l have a 17 foot boat with a 150 horse mercury black max outboard.It has always started well.Now it cranks but wont turn over.Seems like a fuel issue but l have no idea.charged the battery and the voltmeter is reading fine. Sounds like it should turn over.Plenty of oil in the reservoir.Put gas in fairly recently.l had a fuel line leak recently and l changed it out, maybe it is not holding fuel pressure.Thanks for any tips

Have you checked the spark plugs for corrosion and such other evidence of bad shape?

If you think it is not getting fuel give it a prime put a very small amount of fuel in the carb and if it fire you are correct.
Could be the anti-siphen valve on the tank

It can only be one of two things. Fuel/delivery or fire.
Hit the carb directly with carb cleaner spray and try to start, if it sputters or even starts for a moment, then your not getting fuel to the carb.. If nothing different happens then it is a fire issue. likely a coil or magneto.

NEVER put carb cleaner in a 2 stroke engine is carb to clean it. That is the equivilent of taking all the oil mixed in with the gas in the carb & destroying it. No 2 cycle oil in gas means no lubrication, & that equals expensive damage.

If u must do a carb cleaning, u must disassemble, clean & soak the metal parts with carb cleaner, let it sit overnight, then inspect & dry all potential areas where carb cleaner might still be in it. Then reassemble correctly & see if it will run.

premix fuel in a spray bottle instead of starter fluid or carb cleaner, for reasons mentioned above.

Fuel line pressure, u can tell by whether the primer bulb is firm. If the bulb does not get firm when u pump it up, it is either leaking air somewhere, or fuel. Closely inspect all fuel lines under the cowl while pumping the bulb. Engine off, tilt the motor full up & squeeze the primer bulb -- if gas comes out the carb throats, ur float valve needles/seats r worn or sticking & flooding the engine. Carb rebuild will fix that.

Check for spark: remove a spark plug, reattach it to the boot, hold it against the engine block & look for a spark while somebody turns the engine over with the key. Use a plastic or insulated tool to avoid a nasty shock.

Check all six.if u get no spark on some or all cylinders post a question about how to troubleshoot ur ignition. Do not forget to say what year the motor is.

If the plugs r dry after cranking, that generally indicates fuel starvation.

You do not say what year ur motor is; if it has choke plates, make sure they're all closing off completely when the choke is engaged. If it has the primer/enrichener system instead, make sure the solenoid actuates when u engage the choke button, & make sure that it is draining fuel into the engine.

Cranking RPM is -- needs at least 250 RPM to generate spark.