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Latest Notes on QM-II Quarter Midget Q40 and Q500 engine

click here for Jett I (AMA 426) engine recommendations

Thank you for buying a Jett.  We hope it performs up to your expectations.
This engine may or may not have been run.  I run them from time to time as a quality check.  Treat is as a new, unrun engine that needs breaking in.
Set up:  Standard TDC is .150-.151 (all in inches)**.  Standard head clearance is .015-.016.  This is usually .004-.006 of head shims.  Nelson LS shims fit a Jett too.   A Stock head is .140 deep.  Expect this engine to be tight—tighter than you have seen.
Break in:  Since this engine is tight you can expect that break in must be done more carefully than usual.  There is a higher load on the rod and before it seats and wears in properly there is a great danger of ruining your engine if you do not follow these simple rules.
Take a stock 7.4 carbon APC prop and run the engine at 15000 for 5 minutes, 19000-20000 for 5 minutes and finally 22000 for 10 minutes.  Then cut off a Stock APC Carbon prop to 6.0” long and balance.  This will give you high rpms while running rich.  Run the engine at 25000 for 5 minutes.  (If you are at an altitude above 3000 ft or you are using zero nitro, you may want to use a smaller prop)  Never peak the engine hard during this process, but it is acceptable to “zoom” the engine up and down occasionally.  The most common mode of failure during break in is “welding the rod to the crank”.  This is causes by running the engine too hard before it is broken in.

After Bench Break in:  You may now fly the engine.  We use the Carbon APC 7.4 x 7.5.  This prop is currently available from Jett,, and APC (Landing Products)   The QM-1 (not the 1.1) is a better prop for the first few runs.  Otherwise scrape or sand the back trailing edge to get your rpm higher for the first few flights.  You need a prop that peaks above 23500 and preferably 24000 with a hot engine and muffler.  This will give you the best runs in the air.  Any prop that turns at least 23000 can be flown (after break in), but we have not achieved the best speeds with the larger props.  Test a few props and mark the rpms so you will have a reference at the field.  If you are using zero nitro, adjust the prop size accordingly.
Note!!  This engine is not a Nelson.  Do not assume that it will behave like a Nelson.  The peak rpm will be less on the ground, but usually higher in the air. 

First flight:  Use a prop that peaks at 24000 or above on a hot engine.  Start the engine rich, about 1 turn from peak.  Lean the engine in slowly until you find the peak at 24000.  As soon as it nears peak open up the needle and let the engine drop down to 23000. 
For the first flight, open the needle ½ turn from this spot.
  This will give you a rich run so you can trim your airplane and give the engine a bit more break in.  If the engine hits the pipe hard, kill it immediately and go to a richer setting for the next flight.  Before it runs out of fuel, kill the engine and land.  Do not run all the fuel from your tank---ever!!!  On the next flight lean in the needle ¼ turn from the previous setting and fly again.  This time it may hit the pipe for a few laps, but do not run it hard for the entire flight.  After two flights lean in the needle one or two hundred rpm at a time until you achieve a full run at high speed without blowing plugs.             
** 25.4mm /inch      e.g.,   .150” =  .150 x 25.4 = 3.81mm

Remote Needle Systems for QM-II

Note that your QM-II engine does not come with a high speed needle.  Jetts are no longer available with needle valves in the front.  All must be run with one of several types of available systems.  Following is a discussion of your options:

1.  Aluminum Beam Mount with integral remote.  A standard Jett Needle Valve Assembly is built into a Jett Quarter Midget Motor Mount.  This mount has the same spacing and is interchangeable with standard Jett mounts.  This is the best system.  Click here to go to the mount with remote
2.  Exhaust Mounted Remote Needle.   An aluminum bracket, carrying a Jett Needle Valve Assembly, machined to mount to the engine with the rear muffler screw.  This is a popular and convenient system. 
3.  Universal Remote.  This bracketed Needle Valve Assembly is designed to mount to any flat surface, such as the side of any airplane or firewall.  With the Sport-Jett Backplate, it can also be mounted to the rear of the engine, but space is tight.
4.  Firewall Remote.  Same as above, but with the exit fuel nipple 90 degrees to the mounting surface.  This is for applications where you have additional room on the firewall.
5.  Backplate Remote.   Backplate machined to accommodate a Jett Needle Valve Assembly, configured at a convenient angle for the needle valve.  $40.00
6.  Quickie Universal Remote.  Same as the universal remote, but made in a mirror image for mounting (usually) on the side of a Q-500.
Click here to go to the remote needle valves.