1970 Yamaha Mariner 8 high speed issue

[huntleybill]

This motor starts right up. Runs fine and idles good. This issue comes in when the motor runs at full speed. After several seconds at full speed, the motor dies out. When it starts to die out, if I pull the choke on, it stays running. I cleaned out the carb as there was a lot of dirt in the bowl. I see no leaks from any of the fuel lines. I believe this is a fuel issue but I don’t have a lot of experience with Yamaha’s.

Would appreciate any help troubleshooting this.
Thank you
Bill

[billw]

Don’t laugh….is the gas hose connected correctly with the arrow on the primer bulb facing the engine? I have seen people hook the hose up backwards and the engine does just what you say…..

Will the engine stay running if you pump the primer bulb? If so, it’s probably not the carb. Try another tank and gas line first, if possible.* Divide and conquer is the name of the game here.

*Sorry, Yamaha calls the gas line the "fuel pipe." Gotta keep the terminology right….. 😀

Long live American manufacturing!

[huntleybill]

Unfortunately, I do not have another gas tank or "fuel pipe". The arrow is pointed correctly so I guess my next question is Could there be a fuel line air leak? Would that cause the symptoms I have.I was going to replace all the fuel lines next.
Any other suggestions?

[billw]

I guess I’d try pumping the primer bulb when the event is about to happen. That will help circumvent a lot of problems associated with air leaks, restrictions, bad fuel pump, etc. This is, of course, a test, not a solution.

What kind of tank are you using? Modern plastic tanks are often plagued with problems. For instance, a lot of quicksilver tanks have their pick-up tubes loose and/or kinked. If you have one, I would pull the pick-up right out of the tank and give it a look.

If none of the above help, maybe you will then need to revisit the carb. By the way, Japanese carbs in particular can’t take a joke if they have ever had water in them. You can clean them and the corrosion will just keep coming back.

Long live American manufacturing!

[huntleybill]

This is a motor that a guy brought to me to see if I can fix his issue. It is a modern plastic tank. The thing leaks where the "fuel pipe" connector screws into the tank. I didn’t think that was an issue as the tank has to be vented anyway. I have a single line tank for my Evinrude 4HP motor but the connectors are different. Maybe I can reconfigure the "fuel pipe" to fit the Evinrude tank.

[fisherman6]

If there is a sufficient leak at the fuel line fitting connection at the tank that could be causing this symptom. It has to draw fuel from the tank by suction produced by the fuel pump. With a leak between the pickup tube and the fuel pump, it may be starving for fuel at high throttle settings. Sort of like trying to drink through a straw with a crack in the side above the liquid level in the glass. It would be worth fixing the leak or trying another tank.
-Ben

OldJohnnyRude on YouTube

[outbdnut2]

I fixed one of those 8 HP motors with similar symptoms for a friend. The gas line connector clip on the motor end was not sealing well. I bent the part of the clip back a bit that grabs the notch on the motor’s mating connector. It pulled the connector a bit farther onto the motor’s mating connector and fixed it.
Dave

[billw]

That leak may or may not be an issue, depending on where, exactly, it screws in. I would of course, fix that leak for safety reasons, but not until I took the whole stand pipe out and looked at it. If it just leaks around the seal with the tank itself, you’re right….it would be the same as a tank vent. If it leaks where the connector screws into the pick up tube, that’s a whole different story.

Long live American manufacturing!

[huntleybill]

ok…I will address the tank and let you know. Thank you for all the good ideas.

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