bob27s
Posts: 4590
Joined: 4/9/2002
From: Cleveland,
OH, USA
Status: offline
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Hi,
Thank you for writing....
First, if you have any black coming from the engine, that is a bad sign. This occures when metal wears on metal. Compression on an ABC engine may 'seem' ok when turned over cold, or even warm. But when running, a worn-out fit will prevent it from achieving top performance. Black stuff somes from the engine running way too lean, or something else is damaged internally.
If the piston fit has been lost, recovery for this is a replacment piston/sleeve set.
Blowing plugs is caused by one thing: the engine is running too lean. This can be caused by an over-lean needle setting, or fuel system problems that introduce air or bubbles into the fuel.
High speed mixture settings should always be rich. Find peak RPM (Briefly, after the engine has started and run 10 seconds), then immediately back off 400-500 rpm. This is where you should consistantly run the engine. The engine will unload quite a bit in the air, and will require this additional amount of fuel.
Our experience shows that 90% of the 'blown plug' issues have to do with the fuel system. At these RPMs, it is critical to make sure the fuel tank is properly padded and isolated from airframe vibration, or you will most certainly foam the fuel....... creating bubbles and a lean engine run. Even if you do not see bubble on the ground, the engine will unload quite a bit in the air, and often creates foam at those higher rpms.
Turning the SJ-40 engine 18K (or higher) is perfectly ok. The same engine in racing configuration turns well over 20K rpm. Do not worry about breaking it. In fact, you should prop the engine to turn NO LESS than 15,000 RPM..... with a target of 16,000-17,000... on the ground. The Jett-Stream muffler is tuned to run in this power band. The SJ-40 likes lots RPM, more so than the 46 or 50. An APC 9x6, 9x7 work very well. Our baseline prop for this engine is an APC 10x6, which it should turn at about 16,000 rpm (on 15% powermaster). Prop selection is the key factor in good engine performance. If you are only getting 14K with the 10x6, the engine may be worn or damaged. Even with 10% fuel, you should expect no less than 15,000 rpm with a 10x6.
The idle speed screw has an o-ring on it. Idle adjustment is set at the factory, and should not need anything more than tiny adjustments depending on the prop size used. It only controls the engine mixture below 1/3 throttle, and will not affect the top end speed. If the idle screw or its o-ring has been damaged in some way, it may be causing an air leak, that would cause a lean run. I this case I would suggest a replacement part.
As for the OS 46 and jett-stream.....
Jett Stream mufflers work extreamly well on the OS 46.
If your engine is currently turning a 10x6 at 14,400, it will most certainly a ground rpm climb to no less than 15k RPM. You should expect upwards of 15,500 and higher. As noted above, this muffler performs best around 16K-17K rpm. You may wish to drop a prop pitch or diameter size for best over-all performance. One factor that is important here, is that any ABC type of engine has to be broken in at, or close, to the RPM you desire to operate it at. If it is an engine with quite a bit of time on it, it will be use to running in a certain rpm range, so the boost you see from the Jett Stream will be limited, but noticable. You indicated only a 200 rpm increase... Based on what you described, it is difficult to diagnose the situation, or suggest a change.
Some additional helpful material is available on our TECH DOCUMENTS web page
http://www.jettengineering.com/tech/tech.html
and our FAQ page also addresses several key issues.
If you believe the engine requires extensive service, see our web site at [url]www.jettengineering.com[/url] for our contact and service info.
I hope this has been helpful. Feel free to email with additional information and questions.
Bob Brassell
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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Engine Manufacturers Direct Support Forum >> Jett Engineering Support >> old jett 40 needs help 
old jett 40 needs help - 
old jett 40 needs help - 
when they see a very old engine like a junk yard piece they think i must have a magic engine fuel, i was louphing at the other diamond dust flyers