3rd Channel Needle Length Myth?

[David Santistevan]

I have been told by a number of people that you want to keep the fuel lines between the external needle valve and carb as short as possible so that the needle changes are quicker. The longer the lines the longer it takes the change to reach the carb. I went with this for a while on face value but now I am trying to reason this through and it doesn't make any sense. My reasoning on this is the only effect longer lines between the remote needle and carb will have is the amount of time to initially charge the line with fuel. After that, any change you make regardless of the length should be immediate.

The needle regulates the flow of the fuel. Therfore when you close the needle the flow to the carb will only be equal to whatever can flow past the needle to replace the fuel in the line.

Am I on track here or am I missing something??

 

[Kevin Krogness]

I do believe you answered your own question.

 

[stan simpson iv]

I run a remote needle on my mono that has some fuel tubing length to it,when I do adj. to it happens in about a straight away. On my 21 rigger I have the 3rd channel built into the carb,as soon as I touch the needle it is right now. I think if you pulled the fuel line off your engine at the nipple of the carb it would spool up faster than if you just pinched the fuel line at the tank...No?? It works that way with my airplanes... I could be wrong.

 

[moparbarn]

The laws of hydraulics & flow do apply, BUT you have a section of flexible tubing. not a solid pipe......so any flow change has to equalize the pressure created swelling of the tubing. the longer the line, the more the effect. think of it like a balloon. if you could run a solid tube of brass from the remote to the carb, the change would be MUCH quicker.

 

[steve wood]

that's what we thought also,but it turns out we were wrong.went through issues with a greenhead 67/80 in a crapshooter.multiple fuel tanks(different companies,with and without internal tank)multiple needle valves,resealing of fuel inlet and outlet on the needle valve,custom needle base,different fuel lines,enlarged pressure fitting.you name it we did it.boat would run hard/great till last lap(sometimes would finish o k),then die1/4 to 1/2 lap early.still have plenty of fuel left. i made a custom spray bar with the os servo controlled valve mounted in the spray bar.instant problem solved.tank will completely drain,never dies.orginal valve/base was1 1/2 to 2 inches from radio box bulkhead,4 to 5 inches from carb.every boat is different,but i now put the needle on the carb or as close as i can,and have no more issues since

 

[HendricX]

custom spray bar with the os servo controlled valve mounted in the spray bar.instant problem solved.

You should have used a OS-9B carb. Less hassle.

Same quick response with CMB / OPS / Rossi (.90) build-on RC-remote needles.

 

[richard jones]

I have been told by a number of people that you want to keep the fuel lines between the external needle valve and carb as short as possible so that the needle changes are quicker. The longer the lines the longer it takes the change to reach the carb. I went with this for a while on face value but now I am trying to reason this through and it doesn't make any sense. My reasoning on this is the only effect longer lines between the remote needle and carb will have is the amount of time to initially charge the line with fuel. After that, any change you make regardless of the length should be immediate.

The needle regulates the flow of the fuel. Therfore when you close the needle the flow to the carb will only be equal to whatever can flow past the needle to replace the fuel in the line.

Am I on track here or am I missing something??

I could be wrong but I believe the needle reduces or increase the volume of fuel flowing thru a given area weather it is a tube, a square or any contained area, while the pipe pressure remains constant, when leaning the third channel with a few inches or longer distance from the third channel to the spray bar the fuel all ready in the fuel line is different volume than the fuel coming from the third channel depending on weather you have leaned or richned the third channel, untill the new fuel volume reaches the spray bar. The fuel line we use will not enlarge until about 6 PSI with some going up to 10 PSI. Best way to see the difference is to double the fuel line length from the third channel to the spray bar and be prepared to wait longer before the engine changes RPM.

JM2CW

dick

 

[steve wood]

custom spray bar with the os servo controlled valve mounted in the spray bar.instant problem solved.

You should have used a OS-9B carb. Less hassle.

Same quick response with CMB / OPS / Rossi (.90) build-on RC-remote needles.

too small carb for a 67,imo.i use one on a 45

 

[dwilfong]

I have not used one yet but have two to try. thy are the ones with the slot cut in a tube.

thy have different names for them Fuel Doctor , full injector.

I was told that this design came about from watching a gas stove. when you turn the knob the flame changes instantly.

Thy use this type of control valve design.

David

 

[mark voorhies]

I run a remote needle on my mono that has some fuel tubing length to it,when I do adj. to it happens in about a straight away. On my 21 rigger I have the 3rd channel built into the carb,as soon as I touch the needle it is right now. I think if you pulled the fuel line off your engine at the nipple of the carb it would spool up faster than if you just pinched the fuel line at the tank...No?? It works that way with my airplanes... I could be wrong.

Stan, if you get some time, could I see a pic of your 3rd channel needle built into carb on your .21

I like the idea, would like to try that. Or send me some info. on how to? Thanx

 

[Tony Jacuzzi]

custom spray bar with the os servo controlled valve mounted in the spray bar.instant problem solved.

You should have used a OS-9B carb. Less hassle.

Same quick response with CMB / OPS / Rossi (.90) build-on RC-remote needles.

too small carb for a 67,imo.i use one on a 45

Intersting we are using the OS 40-a on our 40 Picos

 

[Tom Foley]

The shorter the fuel lines the better . The sensitivity will be noticed more quickly as the size of the engine is reduced due to less exhaust pressure and smaller lines . This is just what I have found over the years , not an engineer

 

[SayMikey]

I have been told by a number of people that you want to keep the fuel lines between the external needle valve and carb as short as possible so that the needle changes are quicker. The longer the lines the longer it takes the change to reach the carb. I went with this for a while on face value but now I am trying to reason this through and it doesn't make any sense. My reasoning on this is the only effect longer lines between the remote needle and carb will have is the amount of time to initially charge the line with fuel. After that, any change you make regardless of the length should be immediate.

