successful,since the similar problems others have had only seem to be on cold start, or always at idle regardless of temp.

I am experiencing a rough idle only on startup, which will remain until i rev the engine....and it completely clears after a quick rev. The engine then runs perfectly smooth and strong throughout the RPM range (idle to full rpm, then back to idle...consistently). This happens with initial cold startup, as well as startup at full temp. To clarify, I can be running for an hour, or all day, cold and hot, and EVERY time I start the motor, it idles rough and I have to goose the throttle to clear it out. At full temp I can shut the engine off, start again, and have the same rough idle that I need to clean out with a rev.

The motor is a reman 357 Mercruiser, 2016 build, link to specs below. The engine was installed in 2018 at a dealer by the prior owner. I purchased the boat in May of 2019. (Boat is a 1998 Glastron GS 205...in great shape with the newer motor, electronics, all professionally done).

I've used StarTron fuel additive, and have tried carb cleaners. Changed oil, filter, and the water separator. Have put maybe 5-6 full tanks of Premium through the engine since I purchased (a few tanks have been 92-94 zero ethanol marine fuel).

Still having this issue on every startup regardless of engine temp or ambient temp outdoors.

My mind goes to carb adjustment, fuel pump pressure, fuel filter, sticky throttle linkage (checked). But, since this happens on every single startup, hot and cold, and immediately smooths out for the entire time I am running through the rpm range - I am perplexed at where to start.

Thank you to any help that can be provided.

https://www.mercurymarine.com/en/us/engines/inboard-and-sterndrive/remanufacturing/plus-series-alpha-
sterndrives/#specifications

4V carburetor and electric fuel pump

750 cfm Weber 4V with electric choke

Fuel delivery system Fuel injection system / carburetor type

 

[wahlejim]

Sounds like a choke issue that is clearing up after the rev. Sticky choke that is loosened with the airflow.

 

[alldodge]

Its a 1998 boat with a 2016 reman installed in 2018
Your saying its a carb but your link goes to a EFI motor

Its a carb, and I would say its the float level adjustment is a tad to high

When motor is first started its flooding, so rev it and accelerator pump dumps some gas out the bowl and now its not flooding as much, so idle smooths out.

Could also have a worn fuel inlet needle seat leaking, but doing it right after stopping, it leads me to float setting

 

[Rick Stephens]

Knowing that doing a carburetor rebuild is not a simple job for anyone who hasn't done quite a few of them. And not wanting to sound like it is simple as changing a filter, I still think you need to rebuild the carburetor and do a great job of making sure all the ports and passageways are clean and all the check balls are seating on clean surfaces. Double, triple check float height. I can't believe that 'gear heads ' haven't recommended that. Of course, most vehicles are fuel injected and thoughts of carburetor rebuilds is going the way of old farts like me. That's where I would start looking.

Rick