Venturi Vacuum System

Jeff Palmer · Post by **Jeff Palmer** » Tue Dec 28, 2004 6:26 pm

Iâ€™ll abbreviate this to make it less boring. Two 4â€ venturis. 3/8â€ hoses, vacuum regulator, filters. Was loosing vacuum over time. Got to 1.5â€. Instruments started to get sluggish. Venturis very full of corrosion. Too bad to reuse. Replaced 4â€ venturis with new. Vacuum dropped to .5â€ with new venturis. Check vacuum lines. Line on top corroded big holes. Replace all lines and fittings. Now 4.5â€ of vacuum at 120 mpg. Instruments worse than ever. Replace DG and AH with new. New AH shaking very badly. Sent back for exchange. New AH not shaking. Instruments very slow to respond. Bench test at 1â€ vacuum. Instruments work fine on bench test. Adjust regulator to 1â€, 2â€, 3â€, 4â€. Instruments will not perform in the airplane. Ran engine at idle, in a glide, 4â€ vacuum. Remove filters, regulator. Nothing makes them work. Check for leaks, none. Why Me?

N1478D · Post by **N1478D** » Tue Dec 28, 2004 6:32 pm

Hope the leaks and corrosion didn't foul up your instruments.

Jeff Palmer · Post by **Jeff Palmer** » Tue Dec 28, 2004 6:39 pm

The new instruments came last. Bench test OK at 1" of vacuum. I was told to put a chamber in the system to produce a more steady vacuum. That didn't help either. The old instruments worked pretty well with the old corroded system at 2". Very disapointing.

Post by **GAHorn** » Tue Dec 28, 2004 6:47 pm

4.5" of vacuum indicates your venturis are good. (Might get more with regulator re-adjusted or removed?) Now it sounds as if your plumbing routing is incorrect, or you have a kink or serious leak upstream of the vacuum gauge.
Re-examine plumbing routing and compare to the IPC.
(As an example, my dual venturis develop over 6" unregulated vacuum at cruise and during descent.)

N1478D · Post by **N1478D** » Tue Dec 28, 2004 6:49 pm

Sounds like you are not getting vacuum to the instruments. It's very easy to create leaks by over tightening fittings. You might place your vacuum gauge at each instrument location and test in flight to see what you are getting at that location.

Jeff Palmer · Post by **Jeff Palmer** » Tue Dec 28, 2004 11:42 pm

I replumbed the thing 4 times. Blocked off the venturis and tested for leaks each time. One time I eliminated the regualtor and went directly to each instrument from the venturi. I have added a cross over line to link them together. All of the lines and fittings are radiased. I have plumbed it like the manual shows as well as other ways. I tried running only one instrument with both venturis also. I am worried about overspeeding my gyros. Can I run with 5" without dammaging the instruments? I would like to take out the regulator. They warn against that though.

blueldr · Post by **blueldr** » Wed Dec 29, 2004 5:35 am

Jeff Palmer

Your vacuum system is probably plugged. You are getting 4" of vacuum, but it is static. You're not getting any flow through the instruments. That's why a clogged filter will cause sluggish instruments but have a normal suction reading. Try pulling the suction line off at the venturi connection and connect it to a vacuum pump, or even a good strong vacuum cleaner for a ground test.
.Did you ever check for flow through the instrumentsafter installing them?

Jeff Palmer · Post by **Jeff Palmer** » Wed Dec 29, 2004 2:35 pm

I removed the filters and ran rubber hoses out so I could feel the vacuum that is going through the instruments. There is a lot. I relocated the hacuum gage to several places and used 2 other gages in case the one in the airplane was not right. I tried a new industrial shop vacuum cleaner and was suprised to see it only pulls a little less than 1" of vacuum. My FBO has a vacuum pump. I can try the trick hooking the pump up to the system, in my hangar, didn't think of that. It'll take a couple guys to help me pick up and tip the airplane around to check the AH though. I am about to go all electric.

Post by **GAHorn** » Thu Dec 30, 2004 11:49 pm

Running your gyros on venturis without a regulator will not harm them in the short term, but will overspeed them if excessive vacuum is applied (more than 5.5") which will lead to excessive precession.

Jeff Palmer · Post by **Jeff Palmer** » Fri Dec 31, 2004 12:46 am

OK, thanks Okhorn, I was not able to achieve more than about 4.6" at VNE so I suppose it would be safe to remove the regulator. I won't mind that because I like to keep things as clean as possible. I have borrowed a rotometer from work. What I'll do is hook up the vacuum pump, from my mechanic's shop, and measure the actual flow through the instruments. I'll let you guys know the results. The prop does have an effect on the pulsing of vacuum. Both blades line up with the venturis at the same time. I don't know if that pulsing has anything to do with it. Maybe my old rotten pipes dampened the pulsing. Maybe I should drill holes in the pipes! I'm running out of ideas here folks. How do you put these little faces in here?[/i]

Jeff Palmer · Post by **Jeff Palmer** » Fri Dec 31, 2004 12:48 am

Okhorn, where did I get OKHORN?

Post by **GAHorn** » Fri Dec 31, 2004 1:05 am

Go to your Profile, and enable "Emoticons" if you wish to use them.

Jeff Palmer · Post by **Jeff Palmer** » Sun Jan 02, 2005 6:39 am

OK, Tonight I set up the vacuum pump. Ran the system up to 4.5". The regulator popped off to maintain that setting. There is a lot of volume of air going through the instruments. Tomorrow I will measure the actual flow with the rotometer I borrowed from work. Then I will take the stuff off and will plumb it up the way I did at first. It will all be the same except I am not putting the regulator in. My 4" venturis will never produce more than about 4.7" at VX. So that will eliminate a lot of length of pipe. There is one difference that I am not using any rubber hoses like has been recomended. It is all plumbed in solid. The line runs direct from the left venturi, to each instrument, then to the right venturi. That's how I am going to run it. If it still doesn't work, I will probably just go all electric and eliminate the venturis altogether.

zero.one.victor · Post by **zero.one.victor** » Sun Jan 02, 2005 7:19 am

Have you priced an electric horizon & DG? Whew!

Jeff Palmer · Post by **Jeff Palmer** » Sun Jan 02, 2005 4:34 pm

I know, that's part of the reason for using vacuum. It's cheaper. I can't drag any smilies over here and I have smilies checked in my profile. Also, my avatar does not appear in the box on the left????? I wonder if that has something to do with my vacuum not working. It's Voo Doo!

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