I finished building three 6.5 hp longtails, based off of the madmudmotors plans, and tested one of them yesterday on my 1436 flatbottom. The motor worked good, but vibrates like crazy in the mid rpm range.. Very little vibration at idle or WOT. What could cause this? I know it could be a number of things, but how come it vibrates bad at certain rpms and not others? Any ideas would help. Also, could this be bad for the motor or mean something is wrong with it? Thanks.

Don't know what "vibrating like crazy" means but they all vibrate more at one speed range and usually it's a mid range speed. Sort of normal if you ask me. Did you install a center point bushing in the shaft housing? What is happening is some shaft wipe.......in my humble opinion. Duckdon

yes, has 3 bushings, one in each end and one in middle

Mine is a 6.5 gd and does the same thing at 1/2 throttle has since the day I got it so.

All mine do the same thing, I have 1 mudbuddy and a bunch of homebuilt with the same plans.

Have you checked to see if your shaft housing is bent? Put a straight edge or level along side the housing. When you weld metal it wants to pull, if its not straight and bent. Beat it with a wooden block and hammer to straighten it out. Good luck. It doesn't take much on a 60" or 72" shaft to cause vibrations.

thanks for the help, i guess it is normal. Since this is my first experience with a mud motor of any type i was concerned, so its good to know it is common. And yes i think they are all straight. i built a jig to put the motors together and do all of the welding in to make sure they were straight, and checked numerous times. Again, thanks!

Are you using a Harbor Freight engine? Most of those aren't bad engines, but I know of several guys that have bought them and they vibrate bad. They have thinner castings and unbalanced crankshafts. The quality control on those engines isn't very good. They also have fuel leak problems around the carburetor.

yea, the one i tested is a honda gx200, which is the motor i built for me to use, but the other two that i built to sell are the 6.5 hp blue clone from harbor freight. havent run them yet.