Check valves connected to the fuel system vs fuel starvation

[DooYouSea]

First of all, thanks for this excellent forum. This is my first try to solve a technical problem related to my Seadoo (that used to work like a charm for years now) and I found highly relevant information here.

I am currently experiencing what looks like a fuel delivery problem on my 1996 GTS. From a cold start, the engine behaves just as usual and runs great for about 10 minutes. Then, the RPM gets lower and lower at full stroke until it dies. From then on, I need to rest for a few minutes and then, restart the engine that would run like new but only for half a minute or so. Then, the RPM gets lower again and so on... I started to investigate the problem and one of the thing I found out so far is that my check valve for the air pressure regulation on the side of the hull is defective (no air flows across in any direction) but the other one is still working good (air flows towards the fuel tank only) and I guess this may be my problem.

Then I found this very interesting thread started by JPX titled "Troubleshoot Fuel Delivery Problems : low revs, bogging, surging". Since

In this thread, it is mentioned as follows:

7. Check all fuel hose connections are tight and that the check valves freely blow one way, but do not the other (one above the fuel tank and one on the side of the hull from the fuel tank):

So, this may confirm my hypothesis. But thinking about it, I now have a few question.

- Why two such check valves in parallel? Why not only one?
- Why using these two locations (above the fuel tank and on the side of the hull)?
- Could only one defective keep the system from working properly?

My current guess is that, since these valves are check valves with the flowing direction towards the fuel tank, they are intended to keep the tank from collapsing in the event of a severe temperature drop while allowing the positive pressure to build up in the other direction to feed the carburetors from the tank. This would mean that a blocked valved would not keep the system to function properly in a stable temperature environment and this would mean that I probably didn't find my real problem and that I will have to go on with all the process proposed by JPX in his thread (changing the grey hoses and so on...).

But before proceeding as such, if anyone could share his/her thoughts about it to help me understand what's going on, this would be appreciated. Thanks in advance!

 

[Strizzo]

There are two check valves so that air can flow in and out of the tank. In, as the fuel is pulled to the engine and burned, then out, to allow for venting to the atmosphere and release pressure. That is why one goes outside the hull (don't want fuel vapor inside), and the other is inside the hull(in, don't want to pull water into the tank).

When one or both check valves are plugged you can have issues with fuel delivery, either the motor starts pulling a vacuum and can't get the fuel out of the tank, or the tank pressurizes and floods out the motor by flowing too much fuel.

 

[DooYouSea]

There are two check valves so that air can flow in and out of the tank. In, as the fuel is pulled to the engine and burned, then out, to allow for venting to the atmosphere and release pressure. That is why one goes outside the hull (don't want fuel vapor inside), and the other is inside the hull(in, don't want to pull water into the tank).

When one or both check valves are plugged you can have issues with fuel delivery, either the motor starts pulling a vacuum and can't get the fuel out of the tank, or the tank pressurizes and floods out the motor by flowing too much fuel.

Thanks a lot Strizzo. All of this makes very good sense to me. Since this is my outbound check valve that is defective, from what you say, I could have experimented fuel flooding in the motor. But without knowing exactly what I was doing, when I found out the defective check valve, I tried to disconnect the reserve hose from the 3-way fuel selection valve and just hung it high enough to avoid for fuel leakage. My intent was to keep the tank at atmosphere pressure level. If my problem would be fuel flooding from an over pressure in the fuel tank, this would have solve the problem, wouldn't it? Or maybe I then had an low pressure problem keeping the fuel from reaching the motor? What you think? Thanks again!!

 

[Mickirig1]

It's really almost never that easy to solve a fuel delivery problem. The reserve line is not connected to tank till you select it with tank selector. You do not want to do what you suggest anyway.GAS FUMES IN HULL BAD!
Replace the vent valve. they are like $13 or so. Read up on carbs and rebuilding them and tuning. Also fuel line replacement, internal filter cleaning and tricks to rebuilding the carb / carbs. This site is FULL OF INFORMATION AND HELP.

 

[AdamXP800]

Have you ridden this ski with the seat off? Just a thought, could be an exhaust leak choking out the motor. Didn't read entire post, sorry if this was covered already

 

[DooYouSea]

Thanks. Actually, the reserve line trick was for diagnostic purpose only. And I didn't leave the open end into the hull. Just took it out at free air through the hole left open by removing the fuel selector, so, no harming possible. Having said that, I already ordered the vent valve and bought the fuel lines to replace the grey ones. My next step is the carb rebuilding as suggested. Thanks again.

 

[DooYouSea]

Nice try but yes, I already did so (didn't talk about it in my first post). Thanks anyway for the guess!!!