Cox Venom Piston Rod Reset Question

I've enjoyed reading this thread. It's always interesting when someone asks a question about an engine's performance. There always seems to be something to take/learn/consider from the commentary. I'll admit that I've never owned a Cox Venom but I agree with Ron on installing a new reed to re-establish a baseline. The reed-function is crucial. Not sure what reed-system (reed and retainer style) your Venom came with.. but replacement-reeds can vary in thickness by as much as .009" governed by the material used. The older Copper 4-legged star reeds are .001". My Steel (oval) reeds are .002", Mylar (oval and Mylar-star) are .005" and Teflon (oval) are .010". Cox switched the reed-retainer in later years, from a wire circlip.. to a plastic-cap/ring. The cap-style seems to be gapped to use the .005" oval Mylar reed. If your reed-retainer is a wire circlip.. be sure that it doesn't pinch the replacement-reed. Some people thin-out their wire-clips by rubbing on fine sandpaper if they're using a thicker reed. Example; an engine originally having a thin (.001") Copper reed.. being swapped to a .005" Mylar. That thicker reed is going to fit tighter and possibly be pinched when the retainer-clip is installed. Many people (including myself) check the reed-function once installed, by gently sucking and blowing air (with your mouth) on the reed-housing. Sucking should open the reed-valve with little resistance, while blowing should close it with a noticeable sealing-action. It shouldn't "squeak" or whistle. The seat must be clean and smooth. An old reed could have been stuck to the seat from non-use.. (gummed-fuel) which could still be fouling the seat. I use a Q-tip/cotton-swab with solvent to clean the seat before installing a reed. It can take a bit of rubbing sometimes.. and maybe more than one Q-tip, but it's a safe way to avoid scratching the seat. It's really an important area of concern when and if an engine suddenly doesn't run right. A speck of grit or even tiny seeds can be sucked-in through the rear air-intake.. and get caught under the reed. The little mesh-screen helps to prevent this for the most part.. but some people remove them for more airflow. Your Venom has a larger (.082") opening at the air-intake.. and leaving the screen in place is even more important IMO.

I'm partial to running fuel with 25% nitro at least, in my Cox engines. Where your PowerMaster does have the correct oil.. it's short on the nitro that makes these engines run their best. If you do business with "Sig", they have excellent fuel. Their "Champion" blends are available in 25% and 35% nitro and have a 50/50 mix of castor/synthetic oil. It's available in pints and quarts too. It's better IMO.. to buy/keep 1/2A fuel in smaller containers. Running your 1/2A's on low-nitro fuel from (for example) a gallon-jug, could cause trouble.. depending on how fresh the fuel is.. and how soon you can use it up. Your 8cc Venom tank is approx. 1/3oz. A pint of fuel should give you at least 45 runs.. including exhaust-primes. A quart; over 90 runs. Even running a 1oz. external-tank in an RC model will take 128 runs to go through a gallon. Where the Cox engines use so little fuel.. it makes more sense to me, to buy fuel in smaller bottles. Sometimes fuel will go stale.. which can REALLY affect these little engines. You can always use that up in your less-sensitive larger displacement engines.. or for a good soaking-solvent.  

Be careful with the piston (and cylinder..) assuming that it's OEM. You may know that an original uses a lightened piston. The piston; as with most Cox pistons, I believe is one-piece.. (less the con-rod) and has a Copper-plated top. I believe the lightening-process involved removing material from the inside of the lower skirt.

Something to try for running the Venom on the Herr RC model, is running a Cox "black" 6 x 2 prop rather than the 6 x 3. It's a L/H prop.. but the Venom will run equally well in either direction as all cox reed-valve engines will. This would satisfy your concerns of keeping the blade-area outside the fuse.. and also unload the engine more than a 6 x 3. Just be sure to trim for an opposite torque-roll on your maiden.

Good stuff! I have a NIB Herr "Little Extra" ARF.. and also a NIB Ace "All-Star" Bipe kit.. but I've never flown RC.. and neither are beginners models.. especially the Little Extra. My only RC engine is a NIB OS .10 FP which would be ideal for either of them.

rscarawa wrote:Thank you for the advice.  

The reed retainer is one of those plastic jobs.  For that, do you recommend the Mylar reed?  That is what it came with and it has now.  Is one reed retainer style better than another?

