Fuel servo adjustment

[Dream to fly]

A few months ago I had to replace the fuel servo and up until yesterday I had not had any problems.  After flying for a two hour leg I landed, taxied for a few minutes and went to shut down by pulling the mixture.  I got in excess of 300 RPM rise.  I restarted after about an hour and black smoke poured out.  Removed the side panel just to see if the thumbwheel had moved and it didn't and I also ran the pump with the mixture full lean to see if it was leaking like the old one did and it was fine no leaks.  My question is do these things need adjustment continuously or is there a problem starting to rear its head.  Flying at 24/2400 the plane is on the money and I am averaging 10 - 11gal per hour and I am pretty sure I am running ROP.  It just seems that the idle is off.  I am asking before going on a witch hunt for the problem. I don't want to touch what isn't broken and maybe get another month of flying before my annual and co-pilot instrument panel upgrade.

[jetdriven]

Adjust that thumb wheel leaner.  There is a --->R mark on it which shows which way is rich. Go opposite.  You may need to disconnect one clevis and screw or down then reconnect. It can run out of adjustment that way. Mine is set for between zero and 25 rpm rise. Just barely detectable for clean plugs on taxi. 

Edited September 9, 2017 by jetdriven

[MB65E]

Probably less than 1 turn leaner. It's sensitive!

-Matt

[N201MKTurbo]

Did you land at an airport that is a different elevation then your home field?

[Dream to fly]

Did you land at an airport that is a different elevation then your home field?

No, I flew east into the Fargo TRSA then back to my hanger. Was under the hood with my instructor.


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[Dream to fly]

I adjusted it one full turn lean and now the engine idles better at 725-750 and when I pull the mixture I am still getting a rise of about 175.  The other thing I noticed is that my mag drop is 300+ again on the left mag and 125 or so on the right.   Not sure,  but I got a call into my AP and will have him look at it Monday.    I wouldn't think the servo could go bad but now with the mag drop maybe something else is going on and I am missing it.  With all new parts one would think that they would not fail in such a short time.

[MB65E]

Might only need to Clean the plugs. 

-Matt

[Guest]

8 hours ago, Dream to fly said:

A few months ago I had to replace the fuel servo and up until yesterday I had not had any problems.  After flying for a two hour leg I landed, taxied for a few minutes and went to shut down by pulling the mixture.  I got in excess of 300 RPM rise.  I restarted after about an hour and black smoke poured out.  Removed the side panel just to see if the thumbwheel had moved and it didn't and I also ran the pump with the mixture full lean to see if it was leaking like the old one did and it was fine no leaks.  My question is do these things need adjustment continuously or is there a problem starting to rear its head.  Flying at 24/2400 the plane is on the money and I am averaging 10 - 11gal per hour and I am pretty sure I am running ROP.  It just seems that the idle is off.  I am asking before going on a witch hunt for the problem. I don't want to touch what isn't broken and maybe get another month of flying before my annual and co-pilot instrument panel upgrade.

Going from normal rise to 300 says something has changed in a big way.  Normally adjustment is not needed in large amounts, a click here or there and a minor tweak of idle speed.

I would contact the overhauler who did the recent serv work and ask for an opinion.

Clarence

[Dream to fly]

Update-  Mechanic came up to the hanger ran it for a minute or two and just like before black smoke. at start-up and running rich.  Shut it down and turned the thumb-wheel three or four revolutions and restarted.  Ran it at 1800 for a minute or two and it idles perfect.  I am totally confused.  how can it be good then go bad and then be so far out of adjustment?  This seems to be the week link in my power plant.  My gut is saying this is not over but I can't prove it.  Do these engines have issues with valve guides?  I know some of the older pushrod engines in vehicles would stick but that would give a misfire at all rpms. 

[jetdriven]

Do the mag check when leaned to peak RPM. Set it for 2000 RPM leanto peak RPM and then do the mag check, if that satisfactory then keep looking at the fuel servo or other fuel items

[carusoam]

Joe,

For the question regarding valve guides...

