Yamaha 25hp fuel delivery or suction problem??

[condog_aus]

Hi my mates got a 3yo Yamaha 25hp 2 stroke outboard. It starts and runs perfectly at idle and mid throttle. It also runs perfectly at full throttle but after its primed fuel supply is exhausted it goes off song but still runs at idle even if the throttle is held full open. If however you give the primer pump a squeeze it instantly comes back to perfect running at high speed. <br /> <br />We have tried a whole new fuel line and pump so its not an air leak in that section. The diaphragm was replaced by a yamaha service agent and that didnt fix it. Its like the engine lacks the suction to draw the fuel up by itself. If you pump the primer pump it will run perfectly until the fuel pressure is gone. Suggestions please. <br /> <br />All the filters were also replaced by the agent and the plugs. <br /> <br />Thanks in advance.

 

[bigbrownbuku]

Re: Yamaha 25hp fuel delivery or suction problem??

how far is the tank from the engine? do a vacuum test starting at the suction side of the pump working back at every union until you find the problem. does the boat have a water seperator? could be also this housing is cracked. remove the pickup from the fuel tank and check the pickup isnt cracked either.

 

[condog_aus]

Re: Yamaha 25hp fuel delivery or suction problem??

Weve isolated all those aspect even run another blokes working tank and line. Took the seperator off line but nope it still wont suck that fuel up at WOT. We havent checked anything from where the hose connects to the engine inwards.

 

[condog_aus]

Re: Yamaha 25hp fuel delivery or suction problem??

At first we thought it was the seperator cause after you primed it sucked air in but we tightened the clamps either side and now it doesnt suck air in, but we still cant get the engine to suck its own fuel supply at WOT.<br /><br />Id like to find the cause, but would a low pressure fuel pump solve the problem temporarilly.

 

[bigbrownbuku]

Re: Yamaha 25hp fuel delivery or suction problem??

you would notice fuel leaks if the line was cracked/split anywhere. disconnect the line at the carb and plug it. hook up a mighty vac to a short piece of hose with a fuel fitting on it and plug it into the cowl connector.

 

[condog_aus]

Re: Yamaha 25hp fuel delivery or suction problem??

To check the cowl connector.. Weve checked the hoses from te car to the conector, but not the connector yet. What provides the fuel suction in these motors. Eg on my motor cycle its the back pressure created by the air filter and airfilter cage.

 

[Elmer Fudge]

Re: Yamaha 25hp fuel delivery or suction problem??

Check for tank ventilation.<br />tighten the fuel lines and install new fuel pump diaphram.

 

[bigbrownbuku]

Re: Yamaha 25hp fuel delivery or suction problem??

the pump provides the suction. the diaphragm is moved by the pressure/vacuum in the crackcase.

 

[wtrojan]

Re: Yamaha 25hp fuel delivery or suction problem??

Try to isolate tank problems – tank vent or suction tube - just put some gas in a bottle and run the engine with that supply. Make sure the air gets into the bottle freely.

 

[condog_aus]

Re: Yamaha 25hp fuel delivery or suction problem??

Ta we found it using that method and worked backwards. It was the cowl connector. Thanks very much for all the help.