Yamaha 70 Bogging Question

Bought a 1987 16' Wahoo with a 1987 70HP Yamaha about a month ago and haven't been able to get it running right since. A bit of history: When I bought the boat, I tested it on the trailer with muffs using fuel from the built-in tank. It seemed to run fine and I had good, even compression across all 3 cylinders.

After towing it, I evidently stirred up all the junk and water that was in the fuel tank and tried to run the motor. I must have filled up the carbs with crap and it wouldn't run anymore without choke and full throttle. I emptied the built-in fuel tank and haven't even tried that tank since. I bought a portable 6 gallon tank and am using new gas. Had the carbs cleaned and reassembled by a mechanic using new carb kits. Changed the plugs, fuel filter, rebuilt the fuel pump using new diaphrams and gaskets, and changed all the gas lines from the tank to the motor.

Now the motor starts and runs great on the trailer with muffs. When I put the boat in the water, however, it goes like a bat out of hell for about a minute and then bogs down and refuses to rev past 2100 rpm. Today, I shut it off for a second after it bogged and then restarted it. It ran okay for another minute but then bogged down again. After that, I couldn't get it to to run above 2100 at all. It will rev up to full rpm in neutral though. It also seems like the bogging starts as soon as I make a turn or hit a wake (i.e. under increased load).

I'm beginning to think there may be something electrical going on.

Thanks for your thoughts and suggestions on how to test.

[jb123]

if it only ggets toi 2100 then it may be going into rpm reduc. does your buzzer work and what gauges do you have ?

did your mech do water test, run it under load ?

The mechanic and I did water test it for about 10 minutes and it worked fine then, but hasn't since.

There is a single digital gauge which has a tach, trim indicator, and some warning lights. The tach works and the trim indicator does not. I am not sure about the warning lights.

I was guessing that might be the case, although it revs up in neutral. Any thoughts on how to test it?

[jonp]

Hey Wahooyeah

I have a 90 HP few years newer but similar. The lights are for the oil. A buzzer will sound if it over heats (I think steady loud beep). It is Possible that it does not work. Has the water pump been serviced? If it starts to over heat it will go into RPM reduction mode...

[jb123]

does your buzzer work, you can test it by applying 12 v.

are the oil levels ok ?

[Dennisg]

HI, what you are discribing is very similar to a problem that I have had. You say that you are using a NEW 6 gallon tank, is it new, or just new to you? You have a obstruction in the fuel lines, fuel filter, fuel tank, somewhere. Start at the tank, remove the pick up line from the tank, if it has a screen on it, remove it you do not need to replace it if you have a inline fuel filter/water seperator. I like to use Yamaha 10 micron filter, (available from this web site, best price I have found). If you do not have a inline fuel filter/water seperator, you should . there is also a fuel filter on your motor, replace it, even if you have done this receiently. Blow all fuel lines. Clean your entire fuel system.
Let us know how you make out. Dennis

Thnaks for the thoughts Dennis. The tank, all hoses, both filters and fuel are all brand new.

Did you check the check valve in the tank? I have had problems in the past when the valve will not open properly when under load and then starves the engine for fuel. When you slow back down the fuel will catch up and will run fine until you demand more fuel that the tank can supply. Carl