need some help from anybody that has a similar set up to mine. The boat has Mercruiser 475HP twin turbo engines. Its a factory Rayjay draw thru system. its using a Holly 750. Total timing per the factory is 28 deg because there is no waste gate and it just uses a bypass hole in the exhaust housing. I cant get the engine to idle down past 1150 rpm before it sputtres and dies. I have adjusted the idle mixture screws from 2 turns and up to 3 turns. I have adjusted the float levels corrrectly but just can seem to get it to idle. This is a all stock engine that has a at most .530 lift cam Like I said I can get it to idle at 1150 but below that it just dies. I feel like its running on the primarys at that speed. Any suggestions on what to check?

I never have used one if these…so just talking out loud ….Fuel has about a mile of port walls to cling too before it may get to combustion chamber, if it’s lucky to make it that far. Situations like this tend to need way more initial timing. And obviously decent amt of fuel.

What initial timing ? A guess from non experienced me would be easily 16-18 degrees ?

Timing makes the most sense right now. The manual calls out 16 deg at idle with 28 deg max at 3500

Silly question ... but do you have any idea how strong the vacuum signal is at the baseplate of the carbs?

Second silly question, you mention idle mixture screws but not throttle plate position ... as you know, on Holley's there's the 'pure' idle circuit to which you can fatten or lean mixture with the idle mixture screws in the metering blocks but there is also the throttle plate adjustment screw in the baseplate ... it is possible to open the throttle plates too much and move from the 'pure' idle circuit into the transition slots ... is this where you are in your setup?

Assuming ignition timing is sorted and you are dealing 'only' with an air/fuel mixture issue I think you have either a) too weak of a vacuum signal at lower rpm to supply sufficient fuel flow at lower rpms -OR- b) insufficient airflow on the 'pure' idle circuit of the carb(s) ... in this case you 'can' drill a small hole in each of the primary throttle plates to allow more airflow while keeping throttle plates in the idle-circuit only and not utilizing the transition slots. Just keep in mind, that generally speaking ... drilling holes in throttle plates to support more docile idle behavior is a bit extreme ... it does work but is also a somewhat irreversible mod. Also, as a general rule, drilling holes in throttle plates typically only needs to be done with fairly radical cam profiles.

Patrick,
On the Merc 475 turbo package, the plenum under the carb is designed to allow the engine to idle as if it's normally aspirated. There are two round plates that 'float' on a center shaft in that plenum. These plates are normally open and allow idle mixture to travel directly to the intake manifold. At idle, the mixture doesn't route through the turbos as manifold vacuum is pulling through the plates. Under load/power when the turbo speed starts to create pressure, then this pressure pushes those two plates closed and therefore all the mixture is running through the turbos.
I do not believe you have a timing issue. The issue is with the carb and/or vacuum leak. My guess is the carb. With the engine running block off the idle air bleed with your baby finger or use an eraser on a pencil and see if the rpm drops. The idle air bleeds are the outer ones. Call me if you want to talk about it further. Are both engines doing the same?
Andy