I have a 1956 Johnson 7.5 that runs well. I got it running and I got it on my little boat. A Lowe 12 foot V Aluminum. I took it out to my local reservoir to try it out. At full throttle I can only get up to 5 mph, Is that good? The only thing in the boat is me. I'm 6'3" 270lbs. It just seams to...