McCoy 35 Venturi Question

[Colin McRae]

I was cleaning a McCoy 35 (not sure if it is a redhead model) and noticed something that I have not seen before or aware of. This particular engine has some type of liner down in the venturi that is centered w/ the spray bar. It would have the effect of reducing the venturi orifice size a bit to change the air flow. I have an old parts diagram for a 35 redhead and the diagram does not show the liner part.

I know the McCoy 35 had different versions over the years, and this engine has the plug centered in the head.

Might anyone know if this liner is a stock part for the engine, or might a previous owner have added it to change engine performance? And can the engine be run with or without the liner?

Thanks in advance.

[Dennis Toth]

Colin,
This looks like the insert that was used on the K&B greenhead 35's. It allowed the combat fliers to remove it and add pressure fuel feed. I have not seen it on the McCoys, and I think you are correct in thinking it was added to improve fuel draw. This would normally not be needed with the stock sparybar but way back some used a replacement that had a smaller diameter than the stock one so they could have done that. If you use a ST NVA style you should not need the insert for good fuel draw.

Best,   DennisT

[Colin McRae]

Thx Dennis

I do have a ST NVA. I might run the engine in my test stand (without the liner) and see how it performs before I put the engine back on the model.

[bob whitney]

that is deff not a K&B insert

[Brett Buck]

I was cleaning a McCoy 35 (not sure if it is a redhead model) and noticed something that I have not seen before or aware of. This particular engine has some type of liner down in the venturi that is centered w/ the spray bar. It would have the effect of reducing the venturi orifice size a bit to change the air flow. I have an old parts diagram for a 35 redhead and the diagram does not show the liner part.

I know the McCoy 35 had different versions over the years, and this engine has the plug centered in the head.

Might anyone know if this liner is a stock part for the engine, or might a previous owner have added it to change engine performance? And can the engine be run with or without the liner?

Thanks in advance.

    It was a stock part and you want it, if you have the stock spraybar. With a ST or other aftermarket spraybar, you suck up some of the extra choke area, so, maybe you don't need it.

   The previous owner probably removed it to Get More Power. Which has the effect for most people of Get Bad Engine Runs. It was OK as it came, people would take out the insert, then have all sorts of problems  - but it ran like gangbusters on the ground. Too bad they don't give out trophies for "Best Test Stand RPM"!

     Brett

[Colin McRae]

Thx Brett

There are more twists and turns to this story. I got the engine from a guy who did not know it's history. It did not have a NVA. I tried to insert a McCoy stock NVA from another McCoy engine I had, but the hole in the venturi side was not big enough (not sure why) to insert the stock spray bar.  I also tried a Fox 35 NVA. Same thing, spray bar too large for the holes in the case. I had a 35 size ST NVA and it fit. But the engine ran terrible (with insert installed).

I just ran the engine in my test stand w/ the ST NVA (and insert removed). It ran better. Probably OK to fly for sport.

Really weird that a stock McCoy 35 NVA could not be inserted. I guess I could drill out the holes a bit and try the stock NVA (with insert installed). Also, funny that the McCoy 35 parts drawing does not indicate the presence of the insert.

[kenneth cook]

             I own several which have the insert. It's readily noticeable looking down the venturi. I never removed them and tried to use the engine.

[Brett Buck]

             I own several which have the insert. It's readily noticeable looking down the venturi. I never removed them and tried to use the engine.

   If you can, would you measure the ID of the insert?  I checked my McCoy 29 FR that someone long ago converted to CL from RC, it doesn't have the insert. 

     Brett

     

[Colin McRae]

Thx Brett

I did learn a little more while looking at a few of my engines.

I have 4 McCoy 35's. 2 have the glow plug centered in the head, and 2 where the plug is offset from center. On the model where the plug is centered it has the venturi insert. On the offset head plug, no insert. Also, the spray bar diameter is a little different between the two McCoy models.

I also have a NIB McCoy 29 redhead (plug centered in the head). I took the stock NVA from the 29 and it fit on the engine in question. So, I will try the 29 NVA on the 35 and make sure the insert is in the venturi. With this setup the McCoy 35 engine should be stock for a redhead model engine.

(I believe the McCoy 29 and 35 redhead engines use the exact same NVA. Brett, do you know if this is indeed true. I believe it is.)

Thanks in advance.

[Brett Buck]

(I believe the McCoy 29 and 35 redhead engines use the exact same NVA. Brett, do you know if this is indeed true. I believe it is.)

