M20J - Fuel Servo, Opinions Wanted

[d0tnet]

During a 100 hour inspection, it was noted that the fuel servo is at it's limit for adjustment.

The shop tried to re-center the threaded bolt, but reported back that even after doing so and gaining some adjustability, the servo is not acting normal.  After the RPM drop while leaning, there is actually an increase that follows which apparently is not normal.

Only about 500 hours since overhaul, but it seems the prior owners did not OH the fuel servo with the engine overhaul.  I find this to be short sighted, but I digress.

I have a Precision Airmotive servo with Superior Air Parts diaphragm currently installed.

What is the consensus on overhaul vs new and what shops are recommended for overhaul?  Quality Aircraft Accessories and Precision were the two options offered up to me.

What is a typical lifespan of a fuel servo?  Should anything else be proactively looked at - i.e. replace fuel hoses, etc?

Thanks!

Edited October 13, 2015 by d0tnet

[KSMooniac]

Definitely short-sighted to not get the fuel injection system inspected, much less overhauled, during the engine overhaul.  Call these folks:

Aircraft Carburetor and Injection Services of Texas, LLC.
2731 Brookfield St.
Dallas, TX  75235
214-358-2377

and they'll take care of you.  Don Maxwell uses them.

 

[d0tnet]

Definitely short-sighted to not get the fuel injection system inspected, much less overhauled, during the engine overhaul.  Call these folks:

Aircraft Carburetor and Injection Services of Texas, LLC.
2731 Brookfield St.
Dallas, TX  75235
214-358-2377

and they'll take care of you.  Don Maxwell uses them.

 

Thanks for the referral.  I spoke with Bobby (seems very knowledgeable) and the only challenge is the turnaround time.  I just got notified about this 3 days before a trip after plane has been there since beginning of last week...

May need to go with the Precision unit since it is available for install tomorrow unless the consensus is there is a significant quality difference.

[KSMooniac]

I would trust the small shop with very experienced technicians that have been doing it for a long time over the bigger shops.  It will probably be less expensive too, but if you feel like you must make the trip then doing an expensive exchange is likely your only option.  My unit came back looking like new, and I'm finally going to fly it this evening but expect no issues.  We ran the o/h engine on a test stand briefly before installation so I know it functions.

[jetdriven]

Precision owns the Bendix RSA fuel injection. It's the same thing.  Id go with overhauled unless it's the same price. 

[N201MKTurbo]

If the RPM is increasing as you approach idle cutoff it is too rich. When you say you are out of adjustment, are you out of adjustment on the rich side or the lean side?

I would get a second opinion before you spend that much money.

I have found that the Bendix RSA-5 will last for a long time and when it fails it does it slowly and usually benignly.

[d0tnet]

If the RPM is increasing as you approach idle cutoff it is too rich. When you say you are out of adjustment, are you out of adjustment on the rich side or the lean side?

I would get a second opinion before you spend that much money.

I have found that the Bendix RSA-5 will last for a long time and when it fails it does it slowly and usually benignly.

The servo being out of adjustment was corrected by re-centering the threaded screw to offset the adjustment nut to allow for more 'range'.  After doing so, adjustment leads to a RPM that decreases but then spikes back up.  This symptom is the reasoning behind the suggested overhaul.  I don't know how a proper servo should act, so am a bit in the blind here.

For reference, this unit was new prior to 2000 - seems like '98, so a long while in calendar time.  Does this 17+ years align with your experience on longevity?  I try to follow the 'if it ain't broke, don't fix it' methodology, but 1.5k spread over 17 years for peace of mind is seemingly a small price to pay.

[jnisley]

Definitely short-sighted to not get the fuel injection system inspected, much less overhauled, during the engine overhaul.  Call these folks:

Aircraft Carburetor and Injection Services of Texas, LLC.
2731 Brookfield St.
Dallas, TX  75235
214-358-2377

and they'll take care of you.  Don Maxwell uses them.

 

We just had our servo and flow divider overhauled by this shop, all good so far.

[N201MKTurbo]

The servo should be adjusted so there is a 200 RPM rise just before it dies while slowly pulling the mixture control to cutoff. The idle mixture should be slightly rich, so as you pull it into cutoff, it gets leaner as you actuate the internal cutoff valve. As it gets leaner it goes through the Peak EGT (stoichiometric) mixture and the power increases a bit causing the RPM rise.

I set mine to a 50 RPM rise, I think it idles better.

[jnisley]

Ours failed catastrophically several month ago, one minute it was fine the next minute our beloved Mooney was a glider:(

It was putting out more than twice the normal fuel flow!

 

[d0tnet]

Based on feedback, I am sending out the fuel servo to Texas.  Bobby is going to try and turn it same day since he has one he can start on today, which would save my trip.

Thanks KSMooniac for the nudge in the right direction (will go commercial if it comes down to it); after seeing your comments, I did some further research and saw some pretty scary stuff in the NTSB db related to Precision OH units generating no fuel flow directly attributed to engine failure.

