View Full Version : Engine Vibration/Shake Between 1200-1800 RPMs

05-24-2010, 07:59 PM
Boat: 96 Prostar 190 with GM 350 Throttle Body Injected engine

I've had this problem for the last 2 years ever since I had the engine replaced after a spun main bearing on the old engine (replaced with a rebuilt engine). The new engine exhibits a mild vibration/shake, like it is out of balance, but only between 1200-1800 RPMs. As soon as the engine hits about 2k RPMs, it runs perfectly smooth. I have not been able to figure out the problem and am looking for suggestions. I don't think it is a misfire because the engine has a new cap and rotor (although both were replaced with the engine as part of routine maintenance, so it could be that), new spark plugs every year, new plug wires last year, and the timing is set at 10 degrees BTDC. It is definitely not the engine alignment because I aligned it this spring, and the engine also vibrates when revved in neutral.

From browsing the internet, some suggestions I found are:

1. Cracked flex plate: there is no rattle noise, and a new flex plate was installed with the new engine. This could still be the problem, although I don't know why the problem would only present itself at certain RPMs.

2. Bad Engine Mount: no idea how to inspect these on a boat. They seemed fine when I looked at them to adjust the alignment last weekend.

3. Bad harmonic balancer: this seemed like a good possibility to me, but I'm not sure how to tell if is bad. Also a bit skeptical about this because I'd be surprised that it went bad the same day the new engine was installed.

4. ???

The vibration isn't bad, but it is noticeable enough to annoy me, and I don't want to do any damage to internal engine parts if the engine is out of balance. I would also like to diagnose it before the 3 year warranty on the engine expires if it is something internal.

06-01-2010, 04:30 AM
No answers for you, but I have been having a similar problem with an 09 X1 (MCX) since new. A noticable vibration from idle to 1400rpm (enough to shake the tower and steering wheel) - no power problems, pretty smooth above 1600rpm. Now at 180 hours and no answers. Indmar even freighted a new engine over to New Zealand and guess what .......... it still vibrates! (but not quite as badly). So now we are working our way down to the input side of the gearbox. It's a little bit difficult being in New Zealand (other side of the world to Indmar and Mastercraft engineers), so anyone who may have some input on this one??