Guys,

There’s already a lot of information here regarding target AFR - so much so that things get really confusing especially when people start talking about FI and blowers etc.

I had the 311 out yesterday - finally. Idle kind of hangs in the low to mid 12s on starboard, port a little closer to 13. Planing and cruise mid 12s (if I’m remembering correctly). Most RPM seen was about 4200-4300; first time out and wanted to take it easy to get things run-in properly.

Mighty Demon 850 down-leg carbs with 86P, 93S. 4.5 PV, fuel pressure 6 psi.

I’ve seen numbers thrown around of 13.x cruise all the way down into the high 11’s for full operation (Mercury).

So, normally aspirated and carbureted. What are good targets for idle, high-load attempt to plane, cruise, WOT?

Running this setup since 2019 on marina 89 ethanol fuel. 496HO with Raylar 525 kit. Stock Hyper pistons with ring gap opened up.

Sounds like she’s moving along nicely. Plug looks good too.

There’s a lot of stuff to read here on OSO and a number of “differing” opinions. Deep dives, threads going off into different t tangents, etc. Confusing.

In the past I would have just run it, made sure it wasn’t stumbling and/or lean-popping, and checked the plugs to make sure it wasn’t eating itself. Now there’s all this “data” to go through.

Still working out the kinks.

sometimes too much data is as bad as not enough. as for the cruise afr it can depend on the boat. a 10k pound straight bottom at 3500 may require more fuel than a 7k pound step bottom as the load is way different. also keep in mind that Mercury is notoriously rich on their setup for safety and warranty

Mine runs 12.3 - 12.6 @ constant to 3500-4000 rpm
509ci / 850 q-fuel / 9.1 / 7# fuel pressure.
Glad it is going well for you Tom.

Smitty and I saw no gains leaning it out over 12.2 towards peak RPM/HP. I believe we went to 12.6.

Only way to read a plug is to run at a give throttle setting for a few minutes and click the ignition off. Naturally if you've got something fast you'll want to slow as fast as possible to a safe point before clicking it off. Then read the plug. Once you've idle in to a dock all your getting is the idle mixture reading.
In theory 14.7:1 would be perfect. But under load it just doesn't work out. Best WOT power usually comes between 12.5 and 13:1. I honestly don't know how high you can go on an engine fully loaded while cruising and not encounter problems.

Thanks Gary! Been an adventure for sure.

I may be in close proximity now.

I’m not sure I’ll get out to do any testing this weekend… daughter’s bday… but will shortly thereafter.

My thought process here is to watch vacuum and map AFR readings to those in increments… getting on plane, cruising, and then wide open. Once I have that data, I think I’ll have a good idea of jetting and pv function.

Port engine went a little lean with idle at the end of the day… 13.2. I’m not sure of what that’s about but I’ll need to investigate. Hoping my filters are doing their job.

Any other thoughts?

12.5 seems to be the sweet spot per what I’ve read.

I’d rather not find out what the upper limit might be. Safe is better than sorry!