I've had a similar issue with a 36 HP Vanny. Not sure if the 18 has a similar set up or not. I'd start here first.
I ported/polished my intakes and did a few things to increase the air flow to the cylinders. This gave me a really bad lag/bog from idle to WOT. I knew I was running lean on fuel because, like you, I could eliminate the lag by closing the choke plates a little. Once at WOT, ran fine.
I did increase my main jets to compensate for top end. But, the pilot jets are the ones that need to be opened up a little to remedy the problem if you did manifold work...and/or the air/fuel adjustment screws need to be opened to get rid of the bog.
The 36 has air/fuel adjustment screws for each barrel. But, thanks to the EPA, they have these little plastic limit stops that only allow you about a 1/3 turn adjustment. The screws are scored so that if you try to pop the plastic cover off or cut them off, the head of the screw breaks. I ended up using a heat gun to melt the plastic off. I took the ones I broke and popped the sheared shaft out, trimmed the stops off with a Dremmel, then pushed them over the new screws.
You will need to open those AF screws to get rid of your bog. A little goes a long way!
Good luck!
