I found in a Service Book from MerCruiser that said, The throttle body has a 75 mm (2.95 in) bore ( the 496/8.1 also has a 0.100" bypass hole for base model engines and a 0.200" bypass hole for HO [high output] models).

thanks:drink:

so what you are saying is drill out the by pass hole when converting to the ho? if you don't what will happen ? starting problems etc.?

Just the idle. I wound up plugging mine but it's a 90 mil tb.

YOUR IDLE SHOULD BE AROUND SIX HUNDRED RPM,,,i would start small and drill until your idle reaches six hundred

Wasn't sure who you were talking to...

The engine in stock form idles at 650. Certainly no reason to go lower as you increase your chances of reversion.

The bigger the cam, the higher it really needs to be. I try to keep mine at 750 or just below 800 mark.

This is specific to the 600 though.

boatnt

I thought 496's were supposed to idle at 750-800?

Get Raylar in here, he'll know.

All older stock MerCruiser EFI engines 496/8.1 and below idle at 590- 610ish RPM's

As of somewhere around 2007ish the idle speed has been dropped down to 540-550 RPM's on small blocks V6 & V8,

Here is the list for non emissions engines.
Vazer engine is at 600 rpm
4.3 mpi is at 550 rpm
5.0 mpi is at 550 rpm
350 mag is at 550 rpm
377 mag is at 550 rpm
496 mag and HO is at 600 rpm if it has the mechanical shift
496 mag with DTS is 575 rpm (including Axius)
496 mag HO with DTS is at 625 rpm (including Axius)

And here is a list of engines with emissions.
Vazer engine is at 650 rpm
3.0 MPI with emissions is at 700 rpm
5.0 mpi is at 550 rpm
350 mag is at 550 rpm
350 mag is at 550 rpm
377 mag is at 550 rpm
496 mag and HO is at 650 rpm (ALL MODELS)

All the V6 & V8 carburetor engines should be set at 640- 650 RPM's, And the 3.0 TKS should be set at 700 RPM's.

I don't see any reason that you couldn't do them both in a weekend if you have enough room to work in front of the engines.