Rotax 912 ULS fuel pump

[bobcharl]

Would appreciate some advice re the following:

 

At around 200 hrs the fuel pressure in my Sav started a rapid pressure pulsation. Floods suggested it may be to do with the location of the vent tube from the pump and suggested some fixes, none of which worked. In desperation a new pump was fitted (supplied at no cost to me) and the issue was solved. However 50 hrs later the problem has re-emerged. Floods are at a loss to suggest a cause. Logic would indicate that it is not to do with the installation given the outlined course of events.

 

Does anyone have any thoughts as to what is going on?

 

Bob. 

 

 

[Downunder]

Well, it's the floats that control the flow into the carbies and therefore also the pressure. So that might be something to look at. (As well as the needle and seat)

 

There have been bad batches of floats in circulation for a while and I thought it had been sorted but a local Vixen with several hundred hours was found to have bad floats recently. 

 

The flow to the fuel pump in not impeded? No blocked filters, kinked hoses? Disconnected at the pump inlet side, the fuel flows readily out the hose from the tanks?

 

Tank air vents clear?

 

Any sort of restriction on the suction side of the pump, alows the posibility of vapour being pulled from the fuel.

 

As a long shot, the carb vent lines? Generally they give mixture issues if poorly placed in pressure/ vacuum areas.

 

If using a sealed cold air intake, they should be attached to that.

 

How are you measuring the pulsation? Definately no suspicion there? 

 

 

[Thruster88]

Would appreciate some advice re the following:

 

At around 200 hrs the fuel pressure in my Sav started a rapid pressure pulsation.

 

Does anyone have any thoughts as to what is going on?

 

Bob. 

 

I presume that you are seeing this as the needle moving rapidly on a mechanical gauge? This is caused by the normal pulsation of the diaphragm fuel pump at half crankshaft speed. Have seen this when I installed a VDO fuel pressure gauge in my Thruster. To fix I used about 4m of 1/8' od nylon line coiled up.

 

One reason the problem may show after some time is that normally the line to gauge is mostly filled with air providing some damping, as the air bleeds away it allows the incompressable liquid to hammer the gauge.

 

 

[bobcharl]

Thanks for the comments guys.

 

The installation uses a mechanical fuel pressure gauge.

 

D/U, have been through all the things that you mention without success.

 

T88, you may have picked it. Seems very logical to me.

 

I am away for the next month, but on return will check this out and post the results here.

 

Thanks again,

 

Bob.

 

 

[facthunter]

  The position of the vent tube from the CARBY's is critical to how that works. Nev

 

 

[onetrack]

Floscan offer a fuel pressure flow dampener that smoothes out pressure pulsations.

 

http://www.floscan.com/html/blue/accessorydetail.php?aid=98

 

 

[bobcharl]

Thanks Nev.

 

Have checked these and they seem to be fine.

 

Onetrack,

 

Thanks also, if all else fails this may be a good fallback position.

 

Bob

 

 

[facthunter]

The carby vent tube affects mixture. I only mention it in passing for you to be aware of. Nev