The needle regulates the flow of the fuel. Therfore when you close the needle the flow to the carb will only be equal to whatever can flow past the needle to replace the fuel in the line.

Am I on track here or am I missing something??

I could be wrong but I believe the needle reduces or increase the volume of fuel flowing thru a given area weather it is a tube, a square or any contained area, while the pipe pressure remains constant, when leaning the third channel with a few inches or longer distance from the third channel to the spray bar the fuel all ready in the fuel line is different volume than the fuel coming from the third channel depending on weather you have leaned or richned the third channel, untill the new fuel volume reaches the spray bar. The fuel line we use will not enlarge until about 6 PSI with some going up to 10 PSI. Best way to see the difference is to double the fuel line length from the third channel to the spray bar and be prepared to wait longer before the engine changes RPM.

JM2CW

dick

I agree pressure and volume come into play. Remember most if not all the time we are adjusting the mixture at Wide Open Throttle, while the fuel needs metering it also needs volume aka big 5/32 fuel tank tubes

 

[Buckshot]

I had problems with big motors going lean at WFO with short lines. Really noticed it on my 1.01twins. My best guess was that the "needle was getting sucked dry" so to speak. The needles where mounted next to the carbs with less than 1" of line running from the tanks to the needle and less than 1.5" from the needles to the carbs. One day at the pond I decided to coil about 4" of line from the needle to the carb and no more lean out at WFO.

Why is that?

 

[julianconde]

my 3rd channel needles have always been behind the motors on my twin about 4-5 inches from the carb and i use large fuel lines also any changes you make to the needle at that distance has a delay because of the distance the needle is from the carb the closer to the carb the faster the change. the fuel line going from the tank to the needle on my Twin is also about 4-5 inches also. on 21's i have them as close as to the carb as possible seems to work for me but everyone has different results and different ways of doing things..

 

[Tom Foley]

My needles on my lil red Twin (remote but not 3rd channel ) have about 1- 1/4 of line before the carbs and that seems to run very well . LOL Probably more of a function of the type of needle than anything else . Spraybars in the VAC 91 's seem to have nice large holes however I have not measured them .

 

[Joe Warren]

Needles changes are nothing more than fuel Pressure change 1st and a flow change 2nd. Dont forget the venturi in the carb is sucking fuel like mad AND You are making fuel pressure changes that feed it. Modern day Carbs had FUEL LOGS in there applications, that was fuel that had passed thru the fuel delivery system waiting to be used in the carb venturi... Long & large fuel lines from the needle valve to the carb act as a fuel reservoir. You are only making fuel pressure changes behind the fuel when the needle valve is adjusted. This is Very true on engines that can burn 25 oz of fuel in 10 laps. or a twin that can burn 50oz in 10 laps. Long fuel lines also smooth out rapid fuel pressure spikes in the system. The only thing needed to feed the fuel system and complete a HIGH FLOW fuel system for Big Blocks is Open up that Pipe Presssure Fitting to keep the Fuel Pressure Coming consistently.

 

[Tom Foley]

What is being done is no different than changing jets in a fixed jet carb similar to a dirtbike . Venturi suction is what it is dependant on the venturi size and throttle opening except our systems have a bit of pressure due to the exhaust . This is similar to a HP carb setup on a car or boat that has to assist the diaphragm lift pump to keep fuel in the float bowl under hard demand .

 

[dr gary turner]

I run a remote needle on my mono that has some fuel tubing length to it,when I do adj. to it happens in about a straight away. On my 21 rigger I have the 3rd channel built into the carb,as soon as I touch the needle it is right now. I think if you pulled the fuel line off your engine at the nipple of the carb it would spool up faster than if you just pinched the fuel line at the tank...No?? It works that way with my airplanes... I could be wrong.

Stan, if you get some time, could I see a pic of your 3rd channel needle built into carb on your .21

I like the idea, would like to try that. Or send me some info. on how to? Thanx

Mark Bill McGraw had some DJ carbs built and the OS MAX remote needle screws right into it. Ross at Speed Master may still have some.

 

[dwilfong]

I run a remote needle on my mono that has some fuel tubing length to it,when I do adj. to it happens in about a straight away. On my 21 rigger I have the 3rd channel built into the carb,as soon as I touch the needle it is right now. I think if you pulled the fuel line off your engine at the nipple of the carb it would spool up faster than if you just pinched the fuel line at the tank...No?? It works that way with my airplanes... I could be wrong.

Stan, if you get some time, could I see a pic of your 3rd channel needle built into carb on your .21

I like the idea, would like to try that. Or send me some info. on how to? Thanx

Mark Bill McGraw had some DJ carbs built and the OS MAX remote needle screws right into it. Ross at Speed Master may still have some.

The gentleman that made the carb for Bill and speed master was at the pond the last two weekend running his boat.

Sorry I am bad with names. He is back running boats after a long break.

He showed me one carbs he made for his Rossi 61 he is running now and thy are really nice.

I have a speed master MAC .21 carb and it is very nice made carb.

I will talk with him and see if he is getting back into making carbs.

Harry and Ross do not have any old stock left.

David