I will look for a 6x2 prop.  I should probably try many 6x3 and 6x2 props as the pitch is frequently inaccurate.  Some 6x3 might be 6x3.25 or 6x2.75.  It would be nice if they had varying pitch props like they used to for 4 strokes.  Something like a 6x1.5-3  where it was a 1.5 pitch at the root and 3 pitch at the tip.  That would keep from wasting so much thrust against the fuselage.

I also have a tank extender on this motor.  If I keep it on there, I will need to make sure it is drilled to .082 as well.  I am sure it is not at that spec at the moment.  That does not help things.  I assume the longer intake runner also has a perf cost of some amount.

I hear what you are saying about buying in smaller quantities.  Truth be told, I bought the gallon mostly for the can.  My old gallon can is looking nasty and wanted to replace it.  Cans work out better in my RC flightboxes.  I will look into the Champion fuel though.


Thx   Scot

I have both LH and RH snapstarters so either rotation is an option for me.

Hi Scot, Yes.. I'd try a new Mylar reed for sure. Keep all the associated parts/surfaces as clean as possible.. and recheck before final assembly. Both style retainers work equally well. There's less guesswork required with the cap-style using the .005" Mylar reed.. where the older clip-style may require thinning the clip as I mentioned. A while back I measured my circlips' wire-diameters.. and was surprised..





.005" variance. Cox started using the thicker Mylar reed before it changed the retainer-design to a cap.. and may have reduced the wire-size to compensate. It's just a theory I have. Also.. their later plastic "horseshoe" backplates seem like a cheaper plastic. I have two that cracked-open where the clip seats. This likely led Cox to redesign the reed-holder.

The Cox 6 x 2 L/H prop is not being currently produced, as far as I know. It was standard issue on their PT19 Flight Trainer. They are out there though. A flying-buddy having a Cox PT19 might have extra props. If there's any older-established hobby shops in your area.. ask the "owner" if they have anything Cox-Hobbies related. Chances are they might have replacement-props for the most popular airplane that Cox built. You might also find some other Cox-treasures.. you just never know.

The tank-extender as you said; is likely the smaller/std. Bee .062" ID, which is literally choking-away the benefits of all the other high-performance features of the engine. I personally wouldn't use it on that engine. It can be hard enough to get a single tank-bowl to seal. You also need two venturi-gaskets for a tank-extender I believe. If there's anything a high-performance engine "doesn't need".. it's added variables when it won't run right.

Best of luck!

roddie wrote:

rscarawa wrote:Thank you for the advice.  

The reed retainer is one of those plastic jobs.  For that, do you recommend the Mylar reed?  That is what it came with and it has now.  Is one reed retainer style better than another?

I will look for a 6x2 prop.  I should probably try many 6x3 and 6x2 props as the pitch is frequently inaccurate.  Some 6x3 might be 6x3.25 or 6x2.75.  It would be nice if they had varying pitch props like they used to for 4 strokes.  Something like a 6x1.5-3  where it was a 1.5 pitch at the root and 3 pitch at the tip.  That would keep from wasting so much thrust against the fuselage.

I also have a tank extender on this motor.  If I keep it on there, I will need to make sure it is drilled to .082 as well.  I am sure it is not at that spec at the moment.  That does not help things.  I assume the longer intake runner also has a perf cost of some amount.

I hear what you are saying about buying in smaller quantities.  Truth be told, I bought the gallon mostly for the can.  My old gallon can is looking nasty and wanted to replace it.  Cans work out better in my RC flightboxes.  I will look into the Champion fuel though.


Thx   Scot

I have both LH and RH snapstarters so either rotation is an option for me.

Hi Scot, Yes.. I'd try a new Mylar reed for sure. Keep all the associated parts/surfaces as clean as possible.. and recheck before final assembly. Both style retainers work equally well. There's less guesswork required with the cap-style using the .005" Mylar reed.. where the older clip-style may require thinning the clip as I mentioned. A while back I measured my circlips' wire-diameters.. and was surprised..





.005" variance. Cox started using the thicker Mylar reed before it changed the retainer-design to a cap.. and may have reduced the wire-size to compensate. It's just a theory I have. Also.. their later plastic "horseshoe" backplates seem like a cheaper plastic. I have two that cracked-open where the clip seats. This likely led Cox to redesign the reed-holder.

