RE: Welcome to Club SAITO !
Ron:
If the engine rpm is in the right range with the 18x6 prop going to a smaller diameter will increase the speed only by raising the rpm. Better than going to a 17x6 would be a 17x8, equivalent load (for the rpm) but a higher air speed.
On the other hand, if the rpm is low then the 17x6 should help.
You select the pitch for the desired air speed, then diameter to get the correct load on the engine.
I’ve never had a problem with initial sealing on the upper o-ring on the intake pipe, it’s only after running the engine that the problem appears. One failed on the very first run of the engine, usually they last 30 minutes at least, but seldom more than an hour with the early version back plate. The stiffer ones of the early pattern would last two hours. The latest version, part number SAI7217A, are holding up better.
The same method of carb and intake pipe mounting has been used on all the small and mid-block Saitos from day one, there was never a problem until the FA-72 came along with the plastic back plate, too flexible, allowing the pipe to vibrate in and out, sting the o-ring at the top end. First change was a stiffer plastic, the latest ones have stiffener blocks added under the carb mounting boss. Still not a completely satisfactory solution, but much better. In my latest parts order I’m getting one of the newest version to try. We’ll see.
Strangely, the FA-82, using the exact same back plate has not had the problem.
Bill.