Frequently Asked Questions

These are several of the most frequently asked questions.

Question: Why doesn't Jett publish the horsepower of its engines?
Answer: Jett reveals the power of its engines by publishing and guaranteeing the RPM each engine turns a specific propeller. This may be easily compared to similar data from the competition. HP figures are not so easily compared because there is no industry standard with which to test. As a result, each manufacturer tests independently of the other, often on uncalibrated equipment. In cases where competitive advertisements claim power on the optimistic side, Jett is forced to remain silent, or publish equally optimistic claims. Comparing RPM on a fixed and available brand of propeller is the only way to accurately compare engines.

It is usually ok to compare HP numbers generated by the same person, i.e., a tech writer. Just don't compare his numbers to someone else's.

Question: What glow plugs do you recommend for use with your engines
Answer: The basic answer is to use a high quality glow plug. For all Sport-Jett engines, we highly recommed the Merlin 2003 plugs. Alternatives such as the K&B 1L and HP plugs work well too. They operate in the proper heat range for these engines. Alternates include the OS #8, and Hanger 9 high performance plugs.

Question: How do I break in my engine?
Answer: Please see the tech library page Engine Break-In

Question: My throttle response is not what it should be. I have tried everything.
Answer: Please check and read all the tech data and FAQs on setting your carburetor and upon the use of the correct glow plug. These are still the most common cause of slow throttle response and flameouts. Use a hot plug like the Merlin HOT, K&B 1-L or the Mc Coy 59.

If you are not blowing glow plugs and the engine seems to be running right otherwise, you need to check your head clearance. The specifications for head clearance are in the engine operating instructions, located in the tech data section. If you find that the head clearance is not correct, then first try adjusting to the correct dimension. (Read the section on torquing heads in the tech bulletin for your engine). If the problem persists, then try lowering the head .002 to .004". (One or two shims). Sometimes increased compression raises HP and heat enough to improve the mid-range performance. You may need to readjust your needle.

Question: What kind of fuel do you recommend?
Answer: We have always recommended PowerMaster fuels. We do this because this is the fuel we use, and believe that it has all the ingredients and quality you should expect. We test on 15% and use the synthetic/castor blend. We like clear fuels better than "dyed" fuels, but the dye doesn't really hurt anything.

You should use what ever high quality fuel you are comfortable with. Most on the market are top notch products. If you have never had a problem with your present fuel, then Jetts will not have a problem either. There are some problems, however. Let's be frank. The cheapest part of fuel is methanol. The expensive parts are oil and nitromethane. Some fuel manufacturers use other additives or none at all, to reduce the amount of oil and nitro in their product. The trick in making fuel is to get power without heat, and of course, wear. Nitro and oil do it best. Other additives typically generate more heat. Castor oil is typically cheaper than synthetic so if someone is bragging about castor, then you know what they are selling. I recommend that you get yourself a test stand (everyone should have one) and try a few different fuels. You might find that some run differently from others. If the peak RPM is down, then the nitro is down. Don't let them tell you it is because the oil content is higher-that is not true. Some fuels will run hotter at peak than others, causing the engine to surge to a high rpm, then slow down as it heats up. You can see these things on a test stand.

Read our safety instructions before running any engine, in an airplane, or on a test stand.

For break-in, I like to run just a bit more oil than that found in standard fuels. The safe thing to do is take your standard blend and add 4-6 oz. of castor, or synthetic (Klotz is good) oil. After you follow the break-in instructions, switch back to your regular fuel.

One last thing: You should not expect your fuel manufacturer to tell you his formula and ingredients. I do not reveal the HP of my engines. (See the FAQ on the subject) The bottom line is if I say my oil is 16%, which is about average for sport fuel, then there is always someone out there that will say, "mine is 20%". I have a saying; "It is hard to beat the first liar!" It is not good practice to reveal corporate secrets, period. Don't ask, don't tell. Also, don't believe those home remedies for determining the oil content. People don't know what they are talking about. If someone wants to pay for a "three-ball wear test", then listen, otherwise get a test stand and go to work.

Question: Is an AAC engine more likely to wear than an ABC engine?
Answer: Twenty years ago I would have probably said yes. That was the conventional wisdom. In fact, I read not long ago in a racing forum a comment by a prominent racer stating just that. The opposite is true. That is why I make my premium engines with AAC liners. Do you remember how nice it was to just change out a piston ring every so often? Well, using an AAC engines is very much the same. We get engines that look like they have been through the Great War, and when torn down and measured, we find the AAC liner is like new. In most cases, all we have to do is fit you a new piston and you are right back to "good as new". It saves you about 1/2 the cost of rebuilding.

