Early M20J fuel selector, tank, or pump issue.

[jetdriven]

Posted in vintage because this airplane has the Dukes fuel selector/gascolator installed in Mooneys from 1964-1977.

We have a 1977 M20J with the old style fuel selector, and bladders. For the past year, we have never had a problem with fuel.  No water, nothing. Over the weekend, our 3rd partner took the plane to Kansas City and returned yesterday. 

While cruising at 11,500', 2400 RPM, LOP, 7.8 GPH, he switched to the left tank.   After about 8 minutes (1 gallon) the engine began sounding  uneven and like it was getting too lean, the fuel pressure began to cycle rapidly from 10-20 PSI, and the FF began to show the same fluctuation, from 7.8-9.2 GPH, for 5 minutes. It began to run rough.   He switched back to the right tank, ran it another half hour, that was solid 21 PSI, then  went back to the left. The fuel pressure cycled between 15-20  for another ten minutes, then all was well.  He ran the tank down to 8 gallons and refueled before this leg.  He noted the selector felt a little strange on the left tank, like it went slightly past the detent. he did not use the electric pump.

Question I have is, what happened here?

[KSMooniac]

That is puzzling.  At this annual I changed my gascolator gasket and checked the screen (clean as can be) and had some similar indications during my test run after re-assembly.  I chalked it up to air in the system as it seemed to have completely disappeared after a short while.  If the selector feels noticeably different now, I'd suspect you might be leaking air through it, and perhaps it is time to get it overhauled.  

[John Pleisse]

I had the same symptoms and issues. Weber Aircraft (best shop in the world) triaged it well. Mine crapped out in 2005. The factory had a long wait for the OEM part, so they went to plan B:

Installed new "Airight" Fuel Slector valve assy P/N 36100-22, in accordance with Mooney retrofit drawing # 0000001-501.....

It is a great unit...different feel..works perfect and it has been 7 years. Hope this helps.

[jetdriven]

The problem is back, this time it is on the right tank. While cruising at 11,000, switching to the right tank causes the fuel pressure to drop from its steady 24 PSI to about 15 PSI and back continually, which is the lower red band. The engine runs uneven, and the fuel flow meter goes from a steady 7.8 GPH to jumping up to 8.5 GPH. It also runs a little rough and uneven. I assume this is air getting sucked into the fuel selector at the shaft. At 9,000 the fuel pressure drops to 18 PSI, and below 9,000 feet the fuel pressure is steady. Somehow it is related to altitude.

What do you all think?

[320KPH]

How does a J go at 11K?

What MP do you get up there?

[jetdriven]

How does a J go at 11K?

What MP do you get up there?

at Peak EGT and 2500 RPM, around 145-150 knots and 7.8 GPH. MP IIRC was around 18".

[larryb]

I agree on air being sucked in. On my previous plane one time while switching tanks I got a squirt of fuel in my hand. It was fixed by a couple new O-rings on the selector shaft. The selector didn't even have to be removed from the plane.

[N601RX]

If its the same one used in the F varnish and sludge will build up on the ball and seat and allow the drain to leak while setting or suck air while running. It can be took apart and cleaned.

[N201MKTurbo]

This happened on My 77J, I cleaned and replaced all the o rings and it has been fine sense.

[jetdriven]

Sounds like an R+R of the fuel selector with new o-rings is the solution. Does anyone have the part numbers for the O-rings?

[carusoam]

Byron, are you still out island hopping?

Let me know if you need a FedEx package of something shipped somewhere before the holiday really happens!

Best regards,

-a-

[Marauder]

After I read the ENTIRE thread I saw that it was happening on both tanks. It does sound like a selector valve issue.

[jetdriven]

We havent seen the issue since a year ago but we rarely go to 11K for cruise. We did manage to make 775 NM on a 54-gallon J, which takes serious altitude and a tailwind to do it. We used 42 gallons on that 5-hour leg.

[jetdriven]

Byron, are you still out island hopping?

Let me know if you need a FedEx package of something shipped somewhere before the holiday really happens!

Best regards,

-a-

nah we are back in Boca, the Bahamas trip was great.