For one thing, the .91 is a lot more than the M-1 needs, which will fly on a .46 2-stroke or a .56 4-stroke. Have you put a tach on the engine to see what you're turning at idle? Properly tuned, it can idle as low as 1900-2000 rpm, which is just ticking over. A 14-6 isn't too big, I have a 16-5 on my .91 / DR-1. The Bristol is a light plane, and the engine-prop combination is likely to pull it around even at idle. The tail skid isn't going to steer well without getting the tail light and using prop blast against the rudder to turn it, and like you said, it might want to take off, especially on pavement where there's no rolling resistance. I've flown a couple of the Bristols that my friends have, and am building one for myself. I'm putting an .80 on mine, and it will probably be too much, but it's an available engine. I am starting with a 14-4 prop and going from there.

What plug do you have in it? The stock Saito plug seems to keep the rpms the same when you take off the igniter, but I like the OS "F" plug for flying, even though my rpms drop slightly when I remove the igniter, indicating that it's a tiny bit colder than the Saito plug. I also use remote igniter sockets on several of my planes, and the clip seem to stay on the OS plug better. Either one is a good plug, pick one up and change it so you'll know what you have. Ken