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3rd Flight - Low Manifold Pressure

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bkervaski

Hellloooooooo!
Testing
Happy Independence Day!

3rd Flight Today!

Everything seemed to be running smooth, reduced throttle and prop to ease up to 3500, had a good climb rate, other planes in the pattern, engine sounded good.

Once I got to 3500 and went to fine tune the prop I noticed it was only at 19".

My first thought was that the 0.004 restrictor I just installed for the manifold pressure sensor just got clogged or something.

I noticed on my orbit that I was only getting about 100kts at around 2400rpm, manifold pressure was between 17-19.

Temps were fine.

So, I finished an orbit of the airfield, landed, and when I shutdown the engine the manifold pressure went from 16 (1000 rpm idle) to 29 (engine off).

Seems the sensor is reading fine and it's something else.

Demotivating issue and out of my experience level, looking for advice. Any thoughts? (thanks)
 
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Are you suggesting that you were WOT (climb power) yet the MP was 19?

Immediate thought is a major restriction within the induction tract. Debris or linkage is partially blocking the throat to the servo.
 
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Are you suggesting that you were WOT (climb power) yet the MP was 19?

As my memory reconstructs the specific details of the flight I edited the post a couple times. To be frank, I didn't notice any issues on climb and the specific numbers at specific points may not be completely accurate, was more interested in the traffic and making my way back down to the field in case there was an issue. I never saw it above 20" after I noticed it was low.
 
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true confessions....

Is this a change, or something you just noticed?

Like Toolbuilder said, the intake could have inhaled something like a rag, and it's blocking the filter, if you have vertical induction. Less likely with a 'snorkel' & filter at the cooling inlet. I once had a dauber wasp build in the induction inlet (hangared plane). Sloppy preflight, short flight to a flyin, and the 3-finger sized clod came loose on takeoff when I was departing the flyin. This is a really old -4 with no air filter. The clod got crossways across the carb inlet, and I looked like a '50s era jet leaving the flyin.

Did you verify full motion of the throttle butterfly before 1st flight? It should go from appearing to completely block the throat, to being exactly parallel to airflow at full throttle. If there's an issue with linkage geometry, the butterfly might not be opening fully. (Had it happen on my weedeater, so I know it's true....)

A lightly loaded RV, especially with a c/s prop, will climb better than a spam can even on half power.
 
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Are you sure the restrictor is .004" and not .040" I have never seen a drill bit that small. If it is really .004" that isy too small. I would also verify the accuracy of your instrument with a vacuum tool that you can rent at the local autozone. My engine idles at around 11" not 16" That seems high for idle. Sorry, re-read your post and you were 1000 RPM. I can't remember what MAP is normal for 1000 RPM, but stills seems high.

As others mentioned, about the only thing, beyond cam timing, that can create abnormally high vacuum (i.e. low MAP) is a restriction in the intake before the sensor. This should be noticable, as you will not be able to make normal full power.

"I noticed on my orbit that I was only getting about 100kts at around 2400rpm, manifold pressure was between 17-19."

I haven't flown behind a CS prop in some time, but couldn't this condition be caused by the prop being at a finer pitch than desired? Generally speaking, as you go finer in pitch, the RPMs increase and the MAP drops.

Larry

Happy Independence Day!

3rd Flight Today!

Everything seemed to be running smooth, reduced throttle and prop to ease up to 3500, had a good climb rate, other planes in the pattern, engine sounded good.

Once I got to 3500 and went to fine tune the prop I noticed it was only at 19".

My first thought was that the 0.004 restrictor I just installed for the manifold pressure sensor just got clogged or something.

I noticed on my orbit that I was only getting about 100kts at around 2400rpm, manifold pressure was between 17-19.

Temps were fine.

So, I finished an orbit of the airfield, landed, and when I shutdown the engine the manifold pressure went from 16 (1000 rpm idle) to 29 (engine off).

Seems the sensor is reading fine and it's something else.

Demotivating issue and out of my experience level, looking for advice. Any thoughts? (thanks)
 
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Can you confirm you were at full throttle during your low manifold pressure event? If so, then you have an issue somewhere.

Did you check the simple things like your throttle friction? Its easy for vibration to slowly roll your throttle back if the friction isnt set properly tight. Also, do remember that your MP will decrease naturally in the climb if you dont bump up your throttle to compensate. Start with the easy stuff then work deeper!
 
Do you think you got carb icing and the manifold pressure was reduced due to blockage. Or should I ask if your throttle arm is pushed open with a cable? If so maybe the cable is not opening up the throttle all the way. Is your manifold pressure gage analog with hose or an electronic module? Maybe there is a slight leak in the plumbing or the sender has a weak ground wire if module type. Are you pulling manifold pressure from all four cylinders (balanced) or just one at the cylinder intake valve? If so could you have carbon or oil in the line? It's hard to diagnose the issue unless we know the following.
  • Carbureted or Injected
    Push pull cable or solid wire linkage to throttle arm
    Analog gage or MAP module
    OAT
    Where you are pulling MAP from
 
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1. Remove the air filter and ensure that the throttle plate is fully opening
2. Look in the servo past the open throttle plate to ensure the servo gasket is full diameter and not restricting flow. The shipping plate gasket on mine had no hole, the engine would not start. :eek:
 
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