heliothiszea

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Bob - Wonder if you can help me . . . I picked up two Q-500 motors from RCU market place. The motors came with only one muffler. I've run one of the motors a couple of times. They both have good compression, I'm running a K&B L-1 plug with Power Master %15 fuel. I used an APC 8.75 x 8.0 N prop. It just seemed the motor would not turn up. I was getting about 15,500 on the tach.

I was expecting more rpm's from the motor. I'm wondering if the muffler/pipe is the correct size for the engine. From the center of the glow plug to the end of the red part of the muffler is 6 inches. Does this sound about right??

I don't know if you can tell anything from the pictures, but the "star" back plate seems odd to me also. Is this a common back plate? Are these even q-500 motors?

Lastly one of the engines has a broken throttle arm. Is there a trick to replacing these? I tried turning the nut holding the broken arm, but it seemed real tight and I did not want to break anything.

Thanks for any help - Seth

bob27s

My Feedback: (19)

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

heliothiszea

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Bob - Thanks a million. I've wanted a jett engine for a long time. I saw these engines on RCU for what I think is a steal for both of them ($225 with shipping). The seller did state they were not competitive motors for 428 racing. From what you've said the engines may be worn. They really do not have a "pinch" with the plug removed. They do have decent compression with the plug installed. I will tinker around with the engine a bit more, see how a 9x6, 9x7, 8.5 x7 works.

The engines come from FL, so the SEMPRA carb makes sense . . . and I really like that it is user friendly. I'm going to send one engine in for Dub to look at and keep one engine to get "acquainted with". I think this will give me a better chance of not frying the motor when it come back from Dub.

Again, thanks for the help . . . and I'm still grinning from ear to ear with my jett motors, even if they are worn. - Seth

PS - I'll let you what happens with the smaller props.

bob27s

My Feedback: (19)

Very good......

Sometimes the "good deal" ends up expensive for folks. Hopefully this works out.

Bob