venturi tube adjustment screw

[Will Kumley]

My new to me airplane uses a venturi vacuum system. The logbook shows it as an 8" venturi and it is connected to the attitude indicator, directional gyro, and turn indicator. At 100 mph it seems to only pull 3" of vacuum.

There is, what appears to be, an adjustment screw on the trailing side of the mount for the venturi tube but I cannot find anything that says if/how the adjustment screw works.

 

[Bell206]

There is, what appears to be, an adjustment screw on the trailing side of the mount for the venturi tube but I cannot find anything that says if/how the adjustment screw works.

System vacuum is usually regulated by one or more regulators/valves up stream of the venturi tube. The tubes themselves are usually rated at a fixed suction rate like your 8" will draw 8 inches of vacuum. My guess the screw you see is a plug for an alternate vac line install. What is the reason for adjusting the suction level?

 

[Will Kumley]

System vacuum is usually regulated by one or more regulators/valves up stream of the venturi tube. The tubes themselves are usually rated at a fixed suction rate like your 8" will draw 8 inches of vacuum. My guess the screw you see is a plug for an alternate vac line install. What is the reason for adjusting the suction level?

I've got a wonky DG and am trying to determine if it's a failing indicator that needs to go to overhaul or if it is just a symptom of low suction. I'm used to seeing suction values closer to 5" in all the other planes I've rented/flown so this one reading 3" at full speed is making me think an adjustment on the suction would allow the DG to stabilize better.

 

[Bell206]

I'm used to seeing suction values closer to 5" in all the other planes I've rented/flown

In general, it depends on the P/N indicators you have installed that determines the vacuum setting. Older indicators especially the ones with an AN P/N were limited to 3 inches max. The new indicators can run at 5 inches though. And most Turn Bank inds, new or old, usually run around 2-3 inches max. So any adjustments without looking at the whole package can cause you other issues. My 1st move would be to verify the vacuum system is in good condition and to spec: clean filter, no leaks, regulator/relief valve operating correctly (there may be more than one) and so on. Once thats done then look at your instruments. What specific aircraft and model?

 

[Magman]

Disconnecting the AH and blocking the line would provide more suction to DG

to diagnose. Vacuum instruments generally use special fittings to avoid abrupt

changes in direction. Often cutting hose to remove may work better if original

is “ petrified”. As Bell stated the filter could be the culprit. Some units ( AN) have

their own filter. I’ve seen mfg state to replace every 100 hrs!

 

[Dan Thomas]

Like Bell and Magman say, check that filter. If the filter is clogged, the differential between the vacuum and air lines decreases and the gauge reads low. In an airplane old enough to use a venturi, the filter is often no more than a screen on the instrument's air inlet, and sometimes two screens with a layer of felt sandwiched between them. And, since annual "inspections" are often really casual and quick, the filter never gets checked. Ever. If the airplane was flown by smokers, filters and the instruments themselves can get full of tar.

And vacuum hoses get old and crack and leak. Some old airplanes used some aluminum tubing in the system, and I once found a huge hole in the tubing, big enough to stick several inches of a pencil into it and up the tube.

A selection from the inspection sheets for old Cessna 100 series airplanes:







Now, getting at those filters requires getting under the instrument panel, an awkward (and painful for us older guys) job that mechanics prefer to avoid, so it seldom gets done. I have found filters so old that they're literally crumbling with age. That debris gets sucked into the instrument, too. Cheap inspections are false economy.

And that "screw" the OP sees is just a plug.

 

[Will Kumley]

Thanks all,

I was able to take the top panel off today and look behind the instrument panel. There is definitely a regulator just inside the plane from the venturi and it is safety wired so not likely it went out of adjustment. The vacuum lines do in fact have cracks in them so that is a good first place to look I guess and see if replacing the lines helps my issue.

 

[Dan Thomas]

Thanks all,

I was able to take the top panel off today and look behind the instrument panel. There is definitely a regulator just inside the plane from the venturi and it is safety wired so not likely it went out of adjustment. The vacuum lines do in fact have cracks in them so that is a good first place to look I guess and see if replacing the lines helps my issue.

Does your attitude indicator look like this?

If so, on the back is a round thing. It has the filter screens in it. Probably haven't been cleaned in 50 years. Maybe never.

Similarly, an old heading indicator:

And on the back, a screen:

I learned to fly behind this stuff 50 years ago. It's been obsolete a long, long time. Once in a while I still see them in old airplanes.