The secret of AAC lies in the increased heat stability of the aluminum vs. the brass. With chrome on the liner you do not get wear in either case. What you get is warping of the top part of the brass liner due to heat and stress. The brass, when overheated, tends to "bell mouth" and loose its taper. The aluminum does not do that. About the only thing that will hurt the AAC liners is FOD (foreign object damage). Keep your engine and runway clean!

Question: If I use a remote NVA, will I have a delay in engine response between the time I move the needle and when the engine changes RPM?
Answer: This is the most common concern with remote needle valves. The delay is negligible. This complaint has been around for years and comes from the most logical analysis. There can be several inches distance between the needle and intake, so why not? The answer (and the actual delay) is based upon the time it takes the needle to "talk" to the venturi. Just like the delay you hear in speaking or yelling (echo) over long distances, the amount of delay is determined by the speed of sound. In air that's 1000 Ft. (300 meters) per second. In fuel, it's more like 3000 F/sec. (900 M/s). So, if your tubing was one foot long your delay would be only .0003 seconds. In practice, it is slightly longer because your tubing is not completely rigid and "stores" up pressure somewhat, but it is still negligible.

Question: Sometimes my engine locks up suddenly and I can't turn it over. Will the engine be damaged if I force it back off top dead center?
Answer: The Sport JETT engine is a high performance ABC or AAC engine that is hand fitted. The piston and liner fit is much tighter than in standard engines. Occasionally when an engine has raw fuel in it, and you try to turn it over, it will "lock" up. This occurs when the engine dies suddenly on the rich side, and raw fuel has washed lubrication from the piston and liner surfaces. The problem is common and tends to "scare" people at first. It does not hurt the engine to turn it through with the prop. Remove the glow plug and add a few drops of oil into the opening. You might even spin it over without the glow plug to remove excess fuel. This should solve the problem. There is nothing wrong with your engine. Please keep in mind that it has been test run, but only for a few minutes. It still needs break in time (please refer to instructions). After the engine has been run a few times it will loosen up slightly and will be fine. If the engine continues to die suddenly during normal flying, the most likely problem is a bad carburetor setting, bad plug, or a bad tank installation. Please refer to your instructions or to other FAQs for more info.

Question: When I put my piston at top dead center and look in the exhaust there is a gap below the skirt of the piston. I can see inside the engine. Is this a problem?
Answer: Almost all Jetts have this gap. It is called "sub piston induction". Many manufacturers have claimed to use this to enhance power. Unfortunately, it does not make the engine run better. However, during the design of the engine a certain amount of sub piston will allow the engine to be made more compact and thus lighter and stronger. Power is not sacrificed, but a strength and weight advantage is achieved.

Question: My rod is making a clicking sound. I fear it is too loose and about to break. What should I do?
Answer: If your engine is new do not be concerned. The parts are inspected during manufacturing and during assembly. Your engine has also been test run just prior to shipping. Everything is OK.

Now, what is the reason for the clicking? Jett rods have about .003-. 004 (.07-. 1mm) rod-to-crankshaft clearance. This is approximately twice that of an O.S. This is why you see the extra motion. This is not a problem and here is why: During high-speed operation the engine loads the rod in one direction only—the piston pushing down on the crankshaft. This means that the rod is almost always touching at the top of the shaft, leaving a gap at the bottom. The gap is where the lubricant is stored and the more the better. During idle, you may hear the click of the rod moving back and forth on the shaft. The loads are very low at this speed and do not cause a problem.

We have measured rods after years of hard use and find the wear almost zero. In fact, we often say, “rods don’t wear”. Years of pounding are more likely to fatigue the rod body itself, and for this reason it is reasonable to replace them every few years. In racing, once a year would not be unreasonable. Always replace your rod after a shaft run (no propeller).

Finally, do not measure rod play at top dead center. This magnifies the clearance. Place the piston about one-half way between the top of the exhaust and top dead center. You will find a big difference.

Question: How do I make a sucker tube shutoff for my racer?

First, Jett now offers a complete sucker tube assembly. See our Accessories page. Or you can make one yourself.

But importantly... why use a sucker tube? The sucker tube shut off is best because you get 100% positive shut-off without loading your servo. Second, since the servo is not loaded you can turn off your radio without having the tank start leaking. Are you one of those who have hemostats hanging all over your airplane? You won’t need to now.

Use small blue-line tubing. It is large enough for QM and Q-500. Do not use the Prather 1/2A size. Do not use the blue medium or Prather medium unless you plan to use a big, high torque servo. Fold the tubing over itself and measure the width of the fold. (Two times the diameter of the tubing—usually about .300-.400”) Drill a hole this size in a 1/2” round wooden dowel, piece of aluminum, or any other kind of material you have around to a depth of 1/2+ inch. Cut it off. You now have a 1/2” (13mm) long tube. Chamfer the inside of the dowel to make a smooth entrance for the fuel tubing.

