Hi All:

           While flying my HP .40 powered Ring Imperial today I noticed that the engine could not be made rich enough to go into a 4 cycle.   As I turned the needle valve out the engine would slow a bit but if it was turned out a bit more, the engine would quickly die!  I am using a HP nva that was sold a cl option.  This venturi has a throat diameter of .250" and a spigot that projects only until it is flush with the inside of the wall of the throat.  This is supposed to correct.

            The plane is a Ring Imperial with an upright engine and a standard tank, no pressure.  The fuel is 11-11-5.  I am thinking that it may be the plug as that is the only item that may have changed. 

            Comments/Suggestions?

                                                                                                 Tia,

                                                                                                 Frank McCune

   I have an HP .40 set up by George Aldrich... the venturi is .310" it goes as rich as you like . I wouldn't even try a .250" spigot venturi.   There are other things to adjust also ,,, like a different NVA.    What prop are you turning ?
             John L.

The fuel is 11-11-5.

and

The fuel is 20% synthetic with no nitro

  Out of idle curiosity, which is it?

    In any case, that's incidental, because this is very likely to be the problem:

This venturi has a throat diameter of .250" and a spigot that projects only until it is flush with the inside of the wall of the throat.

    A typical range of choke areas for a wide array of stunt engines from 20 to 75 is 0.016 square inches to maybe 0.024 square inches. A 1/4" through hole is 0.049 square inches, more than twice as big as is normally used on stunt planes

This is supposed to correct.

   According to who? It's VASTLY too big. It might get through a flight and be able to be 4-stroking at about 12,000 rpm, which would require something like a 9-4, theres no way it's going to draw fuel (particularly through a very restrictive fuel passage) from a flush inlet at low revs. I would not expect that to work, and I consider it a minor miracle that you even got it started.

    That may well have been what came from the factory (maybe, although I don't recall there ever being a CL version I could be wrong), however, I will categorically state that it will not result in a satisfactory 4-2 break performance, the venturi is FAR, FAR TOO BIG.

   Continuing on:

 

I do have another nva with a .281” hole in the Venturi and a ST spray bar to try

  That has a choke area of about 0.020 square inches, or only 40% of  your current setup, and because of the bluff body in the stream, disproportionately better fuel draw. That will probably work (assuming nothing else is wrong), that's in the midde of the conventional range. although larger than I would have started with.

A Venturi with a .312 throat appears  bit excessive for stunt

   Good that you recognized that - also too big (presuming a ST spraybar), unless you do something like Aldrich and stack in 1/16" of head gaskets. Aldrich engines were fairly well-known for either sagging or quitting outright on the takeoff roll from fuel starvation. But it's still VASTLY SMALLER than your current setup. A .312 venturi with an ST spraybar is the equivalent choke area (~0.029 square inches) of a .195 straight venturi, which is well beyond the capability of an ST46 in normal circumstances. You are trying to run a .250.

     You are all over the place here. We typically adjust engines with increments of .005", which is about .002 square inches of choke area differences. You are jumping around by a factor of 2 1/2.

   A .281 with an ST spraybar is about 0.020 square inches, which, should run OK at these revs on 10% nitro. 
   A .250 straight hole will not work, period, for stunt.
   A .312 with an ST spraybar is far too big for these revs and reliable flights, but will probably let you get it into a 4-stroke.

If you want to run FAI fuel, I would suggest slightly smaller, just to keep the needle from being so touchy. But I think this is a big mistake, there is a very good reason no one uses it unless they are forced to.

     Brett

     

              You mentioned that perhaps a bit of nitro may be beneficial.  The person who made the piston and sleeve informed to use only 20% synthetic oil and no nitro.  In the past, this fuel and Venturi has worked very well.  Forl laughs, I am going to try a different plug to determine if that may help.  Remember Occam’s Razor?

      Indeed. The simplest explanation is that *the venturi is vastly too large*, not some obscure glow plug effect. It's not close, it's not in the ballpark requiring a slight tweak, it's 2.5X too large. The most successful PA75 setup uses a venturi with a choke area about 0.016 square inches. Do you think a mild baffle-piston 40 from 50 years ago will run OK with a venturi >3x as big?

      Even if you get it to run and go into a 4-stroke, it still won't make for acceptable stunt flights. Paul Bugl was a speed expert, not a stunt flier. This venturi would work if you were running pressure or a bladder, round and round in a circle at 16,000 rpm. It won't work in a stunt plane sucking fuel at 8500 RPM.

