Campgems

Comments?

Don

spaceworm

With the exception of ST, my experience has been to direct the slit down the carb throat toiward the crank for glow engines. ST, however, directs that the slit be postioned to point to the closest engine mount hole. Anyone know why?

Sincerely, Richard

beppeVRCS

The observation makes a lot of sense... Did anyone perform any tests with the spaybar in different positions?
Thanks.
Beppe

Campgems

I have two new engines in hand, a Magnum 52 four stroke, and a ST3000. Both carbs are aligned as I have been doing, IEthe slit pointed at the manifold on the four stroke and at the crank on the ST3000.

Iseem to remember that ST recomends rotating the spray bar for best transistion. Interesting differences. Could it be a glow/gas fuel difference?

Don

Motorboy

See at the picture where i marked the place of the hole and how the hole is affected by air flow..
The source is from the book "Model Aero engine encyclopaedia" by Ron Moulton

Mr Cox

On ST carbs you rotate the spraybar to tune the mixture of the midrange.
PAW has two wholes in their spray bar so that none is pointing straight down.

So it seems that most proper manufacturers are well aware of these effect, while the low budget copies might not be. On the other hand, there is still a low pressure when pointing straight down, just not quite as good as the one around 90 degrees.

blw

My Feedback: (3)

I don't know much about fluid mechanics Don, but they are somewhat different than aerodynamics in some ways.

JockC

Thanks, Don, this is something I have always puzzled over, having read texts on fluid mechanics in my student days. I set fixed venturi control line engines up a long time ago with the spraybar hole at right angles to the airflow, with what I thought were very good results, but not measured by a tacho in those days.

For engines equipped with an RC carb, maybe the 'manufacturer's setting' with the hole pointed roughly towards the crankshaft, ie 'downwards' is a compromise to give acceptable results at various positions of the throttle barrel. Who knows?

spaceworm

Looking at the real world flow characteristics of the diagrams above, it seems that ST may have positioned its spraybar holes towards the engine mount holes to put them into the separation point of the boundary layer to disturbed flow point. That kinda makes sense to me. Whadya all think?

Sincerely, Richard

Campgems

From section 2 of chapter 1, Fundamentals, "Matter exist in two states-the solid and the fluid,the fluid state being commonly divided into the liquid and gaseous states" There is a section of the book called "fluid flow about immersed objects" and it is a basic discussion of aerodynamics. The fluid being air.

Don

Campgems

Excellent point.

The air flow through the barrel in relation to the spray bar changes from WOT to idle. At idle or so, the flow would be at 90 degrees to the slit, while at WOT, 180 to the slit. A guess on my part, this would give a better atomization at lower RPMs. Now, why doesn't the Walbro carb do the same? One possible answer is that it is designed for WOToperation, IEleaf blowers, chain saws, etc, which are seldom operated at mid range.

Don

downunder

With CL engines having a fixed venturi it's been found by experiment that the best fuel draw occurs when the hole in the spray bar is pointing around 10 degrees further down from directly across the venturi. A Fox spray bar has 2 holes but they're not drilled 180 apart so it's only fitted correctly when both holes are pointing slightly down and can't be seen. On RC carbs with the adjustable mid range (like ST and Enya) the spray bar outlet can be turned slightly to match it's position somewhat better to the more angled airflow through the barrel for better (or worse) fuel draw. The Enya carb is better than the ST though because it uses a lever to rotate the spray bar while the engine is running and also has an air bleed for idle. I must admit I've never tried altering the Enya's mid range at full throttle though to see what happens to the mixture but I know it's very effective on the mid range.

spaceworm

Maybe because the Walbro has a butterfly throttle plate with the jets fixed in the venturi throat and the glow carbs typically have a barrel with the jets inside the barrel that rotates?

Sincerely, Richard

Campgems

Richard, the Walbro in question isn't a butterfly carb but a barrel throttle. Walbro has a couple series that use this arrangement. The WZ series is the best of the two series for planes as it has an adjustable top end. The WY series has a fixed top end that can only be adjusted by a carb tear down and an orifice replacement.

The WA and WT series are the ones most used for Aircraft engine carbs and while I like the looks of the WZ, the throttle return spring is a major issue along with the barrel ramp which is cast into the housing and the throttle arm follows. The arrangement is going to need a lot of modification to work, so the WA and WT are the most likely for use on a plane engine.

Downunder, you bring up some interesting ideas. I have a Saito 150 that I'm converting to ignition and was looking at a carb for gas. I've been told that it will run on gas with the stock carb, but the mid range is tricky. Perhaps a bit playing with the spray bar may cure that.

Good stuff guys, keep it coming.

Don

spaceworm

Oh yeah, you said that in your original post. My bad. But the rotating barrel are typically fitted to 4 stroke engines, are they not, like Honda and 4 stroke Ryobi? Why have you selected that type when conventional wisdom has chosen twin needle carbs in the WT and WA series for conversions? Thanks.

