why my engine vibration at 1000-1500 RPM
 

any ideal why my engine 350 with lt4 cam vibration at 1000-1500 RPM .engine only 10 hrs after re ring kit, new propeller, still vibration .thanks

Is it a mechanical/rotational induced vibration or a vibration as the result of combustion events (if that makes any sense)? Mounts loose, gimbals, prop shaft, unbalanced prop, prop pitch imbalance etc. In gear or out of gear? Need more details.

when the boat not in gear test it on my drive way with water hose, took it to the lake it is shanking at 20-30mph.i got no vacuum leak, (I check it with starter fluid )

Is the engine running rough- cam lope?

Check your mounts, etc. Is your u- joint coupling tight?

have you checked prop shaft runout

Was your motor rebuilt or taken apart if so - harmonic balancer did someone hit with a hammer for removal or reinstalling , is it in good shape regardless even if motor was not touched

Engine coupler warped - was it torqued down using a proper pattern and torque spec, or engine coupler loose - bolts backing out over time.

need more info plus was the motor rebuilt , is this boat a carb motor or Throttle body injection or MPI, If injected make sure the injectors are properly flowing and correct amount of fuel volume. On TBI engines it is really important both injectors need to match perfectly in spray patterns along with flowing volumes .

unbalance prop blade(s). As other mentioned put a dial indicator on the propshaft and rotate it to check run out.

next compression test solid and or solid leakdown test,

Next are all cylinders firing and have strong spark ?

Check your spark plugs and check them for any signs of water. We see so many improper winterizations that in spring time upon recommision this really causes poor running conditions and vibrations of the engine

I don't think lower unit or propeller
I test it run on my driveway with water hook up
engine idle 700 RPM when I rise up it star vibration to 1500rpm
spark plug new (color dart brown I took off when do compressor test)
compressor all cylinder 150psi only 2 cylinder -140ps
I did not do leak down test(will do )
I used carburetor elderbrock 1409(buy it on ebay)
any input please give thanks for help

same flywheel, same balancer? go back to basics...check firing order, replace dist cap.

need some more info. in your first post you say it has 10 hours since rebuild and still vibrates. did it vibrate before the rebuild or just since the rebuild. my guess is it has either the wrong flywheel or wrong balancer. it should be internally balanced so it would have a neutral balancer and flywheel but as it has been worked on there is the possibility that someone used the wrong one on either end.

the engine I remove from yard (2000 chevy van 3500) balancer same fly wheel used from boats
will check firing order