   Dropping names - Paul Bugl and (implicitly) Brian Gardner, both fine modelers and craftsman - will not fix the problem. I know a lot of names, too. Maybe Randy will weigh in on the likelihood of the engine running properly on a .250 straight venturi, since he supplied the PA75 with something like a .189 straight venturi with half the area.

   My advice is - since you have it, put the .281 venturi with an ST spraybar back in, use *conventional fuel* (Powermaster 10% "air" preferred, SIG Champion 10% will also work, Powermaster GMA 10% will work). If it starts backfiring, kicking back on the starts, or flaming out unexpectedly add .010 of head clearance, then .005 at a time, just until it stops.

   Magic is not real, your air is not 2.5x denser than anyone else's.  But, as always, it's just advice, you can  do as you wish.

     Brett

 

They come out pretty reasonable

Dear Frank,
There must have done some modification in your venturi: original diameter was 5.00 millimeters (.195").
And, as you noticed, the spigot with ca. 1/10 " dia, ran inside to one third, or halfway.  This is approx. equal to 7,5 mm (7/24) dia.  with the usual 4 mm spraybar.
I strongly advise you to check the timing of inlet: by a HP 40 it can be either 180° (no problem), or 200° (!!!). 
By 200° is practically impossible to make an easily adjustable, stable stunt engine, it is better to glue some "U" shaped .5 mm thick soft aluminum half-tube into the inlet window. We did it with many-many engines, the result is foolproof.
As the mechanical basement is done, you can look after the possibly overcooling of the engine: the sudden stopping can eventually point onto the plug: try some brand with idle bar (R/C type), and use nitro, minimum 5%, or more.
Istvan

 
    Even .195 is over the top for a baffle-piston 40 that will wind up spinning 8500 in the air with the prop he is using, but it may solve the immediate problem (drawing sufficient fuel to go into a 4-stroke). Long enough to have the next problem, that is, wild speed variations.

   A stock ST46 venturi (also straight through) is a mere .157, and .173 was pushing it and wouldn't run acceptably on 5%. The most I ever heard anyone using that knew what they were doing was Ted at the 86 NATs, where it told me he used a .186 in the heat of a Lake Charles summer. The HP 40 will be pulling less air through it than an ST46 - which spins a prop the same RPM, same pitch, but 2" larger.

      Brett

               Again thanks for all of the replies!

               One thing that I must make clear is that the HP .40 engine is NOT a baffle engine!  If I dropped some names, it was not my intention.  I try to avoid this type behavior in my writings.

               Today, I swapped plugs, added 7.5% nitro to the fuel and added a venturi with a .281" throat and a ST nva.  No changes were noticed! Tomorrow, I will bench test this engine with an 11-4 or an 11-5 prop to determine if it needs more load to get it to break into a 4 cycle.  In the past, I ran the HP .40 engines in a wet 2 cycle with an 11-4 prop and life was good. The Ringmaster Imperial does not have adequate prop clearance for an 11 inch prop.   Perhaps I should just stop being foolish and fly the planes that already are powered by OS LA .46 or OS FP .40 engines.  For some reason, I am attracted to the HP engines like a moth to a flame! Lol Its a sickness I tell you.  I have 7 of the silly things!

               Oh yes in closing I want to make it clear that the original HP venturi had a bore of 5mm or .195" and not .250" as I stated in error.  Old age is a ?

                                                                                                        Time for bed,

                                                                                                         Frank McCune

               

      Indeed. The simplest explanation is that *the venturi is vastly too large*

Hi Brett.
Without a shadow of a doubt a Venturi from .250 (which now seems to have become .195) can cause "non-optimal" operation of the propulsion system, but I don't think it can explain (alone) the difficulty in switching to mode 4 (during ground adjustments) and not I can believe it is the cause of a sudden engine stop when trying to open the needle.
Moreover, it would be interesting to know what the system parameters are "exactly" ....

Massimo

Without messing with head shims, because I believe a lot of everyday modelers wouldn't resort to that....
I put a CS .36 on the pt-19ARF for testing. It's set up for a bladder for slow combat. It has exactly a .250 venturi with no restrictions. It started and ran ok. It was set to a pretty fast 2 cycle. I could lean it more and make it scream, all the way to a bad sag within about a half turn.
 Conversely, if I tried to richen it, like clockwork, every time, it died about a quarter turn every single run. Because it slowed down and stopped suction. I opened the needle, choked it and fired back up every time. I tried to make a video, but 1 handed was tough.

I cut this dumb little insert. This allowed me to go rich enough to flood the engine, and lean enough to sag the engine. Huge needle range....

So I would still say a .250 with no spray bar restriction, for most people on most set ups is too big to use practically.

