by Alex Varv
How many times, after a virtually perfect carburetor mixture adjustment on the ground, we find the engine running rough in flight (four cycling) in a certain RPM range?
It happens quite often, especially if we would carefully listen to our engine all the time. What we usually do, or should really do, is to land as soon as possible and readjust the carburetor. "Four cycling" is caused by too rich of a mixture, mainly in the low, to mid RPM range. Details about carburetor tuning can be found in my article "Tuning a Diaphragm Carburetor" published in the December 1999 issue of Aero Connections Magazine. In-flight carburetor tuning is necessary not only if we fly at higher altitudes (over 3,000 ft.) but also to fine-tune the carburetor in low flight. Rough engine operation on my Walbro forced me to interrupt a few happy and enjoyable flights.
After the modifications and improvements done on my carburetor, I was looking to find some kind of device that would enable leaning or enriching the mixture in flight. Something similar to those used in airplanes. There is only one system on the market that I know of. It uses a cable that is attached to the HI or LO screw and by turning a knob it opens or closes the given screw. I tried this device out but I was very disappointed.
The core of the cable that turns the HI or LO screws is elastic and the adjustments are far from being precise. Therefore, I decided to design and build my own system that would suit the purposes I had in mind.
From the very beginning, I knew that the principle could not be a rotating cable. I designed a system in which the cable does not rotate but moves back and forth. The fine adjustment is achieved with a threaded mount at the end of the cable, opposite to the carburetor. When turned, a control knob will push or pull the cable and not turn it. This way, the elasticity of the cable is eliminated and the system is very sensitive, precise and reliable.
The cable is attached to a handle soldered to the LO screw of the carburetor. This handle is an L shaped nail and on the end it has a cable stop clamp used on carburetor throttle cables. (see photo below)
Carburetor fitted with adjustment system on the LO screw via a cable end clamp. Notice the handles soldered on the Lo and Idle screw.
The mixture adjustment cable's conduit is fastened to the engine as close to the carburetor as possible. The core (cable) is then connected to the L shaped LO screw handle through the wire stop clamp described above.
This system will permit turning the LO screw about seventy degrees of travel, which is more than enough to fine tune the carburetor in flight.
Before we get into details, I would like to stress that this system can be used not only on paramotors but also on other ultralights that are equipped with an adjustable carburetor, (of course, some modifications might be involved).
Once again, the main purpose of this system is not to tune the carburetor but to perform fine adjustments in flight. This is why I attached it to the LO screw and therefore only minor adjustments are possible but enough to "smoothen" a rough engine. The system is absolutely safe and it can not lean the mixture to the point that an engine seizure will occur. On the other hand, the system can enrich the mixture to such a degree that it can be used as a choke when starting a cold engine on the ground or in the air (after soaring with the engine off as an example)
If this system is attached to the HI screw, the mixture can be adjusted in flight.
Let us asy that the CHT temperature increases due to a very long climb. Enriching the HI mixture we can cooll the engine.
Of course, those who like all the" wistles and bells", my system can be installed on both HI and LO screw for a full in-flight carburetor tuning.
The "first" prototype that I made turned out to work very well but it was too complicated and required too many parts. I mostly fabricated all these parts and they were time consuming.
I decided to try to find ready-made parts, which could be adapted easily. I built the second "prototype" with parts available in go-cart stores. They cost about $19 and are described below:
The photo shows all the components unassembled (top) and the finished control knob and cable (bottom).
Go-cart Teflon cored throttle or clutch cable
Clutch adjustment screw
8mm cylindrical nut
Compression fitting for hydraulic brakes
Wire stop clamp with side screw (to connect the cable to the LO screw)
Small size hose clamp (to fasten the cable to the engine)
Cable ties (to fasten the cable to the engine frame and harness)
At this point, a few very important notes should be made:
In order to achieve the finest possible tuning, very little play is allowed in between the conduit and the cable. A regular carburetor or clutch cable has a lot of play inside. This play will affect the accuracy and range of tuning creating so called "dead turns" of the knob. In other words, when turning the control knob, we will notice that a few turns will be needed to make the LO screw move. They will shorten the tuning range and will cause a less accurate tuning. To improve this, I used an oversized cable, which is very tight inside the conduit. Furthermore, using a Teflon cored conduit will almost totally eliminate any play.
The routing of the cable is vital. The cable MUST be looped or at least bent in a semicircle in between the engine and its frame, otherwise the vibrations will affect the tuning. I found that the best way to route the cable, and keep it in place, is by using cable ties.
The cable is anchored with a small hose clamp in a slot cut in the lower cooling fin at the cylinder base. Next to the nut. Note the vibration dampening loop of the cable.
The end of the L shaped handle on the LO screw should be vertical when the screw is in "normal" position.
I inserted the same picture again to be able to illustrate what I have just described.
Therefore, before attempting to solder the L shaped handle to the screw, the engine should be tuned perfectly. Now we must mark the position of the screw head and remove the screw in order to solder the L shape handle on the screw. This should be done in such a way that with the screw in normal position, the handle will be vertical -upwards or downwards- depending where we can fasten the cable to the engine. The end of the conduit, with the cable coming out of it, should be at the same level as the wire stop-clamp soldered to the L shaped handle. (see photo 2) If the handle was soldered in the correct position and is vertical, it will enable a 1/8 of a turn to the left or to the right. This is more than sufficient for fine-tuning.
When attaching the cable to the wire stop clamp, we should turn the adjustment knob on the system to the FULLEST LEAN position. This means that the LO screw handle is also in the leanest position. Now we can tighten the screw on the wire stop clamp to immobilize the cable.