For other's future reference, make sure you speak to them about the updated mailing address.

ACI

9808 Clark Airfield Drive

Justin TX 76247

 

[jetdriven]

The servo should be adjusted so there is a 200 RPM rise just before it dies while slowly pulling the mixture control to cutoff. The idle mixture should be slightly rich, so as you pull it into cutoff, it gets leaner as you actuate the internal cutoff valve. As it gets leaner it goes through the Peak EGT (stoichiometric) mixture and the power increases a bit causing the RPM rise.

I set mine to a 50 RPM rise, I think it idles better.

that is far far too rich.  It should be a 0-40 RPM rise.  Mine is near zero.  http://www.precisionairmotive.com/Publications/SIL%20RS-67.pdf

[N201MKTurbo]

I stand corrected. The A&P who trained me on the servo said 200. I always thought that was too much. I have an older manual that says 200 also, but I agree with the one you linked to.

Either way the manual referenced above completely describes how to adjust the adjuster to get it in range.

[N201MKTurbo]

Ours failed catastrophically several month ago, one minute it was fine the next minute our beloved Mooney was a glider:(

It was putting out more than twice the normal fuel flow!

 

Did you find out what was wrong with it?

[d0tnet]

Ours failed catastrophically several month ago, one minute it was fine the next minute our beloved Mooney was a glider:(

It was putting out more than twice the normal fuel flow!

 

Hi John - thanks for sharing, glad to hear you are okay!  Definitely could have ended much worse, great job.  How much time did you have on servo and who did the last OH?

Edited October 13, 2015 by d0tnet

[jnisley]

 

Did you find out what was wrong with it?

They found oily contamination on the air side of the regulator, from what the shop told me it can accumulate there if the engine is using excessive oil (before we did a top overhaul several years ago it was using a quart every two-three hours) plus our turbo oil seal failed (not in flight) 400+ hours ago which could have something to do with it??) 

When my mechanic removed air intake ducting and servo there no trace of oil anywhere including the venturi which seems to indicate that it didn't become contaminated recently.

The servo had 1000+ hours, and 13 years (2002) since overhaul which was done by Teledyne Mattituck Services, Mattituck, NY

The shop recommended these servos to be overhauled every ten years.

The shop also told both me and my mechanic to closely follow the directives of Precision Airmotive SIL RS-40 Revision 1  (even though our servos aren't mounted in the updraft configuration)

 

Edited October 14, 2015 by jnisley

[jnisley]

Hi John - thanks for sharing, glad to hear you are okay!  Definitely could have ended much worse, great job.  How much time did you have on servo and who did the last OH?

The servo had 1000+ hours, and 13 years (2002) since overhaul which was done by Teledyne Mattituck Services, Mattituck, NY

[d0tnet]

Great insight, thanks for sharing. Will be sure to reference that directive going forward.

[jetdriven]

They found oily contamination on the air side of the regulator, from what the shop told me it can accumulate there if the engine is using excessive oil (before we did a top overhaul several years ago it was using a quart every two-three hours) plus our turbo oil seal failed (not in flight) 400+ hours ago which could have something to do with it??) 

When my mechanic removed air intake ducting and servo there no trace of oil anywhere including the venturi which seems to indicate that it didn't become contaminated recently.

The servo had 1000+ hours, and 13 years (2002) since overhaul which was done by Teledyne Mattituck Services, Mattituck, NY

The shop recommended these servos to be overhauled every ten years.

The shop also told both me and my mechanic to closely follow the directives of Precision Airmotive SIL RS-40 Revision 1  (even though our servos aren't mounted in the updraft configuration)

 

so in other words, they don't know what caused the failure.  That's concerning 

[jnisley]

so in other words, they don't know what caused the failure.  That's concerning 

According the the shop the servo failed because of "oily contamination In the air side of the regulator" (their words), not knowing for sure where the oil came from is what's disconcerting. According to the shop by "closely following the inspection procedures" (their words) of the above mentioned SIL should alert us if oil does show up again.

[MB65E]

How did the flight go Scott? Glad your back in the air.

 

-Matt

[KSMooniac]

How did the flight go Scott? Glad your back in the air.

 

-Matt

Unfortunately it didn't go...have an RPM indication issue and idle speed/mixture issue to resolve. Hopefully soon!

Sent from my VS985 4G using Tapatalk

[mike_elliott]

According the the shop the servo failed because of "oily contamination In the air side of the regulator" (their words), not knowing for sure where the oil came from is what's disconcerting. According to the shop by "closely following the inspection procedures" (their words) of the above mentioned SIL should alert us if oil does show up again.

Brakett air filter?

[jnisley]

Brakett air filter?

K&N filter

[N201MKTurbo]

That plane has a turbo and the oil came from the turbo, not from the air filter.