Yamaha 70 hp high speed cut out

[Quogue]

Help! I have a 70hp Yamaha on a 1987 15ft Whaler, I belive the engine is the same year as the hull. Engine runs like a charm exept when runing wide open for ten to twenty min. it then dies. It will restart and alow me to limp back to port on one cylender, occasionally after a while it may kick back in to full power. This issue has pretty much ruined my boating season. I have emptied the gas tank and added a fuel seperator but this did not help. Also heat alarm does not go off and engine temp seems to be ok, as **** water seems normal, but I am not 100% on that. Any Ideas? I would like to be able to fix this myself if possible.
Thanks!

 

[BruceAML]

Re: Yamaha 70 hp high speed cut out

Is the Precision Blend oil injection system in place and working?

 

[Ray Neudecker]

Re: Yamaha 70 hp high speed cut out

First place to look would be the anti siphon valve in the fuel tank. Those older 3 cylinder models also were prone to intermittent CDI problems. First you will need to determine whether it is fuel delivery or electrics.

 

[BruceAML]

Re: Yamaha 70 hp high speed cut out

Are you using a portable gas tank? If so, is it metal or plastic? If it is plastic is the vent open? Do you have another tank and fuel hose that you can use as a substitute? What brand is the fuel hose?

 

[Quogue]

Re: Yamaha 70 hp high speed cut out

The tank is a 24 gal. plastic under the seat type, vent is open, I will have to get back to the boat, maybe this weekend to find the type of fuel line, but it never gave me a problem before.
Oil system seems to be working fine.
What does an anti-siphon valve look like and how do I test it?
Is a CDI hard to replace?
Thanks!

 

[BruceAML]

Re: Yamaha 70 hp high speed cut out

Was the fuel pump ever replaced? Did you clean the fuel filter element? The fuel bowl is clear, do you see fuel in it when this happens? Can you borrow a Yamaha portable fuel tank and hose to see if this happens when you switch tanks? CDI is fairly easy to replace. Do you have the correct spark plugs?

 

[djnelson]

Re: Yamaha 70 hp high speed cut out

It sounds like a similar problem to the one I have with a 1985 60 hp. How old is your 70 hp ?

 

[windknotnc]

Re: Yamaha 70 hp high speed cut out

I had a very similar problem with my 70hp Yam. It sounds like the CDI unit (power pack) to me. The problem is this unit is the control for the entire electrical system including spark advance and it costs about $450.00, ouch. You may get lucky and find a used one, but it will be in the $225.00 range and no warranty. Unless you are a pretty savy diagnostition I would take it to a Yam mechanic and have him check it. Putting it on is easy, but if the mechanic is already in there let him do it.