Hello Dave
What size boat do you have ?
I have been working on a 18' SeaSport Sportsman for the last few years. It has a 125b in it.
I was an Automotive tech for over 20 years , picking up info for engine setups / props has been quite sketchy.
The boat sat for over 20 years, the water pump corroded right off the front of the engine, and of course Penta doesn't make that pump anymore.
I was able to purchase 2 250c running cores from a local dealer here in Anchorage.
My first though with everything that I was able to find out about the 125b is that it is massively under carbed
The engine has the A cam in it, I was going to bolt on the dual carb intake off one of the 250's but even so. the solex doesn't have a choke, and here in Alaska I didn't see it worth the time or effort to rebuild the carbs .
I added a redline Kit K861
http://www.carburetion.com/Products/ProductDetails.aspx?Part=K861
Then added an MSD Ignition 6560
https://www.msdperformance.com/products/ignitions/marine/marine_ignition/parts/6560
Just finished a round of test and tune on Big Lake here in Ak, running a Solas 15.3 x 19 cuped 3 blade we got 34.3 mph according to the Lowrance at about 4400 rpm
we also tried a Solas 14 x 21 4 blade cupped and got around 33 mph out of that at about 3750 rpm the trip before, but we had a detonation issue and set the timing back to 8 degs
out last trip we used octane booster but I didn't have time to swap the 3 blade for the 4 blade and play with the timing.
I think I might be able to get 4k with a timing adjustment out of the 4 blade, if so that would really be about the power band I would like to be in, the MSD is multi spark 3k and under, I'm thinking that would be the best for economy, what I can say though is we ran it last weekend for 4 hours on the lake and use 12 gallons of gas.
what pitch prop / speed / rpm are you running now?