Tweet
Our Yamaha T9.9 4 stroke outboard - vintage 1999 - began to fail last season. Looking back through the messages on boating lists showed lots of similar complaints. It first it would mysteriously die at low RPMs....later it would die at any speed. It ran fine at full throttle, but then would quit as though it had suddenly run out of gas. Sometimes it would run good for half an hour, but eventually quit. Though it would always restart after a short rest, docking under power - or under no power - became an adventure.
I'm familiar with engines, and kind of enjoy the challenge of getting one to run right.
So this summer I inspected and rebuilt all the parts from the tank on back to the intake manifold and was disappointed to find nothing wrong...in fact, nothing even suspicious. About $150 later the motor didn't run one bit different. This was becoming less fun as the season wore on. So I took the approach used by many confused mechanics and simply retraced my steps but this time replacing every single part with a brand new one. Including the carb! Ugh $278! After each part was replaced we motored out for a test. Nothing worked until the last part to arrive -the new fuel pump. That is a very different looking piece....Apparently Yamaha has again redsigned the fuel pump with a new version. I didn't know about the change, had just ordered by the proper part number and they automatically sent the new style pump. Physically its a direct replacement for the old style pump. The difference is that the old pump just had the 2 fuel ports; the new pump has 4 ports and is water cooled. To install it connect the fuel lines just like the old pump, and then plumb the "pee water" line through the two brass ports on the port side of the pump....then send the pee water overboard as usual. The new pump comes as a kit with the required hoses and a mysterious clip. Both of my local Yamaha dealers were unaware of the design change and still have not received any tech bulletins about it. In my opinion, the design change is an attempt to cure fuel starvation caused by vapor lock in the fuel pump.
RESULTS: So far, so good....last week we motored all afternoon and even *****ed at idle for a couple hours (fishing). Motor didn't miss a beat.
Notes:
1. Apparently there was an intermediate design change that had a water-cooling block bolted to the port side of the fuel pump. That one can be replaced with this latest design which has all 4 ports in a one-piece aluminum casting. Better cooling.
2. The new carb no longer has the idle mixture adjustment hidden under a knock out; it is now a simple screwdriver adjustment. All else is the same, including jet sizes. They do not include the carb to intake manifold gaskets with the carb or with the carb kit. Best to order two of these gaskets.
3. If you use the Yamaha fuel fittings at the gas tank and motor, note that they will fit either end, but Yamaha makes them slightly different depending on which way the fuel flows. Look on the metal thumb lever to see the arrow denoting fuel flow direction. Being a curious sort, I tried it both ways and it didn't make a difference.
4. Its a good idea to add an additional in-line fuel filter between the primer bulb and engine. If you do so, use a large & low restriction type. Mercury makes a good one with 5/16 fuel barbs.
5. For boats with the ****pit remote control Yamaha makes two grades of control cable. For smooth and fingertip control replace the stock cables with the more expensive ones. Identical fittings, and the difference in price is small.
Finally, thanks to this list, the techs, and to Boats.net for all the help.
Hope this helps someone else out, Roger_Scotty
I'm familiar with engines, and kind of enjoy the challenge of getting one to run right.
So this summer I inspected and rebuilt all the parts from the tank on back to the intake manifold and was disappointed to find nothing wrong...in fact, nothing even suspicious. About $150 later the motor didn't run one bit different. This was becoming less fun as the season wore on. So I took the approach used by many confused mechanics and simply retraced my steps but this time replacing every single part with a brand new one. Including the carb! Ugh $278! After each part was replaced we motored out for a test. Nothing worked until the last part to arrive -the new fuel pump. That is a very different looking piece....Apparently Yamaha has again redsigned the fuel pump with a new version. I didn't know about the change, had just ordered by the proper part number and they automatically sent the new style pump. Physically its a direct replacement for the old style pump. The difference is that the old pump just had the 2 fuel ports; the new pump has 4 ports and is water cooled. To install it connect the fuel lines just like the old pump, and then plumb the "pee water" line through the two brass ports on the port side of the pump....then send the pee water overboard as usual. The new pump comes as a kit with the required hoses and a mysterious clip. Both of my local Yamaha dealers were unaware of the design change and still have not received any tech bulletins about it. In my opinion, the design change is an attempt to cure fuel starvation caused by vapor lock in the fuel pump.
RESULTS: So far, so good....last week we motored all afternoon and even *****ed at idle for a couple hours (fishing). Motor didn't miss a beat.
Notes:
1. Apparently there was an intermediate design change that had a water-cooling block bolted to the port side of the fuel pump. That one can be replaced with this latest design which has all 4 ports in a one-piece aluminum casting. Better cooling.
2. The new carb no longer has the idle mixture adjustment hidden under a knock out; it is now a simple screwdriver adjustment. All else is the same, including jet sizes. They do not include the carb to intake manifold gaskets with the carb or with the carb kit. Best to order two of these gaskets.
3. If you use the Yamaha fuel fittings at the gas tank and motor, note that they will fit either end, but Yamaha makes them slightly different depending on which way the fuel flows. Look on the metal thumb lever to see the arrow denoting fuel flow direction. Being a curious sort, I tried it both ways and it didn't make a difference.
4. Its a good idea to add an additional in-line fuel filter between the primer bulb and engine. If you do so, use a large & low restriction type. Mercury makes a good one with 5/16 fuel barbs.
5. For boats with the ****pit remote control Yamaha makes two grades of control cable. For smooth and fingertip control replace the stock cables with the more expensive ones. Identical fittings, and the difference in price is small.
Finally, thanks to this list, the techs, and to Boats.net for all the help.
Hope this helps someone else out, Roger_Scotty
to the starter of this thread, Scotty. I purchased and installed the revised version of fuel pump 
for not stepping up and doing the right thing in their failure to make an attempt to contact owners of earlier Yamaha outboards to notify them of the redesigned fuel pumps.
Comment