1) you may recognize valve tap when you hear it... it won't be a secret.

2) There is a method to clean the guides called 'the rope trick'.  You can probably find it with a search.

3) Some cylinders have a history of getting really hot without a good engine monitor, oil can degrade and solidify as carbon in the guides.

4) if you find carbon chucks in filters or screens, it may have started in the valve guides.

5) I have had all these challenges with an O360 including a stuck valve on departure...

6) I have also had the oil dripping into the cylinder on a Chevy 350.  The valve guide seals were natural materials and would wear out. Oil would bake on the sparkplug blocking the gap.  Fuel injector and intake cleaner would be a quick non lasting fix...everyday a tuft of blue smoke would come out the exhaust pipe on start up, and clear up in a minute. The drip was really small and oil would collect over night.... slowly the oil usage was increasing....  one cylinder was worse than the others.

Ordinary PP thoughts only. Not a mechanic...

Best regards,

-a-

[Dream to fly]

Well the end result....  We took the servo off Re-centered the metering rod and made absolutely positive the measurements were true from point A to B.  New gaskets and re-assembled.  Tested at 1800 RPMs and no black smoke and no rough idle.  Flew the pattern several times and it runs great again.  I checked everything I can think of and found nothing and I really am no better than when we started but the engine runs perfect.  Mixture full lean to cut off and it has maybe a 30 rpm rise but that is a mechanical tach.

[carusoam]

How old is the fuel servo?  (Age and hours)

What is involved when the servo gets OH'd..?

What else could it be?

Best regards,

-a-

[Dream to fly]

1 hour ago, carusoam said:

How old is the fuel servo?  (Age and hours)

What is involved when the servo gets OH'd..?

What else could it be?

Best regards,

-a-

It was just replaced three months ago.   But it is not a very common problem so it could have been new old stock (NOS).  Maybe the diaphragm is dry.  I am going to fly it again tonight and see.  Just can't figure out why it would need that big of an adjustment.  My understanding is that the servo is adjustable for yearly maintenance and for severe altitude compensation.  

[jetdriven]

I am not aware of any adjustments other than idle speed and mixture for the Bendix RSA-10 fuel injector.  

[Dream to fly]

Flew again tonight and I found a pattern.  I can repeat this over and over.  When the engine is cold the system is primed and two swings of the prop and the engine starts and runs.  after a flight and taxi the engine is popping and idling at 550.  Rich or lean it acts like an ignition misfire.  Idle up to 750 and adjust mixture and the popping subsides and the misfires are every so many revolutions.  Cool it down and repeat exactly the same results.  Plugs are clean almost Lean clean.  Plane at WOT flies awesome  at 24/2400 it is right at 11gph.   I have noticed a vibration that I have not noted before at 24/2400 but it feels like a balance issue not a misfire.    

[dmevans]

On 9/12/2017 at 6:43 PM, Dream to fly said:

Flew again tonight and I found a pattern.  I can repeat this over and over.  When the engine is cold the system is primed and two swings of the prop and the engine starts and runs.  after a flight and taxi the engine is popping and idling at 550.  Rich or lean it acts like an ignition misfire.  Idle up to 750 and adjust mixture and the popping subsides and the misfires are every so many revolutions.  Cool it down and repeat exactly the same results.  Plugs are clean almost Lean clean.  Plane at WOT flies awesome  at 24/2400 it is right at 11gph.   I have noticed a vibration that I have not noted before at 24/2400 but it feels like a balance issue not a misfire.    

Did you ever figure this out? I'm having the same issue. Thanks!

[Dream to fly]

On 4/17/2021 at 11:34 PM, dmevans said:

Did you ever figure this out? I'm having the same issue. Thanks!

I did.  it turned out to be the fuel servo.  I sent it out and they swapped it for another rebuilt.  problem went away and its been running good since.