Manometer question

i am going to build a manometer setup for use with 2 carbs at a time. I will try the single tube filled with ATF. My question is do I need to put restrictors on the tubes before the carb?

 

Not really but if their not real close to start with it's going to suck your atf in before you know what happened.
2 bottles is much more user friendly

 

They are pretty close. I did the methos with 1 gauge and a restrictor, but the gauge starting messing up towards the end. So I want to try the tube method.

So no restrictor if they are pretty close to start with?

 

The restrictor is to damp pulsing. You're going to find it easier to use if you put restrictors near the engine.

 

My curoisity has finally gotten me. I built the two bottle manometer and it worked fine, but was very sensitive. I keep reading about "restricters" in the lines. Can someone explain what they actually do and what to use?

 

The intake pressure on a running engine isn't constant. Think about the four stages in your engine's combustion cycle:

1) Intake
2) Compression
3) Power
4) Exhaust

Only during the first stage is the engine drawing air through the carburetor. The other three stages the pressure in the carburetor will rise. That causes your instrument (manometer, dial gauge etc.) to pulse.

Putting a restrictor (tiny orifice) near the engine leaves a large volume of air between the restrictor and the instrument. Because of the small hole the pressure in the hose cannot change rapidly and the pulse is damped.

The two bottle manometer probably doesn't require a restrictor. Because the hose is rather narrow and there is a large volume of air above the liquid in the bottles the pressure cannot change as rapidly as it will in a U shaped tube. (of course if your bottles are nearly full there is a lot less air "cushion" to damp the pressure fluctuation).

What this is all about is reducing the responsiveness of the instrument, not the sensitivity.

Your manometers are sensitive because you are using a low density fluid. The pressure differential that will raise mercury an inch will raise water about a foot. A water manometer is about 12X as sensitive as mercury. Oil will be a bit more sensitive than water.

The reason mercury is commonly used in manometers is the maximum height you could possibly need is about 31 inches. A water manometer would need to be about 32 feet tall to guarantee you couldn't suck the fluid out.

The water manometer is much more sensitive and would be easier to get a very good result, but not very portable.

 

hanks. That makes sense. I may try a restrictor just to see how it works with the two bottle set up. Anything in particular you would recommend to use as a restrictor?

 

I think I now see the problem with my 4 tube homemade manometer. Mine does "pulse" which in turn puts air bubbles in the line. I use motorcycle oil in the tubes in case it gets sucked in. I have yet to successfully use my manometer before it gets all out of whack.
A restrictor would just be a hole (orifice) in the line a few inches away from the engine? Any suggestions on a fluid other than the oil?

 

The tiny red spray straws that come on virtually any aerosol lube or cleaner make ideal restrictors. You only need a piece 1/2" long.

The restrictors that came with my Morgan Carbtune have a similar ID if I remember correctly (they're now installed in the lines so I can't check.)

I'd stick to a fluid that won't damage anything if it does get sucked in; motor oil, MMO, or maybe ATF.

 

Thanks bigfitz! Just stick a 1/2" piece into 1 side of each tube and it should correct my "pulsing"?

 

"A restrictor would just be a hole (orifice) in the line a few inches away from the engine? " not sure what you were thinking but what you wrote is wrong.
a restrictor reduces the ID of the hose to just a tiny hole, there is no hole in the side of the hose










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Ahhh ok Polock I got ya'. Thanks!

 

I seem to remember in Schuckaholic's vacuum gauge write up, he suggested getting a two ended plastic splice (commonly found at any auto parts store), taking that red spray bottle tube mentioned earlier and JB welding it (on both sides) and then trimming the red tube even with the splice ...the restricter is now inside the splice