Yamaha outboard 150 4 stroke bad vibration at 700 to 1200 rpm run great in nuetral and over 1200 rpm what could it be
is you propeller bent? a dent the size of a quarter will cause a shake. If not that means your engine is misfiring. here are the troubleshooting test and tool list you need to find out. Tools(1) A compression gauge (2) a spark gap tester.(3) A Clymer service manual for your engine. (you can purchase from your local outboard dealer. I know all these are expensive but if your going to be a outboard tech you will always need them. Troubleshooting (1) check compression. without the year model I'm giving you a average numbers. Look at your Clymer manual. all cylinders need to be over 100 lbs and equal each other within 5 lbs.(2) check spark. use a spark gap tester all cylinders should jump a half inch gap. if you have good compression and good spark, fuel is the problem nine times out of ten.Ethanol gas is proven to stay stable in fuel systems for 30 days in 90 + degree heat. Drain gas tank,clean fuel lines replace primer bulb and replace all fuel filters. Yamaha has a gravity filter under engine coul clean it. Finally replace spark plugs, I know spark plugs probably don't look bad but this is a outboard. Fresh spark plugs means less resistance for the ignition system. The ignition system is week compared to automotive. Test drive boat, if the problem is still there then there is further steps to take. Again I don't know the year model so I guessing its fuel injected. Pull out your clymer manual. what is great about the Clymer Manual for Novice techs is it gives detailed info on your outboard. Troubleshooting tips and teaches you how your outboard works.