Sincerely, Richard

Campgems

Richard, I'm a late comer to converting engines. The conversion of Glow engines seems to be one not often tried but some reports are very positive. For me, I would like to know about the spray bar because one conversion is using the stock carb but fine tuning it for gas. The WZ Walbro carbs are a lot like the stock glow fuel carbs only with a pump and regulator (plus items in the equation) but with a really nasty throttle arm setup with doesn't lend it's self to a servo operating it (Real Negative in the equation).That carb is the one that has the spray bar slit at 90 to the venturi.

Also, Ialways like to know if "conventional wisdom" is correct or not.

Don

w8ye

My Feedback: (16)

That midrange rich point on the Saito 150 running gasoline is very noticable. I wish you luck with tilting the spraybar for it could use some help.

blw

My Feedback: (3)

It probably have more to do with intake size/shape, and the rest of the interior if it is intended for whatever reason. There is always something to disrupt stable air flow either for benefit or to work against you.

wcmorrison

My Feedback: (2)

I would note the turbulence factors too.  Could it be that the manufacturer is also looking for a "better mixing" of the fluid (fuel) with the airflow.  It all is smooth, then the mixing too could be streamlined into the crankshaft or up the manifold.  In the case of the crankshaft, it turning could influence the mixing but in a four stroke, if all is smooth, then the point of turbulence would be at the valve opening and that, it would seem to me, to be the least desirable condition.

I think I will stick with the "normal" set up.  I do not need to do such fine tuning.

Chip

spaceworm

I too like to experiment and have three Ryobi 4 strokes with the rotating barrel carbs. I have a SAITO 180 that I am considering to convert to gas, and several gassers to convert to gas/glow. My questions are not to question you but to inform me why, so I can apply that "conventional wisdom" to my own actions. To me "conventional wisdom" is what has proven to work, or not. Thanks for your experimentation and comments.

S incerely, Richard

Hobbsy

My Feedback: (102)

Turbulence needs to occur in the combustion chamber, the intake flow needs to be smooth and steady to accomplish maximum cylinder filling.

Sport_Pilot

This ignores the effects of manifold pressure.  The spray bar is restricting the air flow so the manifold pressure will be higher behind the spraybar.  The area 90 degrees to the side may be lower when at full throttle when velocity is highest, but not at low speeds.  Because of this  you basically know what you have with the orifice pointing straight down the manifold, to the side may or may not be lower pressure.  Also toconsider is any effect if the spraybar is off center, which side would be best?

Roary m

My Feedback: (67)

I have to plead some ignorance here on the fluid dynamics issue; ironic, since I have more STs that I can count. The ability of the spray bar to be manipulated, however, isn't that a way of addressing the altitude at which the engine is to be run? Sorry if I stirred something up here, that's just how I always saw it since a carb throat is a venturi.

Campgems

The whole purpose of this thread was to stir something up. I kept thinking of the barrel carbs in terms of a butterfly valve carb and was having trouble getting my head around the change of air direction over the nozzle depending on throttle position, mainly the metering slit. Keep the pot stirring.

I read an interesting point in the Walbro Diaphragm Carburetor Service Manual on Barrel Valve Carbs. I quote

"Since the barrel restricts the air flow both before and after the main nozzle, the main nozzle is always acted upon by the same amount of vacuum, essentially a variable venturi."

and

"The advantage of the barrel valve design is the it provides for a very even air/fuel mixture thoughout all throttle settings."

Iam in the process of setting up an experiment to see if I can determine if the spray bar alignment has any effect, and if so how much. I probably should have used one of my ST for the project, but I'm going to use my Saito 150, which if you have worked on, you know it is very difficult to get the spray bar to stay in one position while tightening the nut holding it. My goal is to setup the carb for best overall performance, then get RPMreadings for a number of fixed throttle positions. I'm am going to make a ratchet arm setup for changing the throttle so it will be in the same angle for each change of the spray bar, so the spray bar change is the only thing changing. That's the plan, but reality may get in the way of the plan once I have my test bed setup.

As for your point about the spray bar being the way to compensate for altitude, I think that is the job of the high speed needle. The ST instructions are to adjust the angle for best mid range performance after the high and low end are set. This is one of the confusing things that started this thread. Why did ST go to such efforts to make the spray bar easily adjustable in relation to air flow, when other makers are content with a fixed position, or don't seem to care about the orientation?

Don

Kweasel

My Feedback: (29)

A true variable venturi is ideal because it maintains fuel draw at part throttle. Our rotating barrel carbs lose fuel draw at part throttle, the venturi effect is lost as the barrel closes. You should be changing the fuel metering rather than wasting your time with the spraybar. Its easy to do with a Supertigre style carb by grinding the cam slot. Its very difficult to modify a fuel metering slot because of its small size.