odd dips in fuel pressure, pump problem?

Returning to the gascolator isn't ideal, but it's better than nothing, and doing nothing works just fine too.  There are lots of 912s out there with no return, along with a gazillion other carburetor engines that don't have returns.  It's worked fine for a year, in similar conditions, so there's no reason to suspect this is causing my problem. 

I seem to hear a some complaining about the new fuel pumps, but I don't know what the typical issues are.  I know the old pump was 20+ years old and working fine.  I have another engine on a test stand that's over 30 years old with what is likely the original pump.  I do have a new pump for that engine, so I might end up trying it on the plane to see if that solves this odd issue.  It seems like a long shot, but I think I've covered everything else.

All good Rusty.

I am well aware that a fuel return line, is not required to run a Rotax 9, however is strongly recommended, particularly for operations in high ambient temperature (such as Australia where I am from).

Speculation: Plumbing your fuel return line into the gascolator is compromising the effectiveness of the return line. Part of the return lines function is to facilitate the flow of cooler, gas bubble free, liquid fuel to the carburettors.

 I agree with Sean. Pressure fluctuates because as the fuel warms up it’s vaporizing in the line creating bubbles.

None of that has been changed, and in fact the cooling in the cowl was improved since last summer so it should be less likely, not more likely.  The only way it could be vapor lock is if there was a change in the fuel, not the return line.  Maybe they found some winter gas in storage and decided to use it (kidding). 

So from the lack of comments about the mechanical fuel pump, I guess I'm assuming that no one has heard of a problem like this being caused by the fuel pump?

Thanks,

Rusty

hi all

I agree with Sean on this one.  Let's also consider that in 2010 Rotax changed pump suppliers and the Corona pumps have a max output of .5 bar, up from .4.  This can create issues if you do not have a return line.  The installation manual has always called out a return line.  Certified 9 series all come with fuel lines on the engine and a return line is mandatory as part of certification.  

Fuel you get today, auto fuels, are not the same as they were years ago.  it is not all about the octane of a fuel anymore.  It is possible that your fuel is suffering from a high RVP, reid vapor pressure, for your conditions.   in accident investigations with crashed aircraft, when we have recordings on fuel pressure, this pattern is typical of vapor lock.  You need to cool the fuel especially the returned fuel to tank.  the gascolator is not an ideal place given the very small volume it has.  if you had a header tank it would be much better.  As a note the restrictor is also a control for fuel pressure.  Be sure that it is not contaminated and blocked and also that it is small enough (#35 mikuni pilot jet) to bleed off vapor and control pressure.  

Cheers