 

[Magman]

There is definitely a regulator just inside the plane from the venturi and it is safety wired so not likely it went out of adjustment.


That doesn’t mean it’s working properly.

There are at least 2 styles: One with a screen and the other with a foam garter filter. If any crud got in and kept the valve from closing you then have a
Vacuum leak.

You could put duct tape over the screen and it would give you max vacuum.
Not sure if it could be done there. Worth trying to adjust.

 

[Will Kumley]

Does your attitude indicator look like this?

If so, on the back is a round thing. It has the filter screens in it. Probably haven't been cleaned in 50 years. Maybe never.

Similarly, an old heading indicator:

And on the back, a screen:

I learned to fly behind this stuff 50 years ago. It's been obsolete a long, long time. Once in a while I still see them in old airplanes.

Here is the front of the panel.

 

[Will Kumley]

The regulator has a garter filter. Some older hoses in there for sure and they appear to be a little on the large size but if the clamps are tight I imagine it would work with good hoses. I agree the diaphragm in the regulator could theoretically be leaking which can cause some issues. I’m also curious if there is too much connected to the single 8” Venturi. Eventually I plan to install an electric turn and bank indicator so if new hoses doesn’t fix it I may disconnect the turn indicator temporarily and see if that helps. But in my brain, this setup had to work in the past otherwise the previous owners would have already made adjustments.

 

[Magman]

It’s not uncommon to find gyros plumbed incorrectly. INLET would go to the Filter and VAC to the vacuum source.

Markings may vary so it’s well worthwhile taking the time to verify this item.

 

[Bell206]

But in my brain, this setup had to work in the past

This is troubleshooting 101. Has this been an ongoing problem since you bought the plane or something new? Has the system always been at 3" or did it change recently? What do you specifically mean by "wonky" with the DG? Regardless there are additional checks you can perform to narrow down the issue before randomly changing things. One option is to use compressed air or a vacuum cleaner to induce airflow at the venturi for troubleshooting however care must be taken not to cause any new issues.

I agree the diaphragm in the regulator could theoretically be leaking

Not all vacuum regulators/relief valves have a diaphragm. Some even call out to use solvent to clean. Best to verify the type you have installed.

I’m also curious if there is too much connected to the single 8” Venturi

This size venturi is known as a Super Venturi which is designed to handle what you have. In older setups there were multiple venturi to power the instruments.

 

[Dan Thomas]

A majority of vacuum regulators/relief valves do not have a diaphragm.

Most that I have seen have a diaphragm. But the low-vac problem is sometimes a corroded valve seat that leaks constantly.

 

[Magman]

Older piston types have a screen that catches crud.

Sufficient accumulation will render the unit inoperative with resulting HIGH. Vacuum.

I try to position the valve SCREEN DOWN to minimize accumulation and allow inspection.

 

[Will Kumley]

This is troubleshooting 101. Has this been an ongoing problem since you bought the plane or something new? Has the system always been at 3" or did it change recently? What do you specifically mean by "wonky" with the DG? Regardless there are additional checks you can perform to narrow down the issue before randomly changing things. One option is to use compressed air or a vacuum cleaner to induce airflow at the venturi for troubleshooting however care must be taken not to cause any new issues.

Not all vacuum regulators/relief valves have a diaphragm. Some even call out to use solvent to clean. Best to verify the type you have installed.

This size venturi is known as a Super Venturi which is designed to handle what you have. In older setups there were multiple venturi to power the instruments.

The plane is new to me so I'm not sure how long the problem has existed prior to me but its been like this since I picked it up. Wonky is my way of saying the DG precesses excessively, even spinning at a moderate speed on downwind and final then speeding up real fast on rollout after landing. I went back to the plane and looked things over thoroughly. While it appears the system isn't completely done right the DG should still work if its getting enough vacuum. But, its not if I am to believe the gauge and the sticker that shows the requirement on the indicator itself.

 

[Magman]

“Isn’t done completely right”?

There are alternate ways of doing things.

Some work , etc.

 

[Bell206]

While it appears the system isn't completely done right

What makes you think the system is not done right?

DG should still work if its getting enough vacuum.

Yes. If only pulling 3" a 5" DG will not operate correctly. I'd start with a new filter then verify system vacuum vs the indicator.

 

[Will Kumley]

What makes you think the system is not done right?

