TECHNICAL BULLETIN
(ALL SPORT-JETT ENGINES)
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READ YOUR INSTRUCTIONS: This engine is designed to turn! Don't even think of overloading it with propeller. Peak ground rpm (not the rpm you fly at) should be about 16500 to 17000 (17500 for SS) for the .40-.50, about 13,500-15000 for the .60-.76, about 13000-15000 for the .90/.95, and 10000-12000 for the 105/120.
WARNING: (Does not apply to SS carb.) Your barrel stop screw is made from plastic (cheap) and is designed to protect the barrel (expensive) from damage. Thus you must be careful when adjusting the throttle arm. If you put a 3/8 wrench on the adjusting nut without backing up the plastic screw, you will ruin the screw, or worse, weaken it such that the barrel comes out in flight and the engine sucks the barrel spring inside. If you must adjust the throttle arm, remove the stop screw and place a soft rod inside the carb to back up the barrel. Do not over-torque the nut. Replace the stop screw as follows:
ADJUSTING THE BARREL STOP SCREW: (Does not apply to SS carb.) Run the brass nut all the way up the screw. Screw in the stop screw until it bottoms out in the barrel slot. Very carefully tighten the nut until it touches the body, then torque it 1/6 turn (1 hex flat). If the barrel binds, back up the screw slightly, and repeat. When adjusted, there should be zero left-to-right movement in the barrel. This is very important--a loose barrel can cause a battering-ram effect which will shear off the stop screw. Readjust the screw regularly to keep your carb working perfectly.
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.
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. REMEMBER, LOWERING THE HEAD TO INCREASE POWER WILL VOID YOUR WARRANTY. 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 16 inch-pounds for the .40-.50 and about 20 for the .60-120.
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 micrometer, 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 micrometers(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 inch, the .46 is .752, the .50 is .782, the .60-.76 is .890, the .90-.95 is .960, and the 105/120 is 1.080 inch. When measuring the head it is usually safer to measure the bare head and gaskets separately. Metric conversion is 25.4mm per inch.
