Manifold Pressure reading question

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Manifold Pressure reading question

Postby seneca2e » Fri Oct 12, 2012 9:08 pm

Okay I ran her up again today and seem to have the mag drop issue resolved but I still am having no joy on getting the manifold pressure to indicate much off idle. My question to anyone that has run their ships with the outer blades off(which is what I'm doing) is do you get any manifold pressure settings like 14 to 18 inches with the engine run up to 2900 rpm? Or is there not enough drag or flywheel effect so to speak with the outers off to make the mp gauge read higher than idle manifold pressure? The gauge reads ambient pressure when shut off(29 inches or so) and then when you run it sucks down to the bottom of the gauge(like it should with the tthrottle butterfly closed at idle). BUT I can't get it to read above that by rolling the power on up to 2900 rpm(with the blades off). Seems like since it works on both extremes idle and ambient that it should work. I guess I'll answer the question when I put the outer blades on but trying to do all my trouble shooting before I put the outer blades on.
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Re: Manifold Pressure reading question

Postby bryancobb » Sat Oct 13, 2012 4:21 am

Manifold pressure, as you know, is a reading of how HARD THE ENGINE IS WORKING.

Imagine If a bicycle had a manifold pressure gauge hooked to the rider to read how HARD THE RIDER IS WORKING.
When pedaling DOWN a hill, (basically what you are doing) the reading would be very low.
YHO-3BR Pilots International
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Re: Manifold Pressure reading question

Postby seneca2e » Sun Oct 14, 2012 3:09 pm

Yes I'm hoping that's the problem. Took the mp line loose and blew it out both ways and no obstructions noted. Also hooked it back up to the gauge and then applied vacuum at the engine end and it worked fine but seemed a bit sluggish but I'm thinking this is due to the small restricter type line/fittings that probably are there to dampen the vacuum input so the gauge is steady.
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Re: Manifold Pressure reading question

Postby seneca2e » Fri Dec 21, 2012 11:37 pm

Just to follow up on this. It was not as I had hoped the loading of the blades. It still had very sluggish Manifold Pressure readings. I finally found the small vent hole in the aluminum manifold pressure line from the cylinder that makes a u shape which was implemented to keep contaminants out of the gauge. It was about 2 inches from the connector that reduced it back to the original mp line. However there was no problem there.

What happens is the gauge despite even this mod will occasionally get fuel/oil in it and the diaphragm will get wet. In my case simply removing the small orifice at the inlet to the gauge and cleaning it(oil contaminated) and installing it 1 1/4 turns out(trial and error to get damping about right-had another gauge I compared it too) appears to have solved the problem with no disassembly of the instrument required. Otherwise it would have had to went to an instrument shop. Again this job was unbelievably hard. The ship has a nice panel but someone used an365 elastic locknuts to hold the instruments and must have built the panel up in a certain order. Almost impossible to extract the gauge but finally I did it. Very hard to crouch in a Brantly without bending something at the instrument panel level. If you ever put instruments in an aircraft use J Nuts(Grasshopper nuts) and tie them in or Aircraft Spruce now has a very neat solution with built in nuts that back the instrument. It should go back in easier than it came out due to the fact I won't be fighting those darn nuts(couldn't even see a couple of them).
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Re: Manifold Pressure reading question

Postby seneca2e » Mon Dec 24, 2012 1:11 pm

Well I got the mp/fuel flow reinstalled and went to bleed the fuel line connection and there was no fuel at all when I pressurized it with the boost pump! Took the fuel line loose at the firewall and it had fuel there-the line was stopped up between that connection and the gauge. The fuel flow had been somewhat erratic but was working-now I know why. Went to blow the line out and a little air came out and then wham it was plugged tight! If finally yielded to some safety wire and vigorous working up and down 8 inches or so from the gauge connection. Blew it out good from both ends and pressurized it with fuel till it was dribbling out then connected it to the gauge and let it belch just a little before final tightening. Now the fuel flow AND the manifold pressure work good. Went ahead and flew it for an hour on Christmas Eve. I think that is the best present I could have gotten this year!
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Re: Manifold Pressure reading question

Postby Ken_Shea » Mon Dec 24, 2012 2:45 pm

Were you able to determine what plugged the fuel line?

Agreed, that would be a very nice present.

Ken
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Re: Manifold Pressure reading question

Postby seneca2e » Mon Dec 24, 2012 5:52 pm

I think the line was mostly plugged to begin with in hindsight(which is why we had some questionable fuel flow indications). Was never able to see what plugged it totally. Sometimes you get some very fine particulate even past the fuel flow divider when one sets a long time. The line itself might have had a touch of corrosion internally but it looks excellent. Hard to say. If it were to plug again I'd be concerned. All the fuel screens are clean so knock on wood it seems fine right now. I just flew it another 45 minutes in the dark. Was real careful with the landing lights-it has two. There's a SB about painting around them if you're getting blinded by the glare so I was cautious of that. Started out near a couple of street lights and transitioned to the dark side of the ramp.
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