Cleaning the air reference orifice on the fuel servo PA30

[Steve Nieman]

Howdy All,

In the July 06 Flyer, I believe it was Karl Hipp who wrote about his ferry flight to Sweden, where they had lower than normal fuel flow on one engine after takeoff.

I, too, am having low fuel flow probs on the left engine of my PA30, as well as associated other antagonations (I don't think that's a legal word) of having installed a salvaged left fuel selector valve that was badly corroded.

The article discussed pulling off what I assume was the main body Bendix RSA servo, and cleaning the air reference orifice.

Where the heck is that? I'm looking at the schematic on page 74d of the Comanche Service Manual, and I don't see it. I plan to go on the Internet and see if I can find it on any Bendix manufacturing site.

Plus, I've never read anywhere on this site about maintenance of the flow divider on top of the engine. If the flow divider diaphragm or valve is gunked up –– what can you do about it? Does it have to be sent out to be calibrated?

We seem to be getting the same amount of fuel through the nozzles, but I haven't measured carefully over a set period of time.

We have installed a newly overhauled fuel selector 1H7-1 2H from Webco, but the engine will barely run at full throttle. Obviously, we've got some gunk downstream of the earlier installed bad selector valve. Neither the engine-driven or aux electric fuel pump will deliver enough fuel to keep the engine running anywhere near normal.

The engine's only got about 150 hours on it, so the engine-driven pump is practically new. The aux pump is old, however (and probably needs to be checked for proper psi output [I think it's around 28]).

We found some crud in the fuel inlet fuel strainer screen in the main Bendix body, which we cleaned out. We blew out with compressed air all the nozzle lines from the flow divider.

I've read on this site that there could be air in the system that neither pump maybe cannot purge. Any ideas on how to ensure there is no air restricting the proper flow of fuel downstream of the selector valve into the engine?

Any help would be appreciated. Thanks

[Zach Grant L1011jock]

Steve,
Let me guess, engine runs fine up to about 2200 rpm on the ground then falls on its face, and bangs and shakes as you try to advance the throttle. Well, this is THE symptom of a bad fuel selector, and I hate to say it, but once you have gotten this bad, you are in for an expensive ride. Simply stated, all of the stuff the fuel selector/Strainer should havce caught went into the hoses, the fuel pumps, the servo, the flow divider, the spider lines and the injectors. Changing the fuel selector to a known good one helps keep more debris from moving on, but it won't help what is already there. Depending on the capabilities of your shop, all of this must be sent out for OH or replacement. Don't get cheap on this as the debris runs downstream. Start with the hoses and the fuel pumps. Next, OH the servo (and for my money, it is worth it to pull the flow divider/spider lines at the same time and overhaul as a set. If you want a matched set of engines, now is the time to do it, and send the other side in as well and tell them to match the flows). Legally none of these parts are field servicable so they must be sent in to a repair station. Last but not least, if all of your changes and cleanings still do not yield results, or if the problem becomes intermitant (and if it starts to shake and drops a cyl during flight, pull the engine back, or shut it down or you will be buying a cyl too...ask me how I know...) call GAMI and get a set of injectors (a whole set of GAMIs ar about the same price as one Lyc injector...). As long as you are working from the fuel selector out to the engine this can be done piecemeal, or just do it all at the same time and you will be golden.

BTW the uper deck pressure reference line is the hose connected to the open ended fitting on the back baffle, and senses the pressure of the air in the upper plenum of the engine compartment, the same pressure that the injectors see through their air vents. It should be connected to the servo body via a small hose. IF you are a turbo, this line is connected to the box that the flapper valve is in.
-Zach

[Steve Nieman]

Actually it's worse than that. We can hardly get the engine to idle with the throttle wide open and playing with the mixture lever...

I got a reply from Precision Airmotive and Karl Hipp pasted below.

My mechanic and I are going to try a few other things, but we'll probably have to send out both the servo and the flow divider for bench calibration.

Me mechanic thought the servo's finger fuel inlet screen filter would have caught all the bad stuff from the bad fuel selector, but that doesn't appear to be the case. Plus, the servo's inlet screen is downstream of the aux pump and engine-driven pump, which too could have been damaged by the crud in the bad selector. (Which makes me think that I should send both out for overhaul, because they could damage the overhauled servo and flow divider... if they had some residual junk...)

I should have sent that salvaged selector to Webco for overhaul, but I was led to believe that the selector came from a flying PA30 that had just recently been parted out. Plus, that was the time period when Webco was not doing fuel selector overhauls because of their earlier problems with the Feds.

Should have been more careful. Sounds like it's going to be an expensive lesson out behind the wood shed...

~~~~~~~~~~~
Begin forwarded message:

From: Alan Jesmer <alanj@precisionairmotive.com>
Date: July 3, 2008 7:10:40 AM PDT
To: stevenieman@mac.com
Subject: RE: Data posted to form 1 of http://www.precisionairmotive.com/feedback.htm
Reply-To: alanj@precisionairmotive.com

Hi Steve,

Unfortunately, any cleaning or repairs that have to be done to the servo or flow divider will have to be put on a flow bench to get properly calibrated. You will probably fight the problem until someone can take it apart, clean it, and calibrate it. If you go to the web site, click on contact, then
warranty repair centers, I would recommend B & S Aircraft in Wichita (800-835-2961,or G & N Aircraft in Griffith, Indiana (219-924-7110). They can give you a quote. Thanks and have a nice day. Alan Jesmer

~~~~~~~~~~~~~
From: karl hipp <khipp@99victor.com>
Date: July 2, 2008 9:16:38 PM PDT
To: Steve Nieman <stevenieman@mac.com>
Subject: Re: Needin some help on a PA30 engine

The low fuel flow we were getting was due to an air reference hole in the inlet of the fuel servo being partly blocked with oil. The oil was from the new brackett air cleaner elements which had been overly soaked with oil at the factory by a worker there who had stock in the oil companies. Karl Hipp

[Jay]