Troubleshooting stumble at idle in gear

So I bothered to look it up and the spec for a later 225 (based on OP's avatar) calls for 750. But then this isn't a stock set up. For comparison the spec for a 1970 318 is 700, but for a 340 it's 900.

I'm also curious if you (OP) are using gas with ethanol and if so are you using an additive to protect your carb and tank?