What are the most common causes of a motor bogging down when the throttle is engaged?

You are describing a classic symptom of clogged hi-speed jets. the solution: remove carbs, disassemble, soak the metal parts overnight in carb cleaner, blow through passages with compressed air (or aerosol carb cleaner), reassemble with new kits (about $12 apiece on iboats.com), reinstall, link & sync.

before jumping in to the carb rebuild, rule out lost compression and bad spark first. Compression should be within 5 or 7% on all three cylinders, anything upwards of 100 PSI would be good on that vintage, but 90 would be ok if all 3 are equal. Spark should jump a 7/16" gap at cranking speed on an inline tester clipped to the engine block.

bad compression or no spark on a cylinder, post a question for the next troubleshooting steps.

Then make sure you have fresh gasoline, the tank vent is open and clear, the tank is free from water and debris, the throttle valves are all closed at idle, parallel throughout their range of motion, and perfectly horizontal (not beyond) at WOT, and that the choke plates stay open except when you engage the choke.

If the motor dies completely when you advance the throttle, check your idle speed (should be about 800 in gear with the lower unit submerged), make sure the timer base advances smoothly to its stop without sticking, and that the timing advance begins before the throttles start to open.

And stop running this motor until its fixed -- any cylinders starved for fuel are also not getting any lubrication.

you have wasted a lot of money. that's a gas transport undertaking. right here is the least complicated and maximum inexpensive attempt. locate the thank you to droop a one gallon gas tank from a tree or placed it on the roof of your domicile and run a variety of from it directly to the hose going into the carbs. you're arising an "IV" to your boat like interior the well-being facility. this might bypass the gas tank, bulb, clear out and gas pump. If the motor runs nicely whilst hooked as much as the "I V", then you certainly can start up up with the help of changing the gas line with a solid one from a marine keep and not from the Wal-Mart. If this does not paintings, replace all of the gas traces and filters on the motor. in case you nevertheless have issues replace the gas pump. If this does not shelter it, replace the carbs, or have a solid boat mechanic rebuild them for you. solid success!!

After 33 years , maybe it needs the carbs rebuilt. Or the engine is just tired

i have a yamaha 150v6 does the same thing carbs are fine