RE: Help with two Q-500 motors
Hi Seth......
Those appear to be SEMPRA SS-40 engines. Utilized for SEMPRA Q-500 racing down in GA, FL, TN, AL, Carolinas
The carb on these is unique - the SS carb - has no low end mixture, but the engine is capable of idle and throttle transition.
That is the SS-40 muffler shown there. It is the correct muffler for the engines.
The backplate is not a Jett backplate mount. Dub did make an X-mount like that LONG ago, but they were rare. It may be a Nelson backplate. Most of the engines backplates sold by Jett were either round or the firewall-square-confirming type. The SS-40 engine was usually provided with a round backplate.
Replacing the throttle arm - place a hard wood dowel down into the carb to hold the barrel in place. Then use a wrench to remove the throttle arm nut.
As for performance, yeah that is quite a bit off.
The SS-40 engine - or any version engine using the SS muffler (now refered to as the LX muffler) - should be set up to run above 17,500 ground peak rpm.
An SS-40 is designed to turn an APC 8.75x8W up in that range. Anything down to around 16,800-17,000 is still ok and it will still run great.
The engine was also frequently flown with a 8.5x7 or 8.5x7.25 D1 series APC prop. It runs great with these, and usually hits a ground peak rpm up around 20,000 rpm
You have good fuel, the right plug, and a reasonable prop.
You are noting 2000 rpm down from where it should run.
So something is definately not right.
The only thing I can suggest is to start with ensure that the sleeve is installed in the crankcase properly, make sure the bearings are in good shape, and make sure the fuel system is delivering the fuel well. Take the plug out and turn the engine over. You should feel a distinct pinch or compression feel even with the plug removed.
After that inspection.... the easiest thing to start with is......
Install a smaller prop on it... a regular APC 9x6 would work. Run it... it should easily get up over 17,000. The engine has to be able to turn up where the muffler wants it to turn, so this is the first step. If it runs well here, run it a few times, find peak, back off 500-600 rpm .... and let it scream. Then try the 8N prop again just as a test. If it struggles..... the engine might just be worn...... in which case you can run a lighter prop (8.75x7 is good) and take what you can get performance wise.......
Or if it is performing poorly
... the piston/liner fit may be shot - only solution would be to send the engines in for Dub to look at.
Bob