     Hello to all who replied tp this post:

     In my original post, I made a few false statements.  This led to confusion.  This is what happens when one is exhausted. Get a good night’s rest before attempting to write an accurate report!  Oh yes, I am going to keep a notebook and record all information concerning what I am doing with my equipment.  I have seven HP .40 engines and seven OS .40 and .46 engines in use at this time.  Old age has caught with me. Lol.

     I am going to number my engines and keep a written record of what I am doing and not what I think I am doing.

      Thanks for the replies and your patience in this matter.

                                                                                       Be well,


                                                                                        Frank McCune

Without messing with head shims, because I believe a lot of everyday modelers wouldn't resort to that....

But someone clearly has drilled your venturi too big, so I'd doubt that he stopped there🤔.
If you have another that runs better, it will be an easy thing to check. Just remove the plug and measure volume at tdc with a small syringe.
Or, remove the heads and if it seems that combustion chamber has not been raped, just count & measure the shims. L

   Frank ,
  I hope you're having success with your project ,,, I will add that my Silver Star .40 ran better with a tiny amount of muffler pressure but mine was a ring model not a BG high End piston liner.  When I started running Tiger .60s I numbered them as I got them & keep a log book so I know what & where..
 Good luck , John L.

Dear Frank, Dear Randy,
I told that HP-40 has "two and a half" channels...  My aim was to simply, briefly notice that the middle channel is a genial, short one, in my private slang "half channel", since the mixture starts its way from beneath the piston, and goes OUT of the piston, into a very-very short channel, let us say "boost". Naturally, this engine has 3 scavenging ports (3 windows), and one outlet port (2 windows).   
Why I consider this sytem to be genial? Well, the middle channel lubricates the crankpin, upper head of conrod and crankpin holes in the piston, while conventional systems does not.  This is very useful for long life.
Istvan

Dear Frank, Dear Randy,
I told that HP-40 has "two and a half" channels...  My aim was to simply, briefly notice that the middle channel is a genial, short one, in my private slang "half channel", since the mixture starts its way from beneath the piston, and goes OUT of the piston, into a very-very short channel, let us say "boost". Naturally, this engine has 3 scavenging ports (3 windows), and one outlet port (2 windows).   
Why I consider this sytem to be genial? Well, the middle channel lubricates the crankpin, upper head of conrod and crankpin holes in the piston, while conventional systems does not.  This is very useful for long life.
Istvan

Hi Istvan

Yes , and You can see it that way if you like, nothing wrong with that.  I did not speak about the port match in the piston, I just simply stated the fact that is  has  2  Schurle ports , and  the port across from the exhaust is a  boost port.  OS and  many others  have  used  the holes in the piston in their engines.  I have  used those in many engines I have built going back near 40 years.

Regards
Randy

I might add that this flow from underneath the piston crown helps cool the piston as well as the added lubrication of the top end of the rod. These piston ports were very common on many of the older baffled piston engines (Enya, OS, Moki etc) while the Doolings were an extreme example where ALL mixture came through the piston ports. I've often wondered why it's no longer used on modern Schneurle engines. It can't be used on RE engines though because the port would align with the wrist pin.

Oh, 100% piston scavenging certainly works with RE schnuerle too but it for sure caused some head-scratching.
We need only quite small boost port(s), ideally separated to 2 even smaller channels. It's possible to get an unobstructed flow (piston ports are fully open during the whole opening duration of main ports).
In racing they say that holes in piston usually increase instability, so it's allways a trade-off. But I like the idea, it's especially good for a ringed setup.
Btw, just as we speak, I'm drawing a baffle-piston RE with full piston scavenging. THAT is going to be tricky🤕
L

            Venturi was .281 with a ST nva.

            This is what I was attempting to achieve!  Perhaps the warmer weather, 30 degrees, made all of the difference.
         

   ?   You reduced the choke area dramatically, and now it draws fuel. This doesn't seem to be too mysterious.   

      You will never get any idea what you are "fixing" if you keep shotgunning multiple changes at each problem. One change at a time, careful testing, and keep track of the changes.

     BTW, .281 is still probably too big for reliable use, it was just what you had. I would very strongly suggest trying a smaller venturi - while keeping *everything else the same* in all respect. Based on this thread and prior examples, by "smaller", I mean, maybe .005 smaller, like . 278, test carefully, then still smaller,  .273. This should make a small but distinct difference in the results, without broad-jumping into a huge problem. This should require you to run the engine a little bit leaner, and at some point, making it smaller will require you to make it too lean. I doubt you are anywhere close to the "too small" end, but I am unsure what might happen if you get it 2-cycling too hard.

    Small but distinct changes to 1 thing at a time.

    Brett