On my unit ("FLY" Power 115), turning the control knob to the richest position, it will act as a choke when starting a cold engine. As the engine warms up, I gradually turn the knob to the lean side and the engine runs smoothly all the time. This "choke" will work in any other paramotor engine with a Walbro carburetor that does not have a real choke. Even a carburetor with a choke will be very hard to activate in flight, when the engine has been turned off long enough to cool down (after soaring).
Building the Unit
The building process requires soldering. Here some experience is very helpful. To those who have never done this before, I must say it is a very easy operation. A medium size soldering gun will suffice. Besides, soldering is a very necessary skill, not only in PPG but also around the house and in everyday life. It can be learned in no time.
Assembling the control cable. First, we must decide how long the cable needs to be. As mentioned before, the cable MUST have a small loop when routed between the engine and the engine frame to dampen the vibrations. On the other hand, the cable should not be too long, as you want to avoid catching some lines during an inflation and launch. It must also be easily reached in flight with one hand, the one that does not hold the throttle. The throttle on my unit is on the right side thereby the mixture control knob is on the left.
The in-flight carburetor mixture adjustment control knob and cable mounted on the left side of the harness, is easily reached in flight.
Once the length of the cable has been established, the conduit can be trimmed. The best ways to cut the conduit is by using a file or better yet, a DREMEL with a separator. DO NOT try to cut the conduit with a wire cutter. If you cut the conduit with a wire cutter the steel spiral inside the conduit will be flattened!
Trimming the cable is more difficult. The cable being constructed with dozens of steel wires switched together and if cut, will cause them to split and open. Before trimming, the cable must be coated with soldering compound. Also the cut should be performed with a DREMEL separator and not a wire cutter.
Solder the clutch adjustment screw to the conduit. To do this, the plastic covering must be removed with a knife, to expose the spiral. Using soldering paste, cover the spiral with compound. Next, do the same with the adjustment screw sleeve (also the inside of it).
Put the screw sleeve over the conduit, heat up both parts and generously cover the joints with soldering, making sure the hole is not obstructed and it is centered. To achieve all this, we can insert the cable waste (remaining after the trimming) into the screw and into the cable. After the compound has cooled, the cable can be pulled out with pliers. (see picture above)
Solder the clutch adjustment nut to the 8mm cylindrical nut in such a way that the nut will be centered.
Cut a thread with a tap of the same size as the thread on the compression fitting (see photo 1) into the 8mm nut and screw both together.
Note: To eliminate any unnecessary bending of the cable by the control knob, a few layers of heat shrink tubing are recommended near the clutch adjustment screw.
Insert the cable through the compression fitting and into the conduit until the ball (cable end) touches the fitting's rim. Now, using a medium Locktite compound or Teflon tape, screw on the fitting cap until NO PLAY IS FELT ON THE CABLE while pulling or pushing it in. At the same time the control knob MUST turn freely. Check and see if the cable moves smoothly back and forth (while turning the knob) with not turning tendencies inside the conduit.
Note: The Locktite compound on the compression fitting cap threads is a MUST in order to keep the adjusted cap in place. In time, some play may occur and a slight tightening of the cap will be necessary. This is why I did not recommend soldering the cap in place.
With the L shaped nail already attached to the LO screw, the way it was described above, we can proceed with installing the control cable. In order to fasten the cable to my SOLO 210 engine, I cut a slot on the lower cooling fin, by the nut, at the cylinder base. The slot was cut with a DREMEL equipped with a 3mm diameter cutter. The slot is long enough to receive a very small hose clamp. The clamp was opened, slightly straightened and threaded through the slot. Next, it was bent back to its previous circular form and slightly tightened. Once the above operation is completed, the clamp should be tightened to a slightly larger diameter than the cable itself.
Insert the cable through the clamp until the conduit shows through and firmly tighten the screw.
Insert the cable with the control knob already on in the conduit and push it all the way through. Make sure the LO screw is in the FULLY lean position, pass the end of the cable through the cable end clamp and tighten the clamp screw.
The last thing we have to do is to route the cable, looping it between the engine and the engine frame (not the propeller cage) and hold it firmly in place, using cable ties. WE ARE DONE.
Note: The cable MUST not touch any engine parts (moving or stationary). Also keep the cable away from the muffler.
The system mounted on a Fly Power 115, before the first test-flight, which surpassed all expectations.
All the above operations might seem very complicated at first, but I must assure my fellow readers that it is not as difficult as it sounds. This article takes you step by step through the whole process and I promise you, it will be easy and rewarding. I tried out this system and flew many happy hours and every time I fly, it works perfectly. I strongly recommend it to all pilots. Once you try it you will be very glad you have it.
As stated above, this system is safe, it will not damage the engine in any way, since it does only those fine adjustments needed, for a smooth engine operation. I also recommend installing at least a cylinder head temperature probe and gauge. This way, the overall temperature of the engine can be monitored. Again, reading my article about carburetor tuning will be useful in building and understanding how my in-flight carburetor adjustment system works.
Have a safe and enjoyable flight!!
See you up there
Since the parts, building, installing, and using this system is beyond his control, the author accepts no liability for any accidents or damage while using the above presented in-flight carburetor adjustment system.
This article was originally published in the March 2000 issue of Aero Connections Magazine.
The quality of some pictures is low because the article and the original pictures were lost.
Thank all the pilots who helpend me find, restore and re-publish this article on The Inventions Page.