McCoy .35 red head spraybar

[Dan Dalena]

Have an old but not dead McCoy .35 red head. I get it to run on a prime, but no matter what needle valve setting (starting at 1 1/2 open) it will only run on a prime. Fuel system, tank, hoses spray bar are all clear. I prime by covering intake while turning prop (without glow starter connected naturally). I thought (I'm old and can't remember being I started flying in 1959)  the fuel orifice in the spray bar should face forward at 90 degrees to the intake tube. Is this correct?

[Brett Buck]

Have an old but not dead McCoy .35 red head. I get it to run on a prime, but no matter what needle valve setting (starting at 1 1/2 open) it will only run on a prime. Fuel system, tank, hoses spray bar are all clear. I prime by covering intake while turning prop (without glow starter connected naturally). I thought (I'm old and can't remember being I started flying in 1959)  the fuel orifice in the spray bar should face forward at 90 degrees to the intake tube. Is this correct?

    Put it straight down toward the crankshaft. However, I would have expected it to be able to run the way you describe it. Depending on the specific engine, there was a steel venturi insert that restricted the venturi diameter, held in by the spraybar. Be sure that is present.

   Are you sure it is running out the prime, or is it weakly firing for a second and quitting?  Running out the prime, I would expect it to run for a few seconds, ending by speeding up and quitting lean.

   Are you starting it with the cylinder upright?

     Brett

[Dan McEntee]

Have an old but not dead McCoy .35 red head. I get it to run on a prime, but no matter what needle valve setting (starting at 1 1/2 open) it will only run on a prime. Fuel system, tank, hoses spray bar are all clear. I prime by covering intake while turning prop (without glow starter connected naturally). I thought (I'm old and can't remember being I started flying in 1959)  the fuel orifice in the spray bar should face forward at 90 degrees to the intake tube. Is this correct?

    Hi Dan;
     You want to aim the discharge hole straight down the venturi, or maybe just slanted towards the back of the engine a bit, but you should not be able to see it when you look down the throat of the venturi. And the needle needs to be open at least three or four turns to get enough fuel to run. It changes due to air temps outside when you are flying. Make sure you have at least half castor oil in the fuel and at least 25% total oil for the content.  Top it all off with a hot long reach glow plug and 10-6 prop and you should be good to go!
    HAPPY LANDINGS!
   Dan McEntee

    PS to Add;  Take a screwdriver and make sure all the screws in the back plate and head are tight. I have helped lots of guys start old engines that had been sitting a long time and often run into that. I remember one guy that had a McCoy Red Head .35 in a Sterling Flying Fool and was having trouble like yours. I opened up the needle a bit more, got it started and the needle set to a nice four stroke setting, and while we were watching it run, i notice the sound started to change and after a few seconds, I saw it starting to spit out a part of the head gasket!! It quit after a few more seconds. I took the head off, put a thin coating of #2 Permatex on the cylinder and head, put the old head gasket pieces in place and tightened it all down, and then started it up again and it ran the tank out just fine.

[Dave_Trible]

We run a bunch of these.  Open the needle four full turns.  Put your finger over the venturi to choke and pull the prop over a few turns.  You should see fuel being pulled up through the fuel line.  If you do then give the engine a good prime in the exhaust and down the venturi.  It should fire up in a few good flips and be running pretty rich.  If the fuel didn’t come up through the line you have a blocked line in the tank,  an air leak at the backplate or the engine doesn’t have enough compression to draw fuel or run.  Be sure your fuel is at least 25% oil.

Dave

[Gerald Arana]

 the fuel orifice in the spray bar should face forward at 90 degrees to the intake tube. Is this correct?
[/quote]

Almost. Put it just out of sight (91-92 degrees) and either forward or backwards. I've had them in both positions and can't tell any difference.

Cheers, Jerry

[Trostle]

I thought (I'm old and can't remember being I started flying in 1959)  the fuel orifice in the spray bar should face forward at 90 degrees to the intake tube. Is this correct?

This question about the spray bar orifice location has been raised on several occasions.  In an "R.C. Channel Chatter" column in the April 1970 issue of Flying Models, Maynard Hill wrote a multi-page article on "Throttles -- How do they Work?"  In that article, Hill showed a diagram of the pressure distribution of air moving past a spray bar as in a venturi.  As can be expected, maximum pressure is measured at the very leading edge of the spray bar.  Minimum pressure is measured at 120^o from the air flow.   In other words, the orifice should be just past right angles (like 30^o) to the airflow through the venturi.  The diagram shows the airflow is turbulent at the trailing edge of the spray bar and is not probably the best place to place the orifice.

(For those not familiar with Maynard Hill, he is a recognized expert in the RC world and has set world altitude records with his RC models among other accomplishments.)

