Quickie JETT QM40
Technical Info and Operating Tips
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PROP SELECTION: Piped engines are much more sensitive to prop selection than you have been used to; therefore the wrong prop can cause many symptoms which lead you to believe you have other problems. For example, two props that vary just 500 on a non-piped engine may vary 1500 on a piped engine. Also two engines which are just 400 different on one prop may vary 1000 on a larger prop, yet their air speeds are insignificantly different. Port timing and pipe length also play a part in confusing the situation. In summary, use a small prop (at least 26000) to compare engines, not the biggest one you have.
These engines are timed and piped for at least 24000 RPM. Get several different sizes. Balance the props. It should require very little. Don't use it if it pegs your balancer. Test them all, making sure you don't overheat your engine (see next section). You will find that there is a large variation in wood props.
Don't run a prop that does not turn at least 23000 at peak on a cold muffler and engine. If you must try a larger prop, get the engine set on a smaller prop, then make the switch without changing the needle. Remember, those big props can fool you into setting the needle too lean. (read on)
Needle Valve Setting: Before you start, you should have a prop that you are sure is not too big, and a well calibrated tach. Open the Needle 5 turns and start the engine. Leave the battery connected and slowly lean in the engine. The engine should stage to 22000 quickly while still rich. Slowly continue leaning it until the engine peaks. You will know the peak when the engine slows down rapidly. At this point quickly open the needle about 1/2 turn, or until the engine crackles rich. Kill the engine and let it cool. Trying to set the needle after you have taken it past peak is fruitless and can do damage. The pipe will not stage properly and your needle could easily be off by one full turn (lean). Finding the peak and how much to back off from the peak is the most important part of needle setting. This is not easy, but with practice you can get quite good and will be able to put in flight after flight without missing the setting.
ADJUSTING COMPRESSION: From time to time you may feel that it is necessary to raise or lower your head for better performance. There are several rules of thumb which others may give you for this. In general, ignore them and use the time honored system "when in doubt don't". Most often as you travel, or experience different weather conditions you will see a corresponding change in engine performance, but in most cases, this can be fixed by changing the prop. Your engine was shipped to you with the head set for optimum performance in average conditions, but in 80% of the cases a change will not measurably affect your power. In all cases, a change of 2-3 thousandths is all you should consider. More, and you should look elsewhere for the problem. For a QM on 15% fuel, the operating range should never exceed .009 to .015.
Look to the glow plug to tell you what to do: If you never blow one and the wire shows no sign of heat (a rough, semi-melted look), then you may want to try to remove a .002 shim. Conversely, blowing plugs often may indicate that you need to add a shim. Before you do, though, you must be certain that you are getting good needle settings. A lean run will wipe out a plug regardless of the head clearance.
Tips: In very hot (< 95deg.), or in very humid, hot (< 85deg.) weather you might need to add a shim. A lower head will give you better needle range on a big prop. Remember, there is no free lunch, the lower the head the higher the maintenance costs.
TORQUING YOUR HEAD: If you have a torque wrench, be careful how you use it. It has been shown that many of these tools are not as consistent as you are with your fingers. Use a screwdriver handle type hexdriver, not a T-wrench.
Make sure all of you bolts are clean, and will screw completely down without sticking. If not, run them in a few times until they loosen up. Clean all the critical parts with a paper towel: The cylinder flange. The head surfaces. The gaskets. The head clamp where it touches the head button. The screw holes and head bolt seats. Next, seat the gaskets and head button on the cylinder and place the head clamp on top. Grasp the head clamp with two fingers and hold it down tightly while you spin the bolts down with the hex between your fingers--don't touch the handle yet--just your fingers on the small hex. When the bolts touch (you are still holding the head down with your fingers) go to the next one across, using a 1-4-2-5-3-6 crisscross until you have them all touching. Work the circle twice. Turn loose of the head clamp and tighten once again with your thumb and index finger on the handle. Always work the circle twice. Finally torque the head bolts to the desired amount with your torque wrench or by hand. About 20 inch-pounds.
It is more important to do step one - seat the bolts - than any other. If you skip it you will find that you torque wrench will give you as much as 100% variation in torque, which can surely ruin your engine.
MEASURING HEAD CLEARANCE: If your engine is not overly tight, put the piston at top dead center before removing the head. Measure the height from the top of the sleeve to the piston with a good depth mic, not calipers, then take note of the sleeve: Does it lift up off the case as you rock the shaft back and forth? If so, you can measure the amount it rocks with your depth mics (usually about .001-. 003) and subtract it from your reading. You can also measure bottom dead center and subtract the stroke. This method is foolproof, but you do need to know the stroke within 1/2 thousandths. The Jett .40 is .707. When measuring the head it is usually safer to measure the bare head and gaskets separately.
