Yamaha 70 HP 1996/7 2 stroke Loses Power under load

[Melbournite]

Went out in boat, fixed overheating problem but got boat up to around 3500 revs and power died boat still managed to tick over but when in gear and 3/4 throttle there was no power at all. Bought throttle back to idle then back up to low revs and boat moved forward. I tried revving motor and it did rev up but seemed to be very inconsistent. Thinking it may not be getting enough fuel. Any suggestions on what should be checked?

 

[robert graham]

Have the carbs been cleaned?....could be debris/varnish in carb jets causing loss of power....

 

[flyingscott]

Do a compression test

 

[Melbournite]

Had the carbs cleaned about 12 months ago so would hope they were still okay, also have done a compression test and got 117,115 & 110 from top down. Was wondering about the primer bulb as I noticed this wasn't hard when power was lost but didn't seem to make much difference when i primed it.

 

[eddie haskell]

Clean the carbs again. Do you clean them yourself or pay to have it done?
If you pumped the ball while it was losing power and it made no difference then you can nix fuel pump.

 

[Melbournite]

Carbs were cleaned by a Yamaha dealer. New spark plugs have been put in but come to think of it when boat had been standing awhile it was hard to start. Did the usual priming and the bulb took sometime to pump up. Then it seemed to take awhile before the engine fired,Once it did then we had to When it did it seemed to help getting it started by pumping the primer bulb. Don't think the petrol filter has been changed so intend to check & change that,, check the fuel pump to make sure it's working and check the primer bulb to see if that works properly, also the fuel tanks to ensure they are aerating okay. If they are all okay then will recheck the carbs.

 

[Melbournite]

Been awhile but back to my post, I was speaking to a car mechanic who suggested the coils could be breaking down when the motor warms up. Have checked the resistance on the coils and all seemed well, however have found out that the spark plug boot (connector between HT Lead and spark plug) has a break in it on the first and second cylinder. Have just gotten new leads and boots for all 3 cylinders so will attach and test and see how things go from there. I've also fitted a new fuel pump but as yet untested in the water.