Drill a .128” (#30) hole in the firewall at a convenient spot for the shut-off. My favorite spot is exactly in the bottom of the airplane, below the mount. (Q-500 is different) Leave room for the 1/2” dowel beneath the mount. Take yellow nyrod and run it from the hole to your shutoff servo. CA the nyrod to the firewall.

Using 1/32” (.8mm) music wire, make a loop on one end slightly larger than the tubing. I use a #1 screwdriver and wrap two wraps around the shaft and cut it off smooth. Bend the circle as required to make it go smoothly in and out of the dowel without binding. I usually solder the loop, but most people don’t bother. Just make sure it won’t catch going in and out.

Place a short piece of fuel tubing in the loop and pull it inside the dowel. Does it seal? If not, the dowel is too short, or too big. Adjust if necessary. When all is well, then leave the assembly together and feed the wire into the nyrod, pull it up against the firewall and glue the dowel to the firewall with 5-minute epoxy. Make sure it works freely and still seals. Adjust your servo (1/2A size will work) such that the tubing pulls all the way inside the dowel when shut off, and comes all the way out when open. Use a long servo arm to achieve this. Grease the tubing around the loop.

Question: My front end wobbles when I put the prop on. Is the shaft bent?
Answer: In most cases, Jett crankshafts do not bend. They are more likely to break underneath the front bearing during a crash (very rare), than to bend. What usually happens is that the front removable stud bends. This is what it is designed to do. The stud should always be replaced after a crash or runaway (sheared propeller), and should be replaced as a maintenance item every year or so.

If the front of your engine wobbles after you tighten the propeller, then there are several things to look for. First and foremost, remove the stud, clean the threads in the shaft and replace the stud. Screw the stud all the way in, then back it out two turns so that it rotates freely. If you screw it in tight, then it cocks slightly as it tightens. Second, remove the prop drive and lock cone. Clean them carefully, replace and fit the front end with a new propeller. Tighten the spinner nut and inspect. Unless there is a problem with the fitted parts, it should be aligned. During a crash, it is also possible to damage the cone and prop drive. They may need to be replaced.

If the stud still has run out after being cleaned, and fits tight in the shaft housing, then this is the most likely problem. The crank may need to be replaced (very rare). The stud should be loose enough to self align as the prop is tightened. You should return the engine to Jett for inspection.

Question: How can I adjust my throttle arm without changing the idle needle setting?
Answer: Since the position of the idle needle is very critical, this is a very important process. First, can the throttle arm be bent to make a small adjustment? It is made from special steel that allows it to be bent around without damage. It also is a very inexpensive part, should you decide to replace it. If you want to shorten the arm (to give you more stroke) it is best to bend the arm over itself, making a sharp "Z". You can avoid some drilling and cutting this way.

Now back to the question. Make sure you have a brass stop screw!!! Early Jetts had a black plastic screw. Do not follow these instructions if your stop screw is black!! Follow the instructions included with your engine to avoid damage to your engine.

If you have a brass stop screw, then take a marker or pencil and place a small dot on the idle needle face and a matching mark on the brass 3/8" nut. This will tell you if the idle needle has moved during the adjustment of the arm. Loosen the 3/8 nut. Adjust the arm to the new position. Tighten the nut. Since the barrel will rotate during tightening, you may have to use trial and error to get it right. Tighten the nut with about 30 inch pounds of torque (not an excessive amount).

If for some reason you wish to remove the brass nut completely, then you must take special note of exactly where the idle NV is with respect to the face of the nut, e.g., one turn inside the nut. It can actually be just about anywhere.

After your arm is set, then check to see if the Idle N.V. has moved. It should not have, but if it did, then use a small screwdriver to readjust it to the mark.

If the carb is out of adjustment, then read the TECHNICAL section on setting the Jett carbs

Question: The exhaust port on the OS seems massive in comparison to your engines and the inlet on your mufflers. Doesn't this smaller opening restrict power? Do you make a different muffler adapter/header with a larger opening?
Answer: We get this question so much that I had to include this FAQ on it.

Look at the opening on a Moki. It is small. It runs as good or better than the OS. Large exhaust openings are just a way to make the engine lighter and cheaper by using less metal.

Exhaust back pressure is better in most cases, especially on a tuned system.

The exhaust openings on JETT engines are very small. Since we beat the pants off of the competition, this pretty much speaks for itself.

The opening size in Turbo-JETT and Sport-JETT mufflers WILL work properly with your engine. We make a headers for many different brands and sizes of engines. For example, take the Moki 180 and OS 160. In this case, the only difference is the span of the screw holes. If you own the muffler for one engine, you can either change out the header or slot the screw holes to fit the other.