dxbman

hi,

i got a sport 40 engine from ebay, it was used but had good compression. it was running great but it was getting 14K rpm using 10X6 prop. it was blowing plugs after each flight , i raised the tank and everything got OK. i once got 18k using 9X6 but i didn't want to lose the engine.

now it dosn't reach the peak rpm the scream sound i re adjusted the fast idel lots of times it didnt work.
Note: i added teflon tape to the idle screw because it wasn't holding and it was getting lose. the plugs are melting.
i can see Black oil on my plane, the compression is OK.
There is lots fuel drops is coming out of the front bearing why?
i used model technic fuel 10 % nitro and 18% castrol oil.
I am going to take it out of the plane (diamond dust )and check it on a test stand, after i check each part of it.
if you want any pics of the engine parts i will send it to you.


ferthermore: i go the jet stream tuned muffler for the OS 46fx,
I tached it with the stick mufler 14.400 K
than i changed the mufler with the jett one got 200 rpm more. i was disapointed from the results. i want to know where i went wrong.

thanks

bob27s

My Feedback: (19)

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 www.jettengineering.com for our contact and service info.

I hope this has been helpful. Feel free to email with additional information and questions.

Bob Brassell

dxbman

thank you for your answer,

I am going to take it a part and check it . I will change the needle valve and the O ring .

how can i adjust the idel needle after replacing it.
Do you recomind sendding the engine to the servece center to be fully checked.

thank you for a gret service , which i think nobody can beet.

thanks

bob27s

My Feedback: (19)

Hi.....

Glad I was of some help, and thank you for the compliment.

For carb and needle setting, go here
http://www.jettengineering.com/tech/techcarb.html

Set the idle needle flush with the end of the throttle arm nut (for starters).

Piston/sleeve assembly and needle valve change out can be done on your end. They are not difficult if you have some experience with engine assembly. That is up to you, and will depend on whether there are other concerns with the engine. If you wish to send the engine in for service, we can inspect and repair the engine, and return it to like-new performance.

Service may be obtained from:
Jett Engineering Inc.
6110 Milwee Suite J
Houston, Tx. 77092
USA

Bob

dxbman

hi bob,


thank you,
i just finished dismantled the engine, the big bearing it definatly bad. so i'll change both bearings, i'll use the proceture inthe jett site.
and the idel O ring is hard, that will be changed to.
i have attached picture of the engine .. you can see the idel adjusment , and the carburature is defrant than the one in the jett site, what type of engine i have. ?
thanks for the great help agian

arif

bob27s

My Feedback: (19)

Hi....

Wow..... you have a classic on your hands there

That is a very old SJ-40. About 1994 vintage. That type of idle needle was only produced for about a year during the transition from the original (1993-ish) RAM-Jett carb to the more current version.

I would imagine that, based on the age of the engine, your performance issues may lie a bit beyond a bad bearing and an idle needle. You will want to check your existing piston and sleeve for fit, and to check for any scratches. Depending on how it has been used and serviced over the years, a scratched or worn piston would certainly provide a loss in performance.

Drop Dub an email at [email protected] and inquire if there may still be piston/sleeve parts available for the engine if they are required. The current parts are not interchangeable.
Bearings and idle needle should be available.

dxbman

hi,,

I just want to say that jett engines quality is perfect, as you said a have avery old angine and my piston and sleeve have lost compression. well jett engine prove it self in my field. i replaced all the orings on the carb and fitted it with 9X6 APC. now the engine screams . my daimond dust is so fast poeple are asking a bout the engine... 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 ..

I want to thank you for a great engine and soon i'll get two of them for my other projects...

i have one more inquary . i am thinking of using thies props on the engine
8X8
7.2X8
what is the recomended prop to go as fast as possible ? i read some where that i have to use carbon fiber 7.8X8 i think to get the most out of the engine ..

thanks

bob27s

My Feedback: (19)

Hi

I'm glad your engine is back up to full speed! Thank you for the kind words.

As for a prop for that engine, the same basic rpm rule applies with that engine. Try to prop the engine to run around 17,000 rpm to get the best speed. An 8x8 or 8x9 may work well.

Having said that, the SuperSport 40 we use for racing is essentailly the same engine you have. Best prop for Q-500 we have found is a APC 8.5 x 7.25. This will turn up in the 20k range. Although the sport engine and muffler combo was not specifically designed this way, the muffler seems to have a sweet spot right around there. The standard SJ40 muffler has a similar sweet spot.

As for the QM40 (7.8x8) carbon props, those are designed to run upwards of 24,000 rpm. Your SJ40 with current muffler will not likely give you the perforance you need to get the most from these props, but feel free to experiment. The engine can take it... no problem there.... it is more a function of muffler and engine timing.