The Cox 6 x 2 L/H prop is not being currently produced, as far as I know. It was standard issue on their PT19 Flight Trainer. They are out there though. A flying-buddy having a Cox PT19 might have extra props. If there's any older-established hobby shops in your area.. ask the "owner" if they have anything Cox-Hobbies related. Chances are they might have replacement-props for the most popular airplane that Cox built. You might also find some other Cox-treasures.. you just never know.

The tank-extender as you said; is likely the smaller/std. Bee .062" ID, which is literally choking-away the benefits of all the other high-performance features of the engine. I personally wouldn't use it on that engine. It can be hard enough to get a single tank-bowl to seal. You also need two venturi-gaskets for a tank-extender I believe. If there's anything a high-performance engine "doesn't need".. it's added variables when it won't run right.

Best of luck!

Hi Roddie, it's very interesting all the comment you make. I am very surprised by the large number of variables that make a Cox engine work well, regular or poorly, or do not work directly. I think Cox, I reach almost every corner of the world, what happens is that many people left because of lack of knowledge for the proper functioning of them.
Today that I have taken up the theme of the Cox engines, I am overwhelmed by the number of variables that can happen in a single engine so that the result is very good or disastrous.
I fully agree with you that the valve assembly and retention ring are a vital point in all engines that have this system, as even the Pee Wee .020, the latter is much more critical - delicate - even though the line Of the 0.049.
I remain attentive to such good information! Regards....

MauricioB wrote:

Hi Roddie, it's very interesting all the comment you make. I am very surprised by the large number of variables that make a Cox engine work well, regular or poorly, or do not work directly. I think Cox, I reach almost every corner of the world, what happens is that many people left because of lack of knowledge for the proper functioning of them.
Today that I have taken up the theme of the Cox engines, I am overwhelmed by the number of variables that can happen in a single engine so that the result is very good or disastrous.
I fully agree with you that the valve assembly and retention ring are a vital point in all engines that have this system, as even the Pee Wee .020, the latter is much more critical - delicate - even though the line Of the 0.049.
I remain attentive to such good information! Regards....

Hi Mauricio, Yes.. it is interesting to learn about more ways to get consistent or "better" performance from the Cox engines. I found that installing "larger" fuel line for an external tank, helped to get fuel to draw through the line using a "Killer-Bee" (large venturi) backplate on a suction-only feed. This might help the Tee Dee to draw fuel better too.

I made "compression-fittings" to squeeze the ends of the fuel-line smaller.. for a tight-fit on the smaller NVA nipple/tank fittings.




Smaller line was developing "air-bubbles" when the engine ran-off the exhaust-prime. This cured that issue.

roddie wrote:

MauricioB wrote:

Hi Roddie, it's very interesting all the comment you make. I am very surprised by the large number of variables that make a Cox engine work well, regular or poorly, or do not work directly. I think Cox, I reach almost every corner of the world, what happens is that many people left because of lack of knowledge for the proper functioning of them.
Today that I have taken up the theme of the Cox engines, I am overwhelmed by the number of variables that can happen in a single engine so that the result is very good or disastrous.
I fully agree with you that the valve assembly and retention ring are a vital point in all engines that have this system, as even the Pee Wee .020, the latter is much more critical - delicate - even though the line Of the 0.049.
I remain attentive to such good information! Regards....

Hi Mauricio, Yes.. it is interesting to learn about more ways to get consistent or "better" performance from the Cox engines. I found that installing "larger" fuel line for an external tank, helped to get fuel to draw through the line using a "Killer-Bee" (large venturi) backplate on a suction-only feed. This might help the Tee Dee to draw fuel better too.

I made "compression-fittings" to squeeze the ends of the fuel-line smaller.. for a tight-fit on the smaller NVA nipple/tank fittings.




Smaller line was developing "air-bubbles" when the engine ran-off the exhaust-prime. This cured that issue.

Roddie,That good information !, true, air bubbles !, so when more diameter tend to disappear !, good data !.
You know that in my experience I've noticed ....: the engine with its own vibrations at high rpm, makes the valve flapper move and automatically vary the setting and engine turns ..... God !, Pee Wee 0.020 if He is easy friend!