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Question Manifold pressure split at idle

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17 Feb 2024 18:59 #4217 by STEVE ELLS
Replied by STEVE ELLS on topic Manifold pressure split at idle
Hi Bob;
If your engines are close to stalling at 600 rpm, that's evidence the idle mixture needs adjusting.
I suggest you get your mechanic is back off the idle stop screw a turn prior to starting the engines.

Then get the engines up to temperature. 

Turn on the boost pump then do the following idle mixture check.

After the engines are warm and back on the ground, slowing pull the left and right mixture knobs. Note the rpm just before the engine cuts off. If it falls flat then the idle mixture is too lean. 

If the engine gains more than 50 rpm, the idle mixture is too lean.

Once you know the idle cut off rpm behavior (fall flat or rpm rise) then shut the engine down and adjust the idle mixture adjustment screw a turn or two to move the mixture richer. 
If the rpm rise is more than 50, shut the engine down and adjust the idle mixture a turn or two toward lean.

Then start the engine(s) run the rpm up to 2000 for 15 to 30 seconds, then pull the throttle back to the stop. You should be getting an idle rpm in the 600-700 range. 

Once the idle has stabilized, slowly pull the mixture know toward cut off (shut down) 

Note what happens just before the idle cuts off. Then adjust the idle mixture screw to adjust more rich or less rich as needed until you get 50 rpm at cut off.

There's one other thing that my Twin Comanche guru told me about. There are two sniffle valves in the lower part of the intake sys on each engine. Those are supposed to automatically close during engine operation. If they are stuck open you will get a ragged idle and won't be able to get slow idle speeds. 

I'm not sure I understand the following sentence: I have used the button before for hot engine "throttle shutdown " seems to work well on "engine run-on" when hot and using mixtures."

Do you mean the boost pump button??

Check those sniffle valves and repair as necessary. Then do the set up as described above.

Best,
Steve 

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  • Bob Rooney
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17 Feb 2024 18:04 #4216 by Bob Rooney
Replied by Bob Rooney on topic Manifold pressure split at idle
Thanks Steve,
Clean injectors today. The 900rpm wasn't idle. idle rpm is 700 to 800. Engines are close to stalling lower than that. Explanation... pulled all the way back holding them occasionally i see 680rpm but the are close to stalling. I have used the button before for hot engine "throttle shutdown " seems to work well on "engine run-on" when hot and using mixtures.
For my clarification... run engines until warmed up. Are steps 1-4 done with engine running? If so how do you keep a screw driver on the moving throttle body.

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16 Feb 2024 10:01 #4212 by STEVE ELLS
Replied by STEVE ELLS on topic Manifold pressure split at idle
Hi Bob;
Thanks for the explanation of the testing you've done and thank you for including your N number so I could look up what airplane is involved. I'm grateful to know that.

My first thought was that, based on you troubleshooting that the problem is at the fuel injection idle speed and idle mixture settings.

Question. Why is the idle set so high on your engines? Normal idle speed is 600 rpm. 900+ idle rpm translates into longer than book landing distances because there's always some thrust at throttle closed position.

The process for setting idle speed and mixture is as follows:
1) engine at operating temperature
2) According to the service manual,  "The idle mixture must be adjusted with the fuel boost pump ON", 
3) set idle stop screw so engine idle screw so idle is between 600 and 700 rpm.
4) Once speed has stabilized, slowly pull the mixture aft until the rpm drops off. You are looking for a 50 rpm rise. If you have an analog tach, you may not see the needle move but you should hear the rise.

If the rpm goes up more than 50 the mix is too rich, screw the needle in a turn; then start the engine and run it up to 2000 rpm and pull it back to the idle stop again. 
5) Repeat until you get a 50 rpm rise at 600 to 700 rpm.

Set up your fuel system to these values; then please let me know if that lessened, or eliminated the problem.

Best,
Steve



 

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15 Feb 2024 11:52 #4207 by Bob Rooney
Manifold pressure split at idle was created by Bob Rooney
Greetings,
I have a manifold pressure split at idle. RPM LE  917 RPM RE 913 LE MAP 11.5" RE 14.5" TO, Climb, Cruise no issues ...match perfectly. Airplane seems neutral trim in yaw. I lean to equal FF's engine parameters EGT's CHT's seem all reasonable with eng to eng comparisons... so I'm assuming same power output. Did a line check to MAP gauge from wher the hard line attaches to rubber line just aft of #4 cylinder. Pull a vacuum to a set suction on my vacuum pump to "x" inches. it showed 21 " on cockpit gauge for LE. The same procedure for  RE, RE held at 21". So I assume the MAP gauge and lines are good from starting point of test! Any thoughts or ideas?
Thanks Bob N284TC 

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