The way the turn & bank indicator is connected seems off to me. Am I wrong to think it should be connected directly to the vacuum line with its own regulator? It already has its own regulator and by being split off the filter line instead of the vac line I think it limits the effectiveness of the vacuum being pulled through the turn & bank. Maybe I'm overthinking it, but I would imagine the vacuum would just pull through the main vacuum system filter and effectively bypass the turn & bank. Of course, this doesn't mean anything for a DG that isn't working (although I've noticed the turn & bank isn't effective either). If I had to choose to have only one working for now it would be the DG.

Although the filters look clean I looked through the logs last night and the last time they were documented as replaced was 9 years ago so that could be the simple solution. While talking with the previous owner, I'm sure he said he replaced them but there are no logbook entries saying it was done.

 

[Dan Thomas]

The way the turn & bank indicator is connected seems off to me. Am I wrong to think it should be connected directly to the vacuum line with its own regulator? It already has its own regulator and by being split off the filter line instead of the vac line I think it limits the effectiveness of the vacuum being pulled through the turn & bank. Maybe I'm overthinking it, but I would imagine the vacuum would just pull through the main vacuum system filter and effectively bypass the turn & bank. Of course, this doesn't mean anything for a DG that isn't working (although I've noticed the turn & bank isn't effective either). If I had to choose to have only one working for now it would be the DG.

Although the filters look clean I looked through the logs last night and the last time they were documented as replaced was 9 years ago so that could be the simple solution. While talking with the previous owner, I'm sure he said he replaced them but there are no logbook entries saying it was done.

All three instruments should be connected with their "Vacuum" ports to the regulated vacuum line. The turn-and-bank might have a needle valve to limit the vacuum to three inches or so.

And the instruments' "Air" ports should go to the filter. That T&B might have its own filter.

The system is likely misplumbed. Stuff happens, especially when owners fool with their airplanes. It's one reason why an A&P ticket is necessary for any but the most simple tasks. People die when stuff gets screwed up.

Three vacuum instrument systems disappeared a long time ago. If you're in IMC or on a dark night over rural areas, you could be in trouble when you lose everything. So electric T&Bs or turn coordinators have been required for IFR and most night flight for a long time now.

 

[Bell206]

Am I wrong to think it should be connected directly to the vacuum line with its own regulator?

Depends. What year and model aircraft is this?

 

[Will Kumley]

Depends. What year and model aircraft is this?

64 Piper Colt. It had a vacuum pump a long time ago but a previous owner opted to remove it for a venturi. It's had the venturi for the better part of 40 years. The plane is new to me and flies great, this is the only real problem with the plane. The DG was replaced a decade ago and my best guess is that is the last time the plumbing was touched.

 

[Bell206]

It had a vacuum pump a long time ago

Given its age and looking at your pics I got 10 to 1 your aircraft originally came with only a T&B, small venturi, and that pin valve. Then they added the vac pump and simply tied in the T&B where convenient. If you're sure your T&B is also under performing now would be a good time bring your entire system up to spec.

 

[Will Kumley]

Given its age and looking at your pics I got 10 to 1 your aircraft originally came with only a T&B, small venturi, and that pin valve. Then they added the vac pump and simply tied in the T&B where convenient. If you're sure your T&B is also under performing now would be a good time bring your entire system up to spec.

If bring up to spec is getting the vacuum system working properly that's all I'm wanting to do right now. As the plane is great otherwise and I only need it for VFR at the moment I just want to get what's in the plane working so I don't worry about it. After I've spent some time flying it and enjoying the freedom of ownership affords upgrades will happen. I'd like to add an engine monitor, electric T&B, GPS with traffic, updated transponder, and a myriad of smaller items. If adding an Aspen dual indicator is in the cards- even better. But I'm also not trying to break the bank in the first year of ownership.

 

[Magman]

The Colt is a great aircraft that has a “ Rodney Dangerfield Complex”.

Until someone has sufficient time in one the the x/c and short/ soft field capability’s

are not appreciated. But it IS different than the spam cans.

The initial order from Piper dictated what optional equipment the aircraft came with.

This included avionics but also number of fuel ranks and likely the vacuum system.

My first Colt had no Vacuum Pump and the engine had no provision to accommodate one.

Possibly your pump went away when an engine was changed?

Per this conversation; there has been neglect.

it may be due to unfamiliarity with these aircraft.

One area that is often neglected is the battery box,

The overboard drain tube breaks off and acid is dumped in the belly.

If addressed in time repairs are not too difficult.

Find a Tech that knows these aircraft and you will love it.

 

[Dan Thomas]

Find a Tech that knows these aircraft and you will love it.

That there. The cheapest repair is the one done by someone who knows what they're doing.