1992 9.9 4 stroke Hi Thrust Yamaha

[the machinist]

OK guys, a long time lurker & sometimes contributor on the Johnson page, now I have a problem here.<br />The motor in question is a trolling motor on a 20' Tiderunner & the mainpower is 150 Yamie. I have owned this boat & motor for 7 years & had to replace the fuel pump about 5 years ago. Now my engine will not run unless I pump the fuel primer bulb. I rebuilt 2 fuel pumps,(diaphragm & check valve) & still have the old original "BAD" pump. I replaced the fuel filter. Even replaced the fuel line connector O-Rings. Vacuum checked the fuel line to the outlet of the fuel pump & all OK. Reset the valves also. But still no run unless I pump the bulb every 3 min. This happens when connected to my main 60 gal. tank or a separate 6 gallon tank using seapareate hoses.<br /><br />Now this is what I found that makes things interesting. The fuel pump mounting on the valve cover base to the rocker arm pump plunger pad at the full highest position is .662" lowest settign is .806". The fuel pump has a lot of slack in the plunger system before the final activating the diaphragm, so much so that the rocker arm can not even take out the slack to where it starts to pump while on the motor. The plunger needs to be about .100 longer to even reach the rocker arm's activation point. <br /><br />I made a manual adjustable pump simualtor & by putting on a 16" clear hose on each outlet of the pump, filling them with fuel, & setting it to the .662" dimension & activating my lever which activates the pump plunger, I get no pumping action at all. I made a .100" adapter cap & with this in place & without changing the settings, I get pumping action. The pump diaphragm only moves about .050 total in this pumping mode. This is not just one fuel pump, but 3 different ones. I even put the old original "BAD" pump on my tester & it pumps.<br /><br />Are there some inhearant problems with the fuel system of motors?<br /><br />My question I guess is just how in the heck did this motor work for the last 13 years unless there is sufficient suction thru the carb to suck fuel once the system is pumped up by the bulb?<br /><br />One mechanic said these fuel pumps are weak & will not usually suck fuel from a belly tank, & needs a separeate tank closer to the motor.<br /><br />I am going to epoxy my spacer cap onto the fuel pump plunger & I will bet it will run.

 

[rodbolt]

Re: 1992 9.9 4 stroke Hi Thrust Yamaha

the carb will definatly not suck fuel from the tank. the fuel pump is not as strong as a car pump thats for sure. but a lot of my customers draaw from the main tank with them. I would suspect maybe a worn pump actuator. or just aa bad pump. could also be a small air leak someplace. but you are definately dealing with a pump issue. if you epxy that extension on it will fall off. epoxy most likly wont take the heaat and vibration. would be better to thead the ends and use a permmant loc-tite. or maybe weld a pad on the rocker.<br /> my question is if it worked for 13 years why do you have to modify it now?

 

[tevensa]

Re: 1992 9.9 4 stroke Hi Thrust Yamaha

Hello, I have the same motor and had the exact same problem!!! When a put a seperate smaller gas can near the motor the motor ran great! The pump does not have enough power to suck fuel and over 3-5 mins, it sucks the gas out of the bulb, stutters and shuts off. You could also install an axillary electric pump to pump gas to the engine if you have not already tried it.