Keith

[BillLee]

This question about the spray bar orifice location has been raised on several occasions.  In an "R.C. Channel Chatter" column in the April 1970 issue of Flying Models, Maynard Hill wrote a multi-page article on "Throttles -- How do they Work?"  In that article, Hill showed a diagram of the pressure distribution of air moving past a spray bar as in a venturi.  As can be expected, maximum pressure is measured at the very leading edge of the spray bar.  Minimum pressure is measured at 120^o from the air flow.   In other words, the orifice should be just past right angles (like 30^o) to the airflow through the venturi.  The diagram shows the airflow is turbulent at the trailing edge of the spray bar and is not probably the best place to place the orifice.

(For those not familiar with Maynard Hill, he is a recognized expert in the RC world and has set world altitude records with his RC models among other accomplishments.)

Keith

+1

Plus: empirical measurements have shown 15%-18% improvement in "fuel draw" if the orifice is just at that right spot. I wrote this up back in the 70's in my CL Racing column in Model Aviation, and I think Frank Williams replicated the testing in the PAMPA newsletter as well.

[Dan McEntee]

+1

Plus: empirical measurements have shown 15%-18% improvement in "fuel draw" if the orifice is just at that right spot. I wrote this up back in the 70's in my CL Racing column in Model Aviation, and I think Frank Williams replicated the testing in the PAMPA newsletter as well.

    I remember reading something like this at some point and always wondered how in the heck they measured this and if they used some sort of flow meter or flow bench. I didn't focus on this in my answer figuring he wanted to just get it running. But in my journeys through running engines in stunt I have seen improvement in performance with the orifice in just the right spot. I think it helps to work your way up  to this point of fine tuning. It helps to have the proper tools to adjust and tighten down the spray bar when getting this precise. I gave up needle nose pliers along time ago when I found Jim Lee had some nifty socket tools for the job!

  Type at you later,
   Dan McEntee

[Trostle]

+1

Plus: empirical measurements have shown 15%-18% improvement in "fuel draw" if the orifice is just at that right spot. I wrote this up back in the 70's in my CL Racing column in Model Aviation, and I think Frank Williams replicated the testing in the PAMPA newsletter as well.

Hi Bill,

I know there is other material on this subject.  I remember your note in MA and the work that Frank did in Stunt News, but could not find those.  There is also an article I have where someone actually ran tests and measured the pressure distribution around a spray bar.  I have it somewhere, but I could not find it before submitting the information from the Maynard Hill article above.

Keith

[BillLee]

Hi Bill,

I know there is other material on this subject.  I remember your note in MA and the work that Frank did in Stunt News, but could not find those.  There is also an article I have where someone actually ran tests and measured the pressure distribution around a spray bar.  I have it somewhere, but I could not find it before submitting the information from the Maynard Hill article above.

Keith

Memory review: My tests were written up in my June 1982 MA column, not "the 70's". If anybody is interested in what I showed  and how I did the tests, you can visit the TechTopics on the NCLRA website http://nclra.org/TechTopics/index.php and find "Where Does The NV Hole Go?" or http://www.nclra.org/TechTopics/NeedleValvePlacement/NeedleValveAlignment.pdf

Briefly: a simple manometer and a source of suction (a shop vac).

[Gerald Arana]

This question about the spray bar orifice location has been raised on several occasions.  In an "R.C. Channel Chatter" column in the April 1970 issue of Flying Models, Maynard Hill wrote a multi-page article on "Throttles -- How do they Work?"  In that article, Hill showed a diagram of the pressure distribution of air moving past a spray bar as in a venturi.  As can be expected, maximum pressure is measured at the very leading edge of the spray bar.  Minimum pressure is measured at 120^o from the air flow.   In other words, the orifice should be just past right angles (like 30^o) to the airflow through the venturi.  The diagram shows the airflow is turbulent at the trailing edge of the spray bar and is not probably the best place to place the orifice.

(For those not familiar with Maynard Hill, he is a recognized expert in the RC world and has set world altitude records with his RC models among other accomplishments.)

Keith

That is exactly my point Keith! And it can be 180 degrees to that diagram (on the other side).

Cheers, Jerry

[Brett Buck]

Dan, while you might get more fuel draw by positioning at exactly the right angle, that range is pretty narrow. If you get it too far around towards the inlet, it will not work at all.

    In any case, the engine should certainly run with the hole pointed toward the crankshaft - which is not at all narrow range  nor does it require any sort of tight tolerances.

   So, despite the generally correct information about how the angle affects the fuel draw, I still recommend that you *point the hole at the crankshaft*, that way you know it is at least in an acceptable position. Once you get the basic problem solved, get it running at all, then, maybe experiment with different position.

   This is a case of "perfection is the enemy of good enough".

     Brett