   As far as I know, yes. But I am not the world's expert, just observing from engines I have acquired somehow over the years.

      Brett

[Colin McRae]

Thx Brett

The proof will be in operation. I have a few stock McCoy 35's that run great, so I know how they are 'supposed' to run.

So, it looks like my original problem was simply the use of the ST NVA for that particular engine. The larger ST spray bar diameter (compared to the McCoy stock NVA) was not letting in the correct amount of air and the engine would just not run right.

Should be good now.

[Brett Buck]

So, it looks like my original problem was simply the use of the ST NVA for that particular engine. The larger ST spray bar diameter (compared to the McCoy stock NVA) was not letting in the correct amount of air and the engine would just not run right.

  I am a bit skeptical that too little choke makes it run oddly. Maybe weak, but should be very steady. Might have been something else.

    For reference, I checked my 29FR with the original needle and spraybar, the threads are 8-32, with the thread form blunted so the thread diameter is .153. The hole in the engine is .157, so the ST would just fit very closely, nearly interference.

     In the choke, the spraybar is .125 (nominal 1/8"). So, if we had the ID of the insert, we could figure out the original choke area.

     Without the insert, the venturi diameter is .329. With the ST spraybar and no insert, the choke area would have been .0354 square inches - which is astronomically large. As I found it, it had the stock spraybar, which is even more excessive, .0443 square inches, hopeless on suction.

    Brett

[Dave_Trible]

The center plug version with the venturi insert came from the first runs of the engine with Dick McCoy at the helm.  I think these were fit better and are superior to latter slant plug engines.  I think the change came about when Duromatic took over production.  Don’t know why they did away with the insert- likely to save a few cents.  The pictured engine COULD be the case where someone toasted the engine and brushed the burn and red paint all off.  I’d be suspicious of its condition.

Dave

[Paul Walker]

Nit to disagree with all here, but my two McCoy 40's have inserts. I discovered at VSC years ago about them. The 40 ran well, but tge plane was taxing it a bit. I then removed the insert  and all I can say is WOW!  Instant power. It ran 4 cycle throughout the pattern and ran great with an awsome sound. No muffler of course!  It used a stock McCoy needle valve assembly.

Remember to run LOTS of oil in the fuel. Give it a try if you are down on power.

[Colin McRae]

Thx to all for the info.

I have both versions of the engine, and both run well w/ it's stock NVA. But on one particular engine (with insert) I had a broken needle, so I tried the ST NVA. The engine would run, but only when run very rich. If I set the ST needle right at the 4-2-4 break point, and tilted the model up, engine would instantly go lean and quit.  So the fuel/air ratio setting on the ST needle was overly sensitive.

In any event with the McCoy 29 stock NVA used I expect that things will be back to normal. Will run the engine again and see.

And I do run 5% nitro / 29% all castor in my McCoy's.

[Colin McRae]

   As far as I know, yes. But I am not the world's expert, just observing from engines I have acquired somehow over the years.

      Brett

Brett, I was able to look at both parts lists for the 29 & 35 (center plug version) and the McCoy part numbers for the spray bar and needle are identical. So, the 29 NVA I have will end up making the 35 stock. Should now run fine.

[John Park]

I have an ancient (circa 1959?) McCoy .19 redhead, which also has the venturi insert, obviously an original fitment.  With the stock NVA, it ran well and was fully aerobatic.

[Colin McRae]

In case anyone is interested. Here are the 2 engine manuals I have. The 35 does not have an insert. The 29 does.

But same part numbers for the spray bar and needle.

[Colin McRae]

On my McCoy 35 engine run problems, one thing I did not think about (and I should have!!) is that I have been using a tongue muffler that I got on Ebay as my flying club required mufflers. The muffler is 'supposed' to be designed for a McCoy 35. On this particular muffler there are (19) 1/16" dia exhaust outlet holes (0.0583 sqin total exhaust area). My guess now is that the muffler is too restrictive contributing to the poor run quality. I am going to enlarge the holes to 5/64" (producing 0.0911 sqin total area - 60% increase) and see how she runs. After all, the McCoy's were originally designed for no muffler. And can even enlarge more if needed.

As a comparison, I also have a different tongue muffler that I got from Brodak for the McCoy 35. My engines run well w/ this muffler. It has (10) 3/32" dia holes for a total exhaust area of 0.069 sqin.

Sometimes the simple things are not obvious !!