84 70 hp yamaha

[chuckclay]

I recently bought a 70 hp yamaha. When I take it out I can run it full throttle for about 5 minutes and all of a sudden it will lose rpms. It doesn't die just goes back down to around 1500 rpms. I put it in neutral and it goes thru the same process, runs fine then for a while then idles back down suddenly. I just had the carbs rebuilt. Before the carb job it did the same thing but instead of running full throttle for 5 minutes it would only last a minute or two.
Any ideas? Motor has good compression 115 + on all cylinders, starts fine, and idles fine.

The best thing I could think of is maybe the fuel hose is collapsing.

 

[drewmitch44]

Re: 84 70 hp yamaha

Any warning lights on dash or buzzers going off? It sounds like something is going on like the cooling problem. If its overheating or if its not mixing oil correct with the gas ive heard that the motor will limit the rpms to a low level like that so that way you don't blow it up. Someone else with better info will be along here soon. Welcome to iboats

 

[mtown]

Re: 84 70 hp yamaha

I have a 1985 70 hp yamaha. Had very similar symptoms to yours a couple years ago and it was the squeeze ball that had internally come apart and the ball bearing inside was being sucked into the outlet side and blocking the fuel line. I realized it when I propped the squeeze ball in an upright position and the problem went away. New squeeze ball end of problem, NOT. The next year same problem crops up and I am amazed that the new squeeze ball is defective so fast. I remove it from the fuel line and still have the problem.
Took to local repair shop: Here is the report.
Bottom carb not getting proper fuel. Tech found fuel leak in hose to that carb. Bottom carb needle stuck and we got the float needle unstuck. Ran engine and sounds OK. Removed fuel connector and let it run out of gas. Filled bowls with engine tuner and let sit overnight. Re-ran and adjusted carbs. All has been fine since.
ps this is by far the best motor I have ever owned. It has been abused since new, run in mud,sand,and ocean salt. I change lower fluid and plugs each year and not much else.

 

[triumphrick]

Re: 84 70 hp yamaha

One more thing you need to consider in addition to the other posts. Those 3 cyl Yamahas had a habit of building corrosion in the water jackets. The overheat conditon will throttle the motor back and after a few minutes will let u get back up on plane and repeat all over again. It was not an expensive fix as I recall. Pulled the head, blasted the passages and reassemble. Have you used or do you use Salt Away? I would try a few rigorous treatments and see if that helps. Also, if you boat in salt water I think it is a must....For $30 you may be able to break enough of that residue down to do you some good. Usually the fuel related problems mentioned rarely give you an alarm. Good luck!

 

[chuckclay]

Re: 84 70 hp yamaha

Any warning lights on dash or buzzers going off? It sounds like something is going on like the cooling problem. If its overheating or if its not mixing oil correct with the gas ive heard that the motor will limit the rpms to a low level like that so that way you don't blow it up. Someone else with better info will be along here soon. Welcome to iboats

I don't have any gauges and I am not getting any alarms. I've heard some of the motors will limit the rpms but wasn't sure if the older motors also do the same thing. I'll try the salt away. The motor was sitting up for 4 years before I bought it. Any way it could be the fuel pump. Thanks for all the replies. This is my first yamaha motor. Before I have only dealt with Johnson/Evinrude.