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Old Oct 03, 2009, 10:57 PM
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Joined Oct 2009
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Help!
JBA .61 does not throttle nicely

Hi there.

Im fiarly new to nitro engines (i have a nitro rc car and thats it) and even newwer to nitro flight.

the other day i went i bought a tidy second hand trainer and a JBA .61 to go with it.

i broke the engine in and everything and it seems to be running fine except that when the throttle is opened quickly then engine dies instantly which is a problem in the air... the engine can also have a temperamental idle and will will sometimes drop off when idling and has a very bad pick up if it has been idling for more than 10 seconds.

Im using a os A3/#6 (hot) glow plug and 15% nitro fuel.

Any help would be greatly appreciated
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Old Oct 03, 2009, 11:03 PM
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Does it lean out and die or load up (rich) and sputter to a stop?

There is a little idle adjustment screw on the side opposite the needle. (in the centre of the throttle arm)
If it is leaning out then open it a tiny bit. If it is going rich then close it a tiny bit. About 1/8 of a turn at a time.
Test and repeat.

Also make sure you are not just trying for too low an idle.


Pat MacKenzie
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Old Oct 03, 2009, 11:13 PM
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Im guessing it loads up because i had the leaning out problem before.

but in both cases it doesnt even get to full throttle before dying, it just sputters and stops as soon as the throttle is opened quickly
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Old Oct 03, 2009, 11:16 PM
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If it "sputters" then it sounds like it is rich, so try turning in the low speed needle.


Pat MacKenzie
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Old Oct 03, 2009, 11:57 PM
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Also try to make very little movement during adjustment every time.
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Old Oct 03, 2009, 11:58 PM
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I think you should use an OS #8 in this engine, here is my take on this
Quote:
throttle is opened quickly then engine dies instantly
this could be due to the idle being too rich, you are basically putting out the glow plug by flooding it
Quote:
the engine can also have a temperamental idle
another sign of being too rich
Quote:
has a very bad pick up if it has been idling for more than 10 seconds.
rich again however this is also a sign your tank is too high in relation to your carb REMEMBER CENTRE OF TANK TO CENTRE OF CARB IMO ok how to fix it, read the stick on engine tuning then try the following
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Old Oct 04, 2009, 12:07 AM
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1 increase the idle rpm until it will stay idling, pinch the tube going to the carb and watch what the engine does, if it improves and speeds up then yes its rich, if it just dies then it was lean, now this just proves what is going on next do this.... open the throttle fully and listen to your engine, if it sounds like a two stroke then slowly increase the main needle until it sounds like a 4 stroke, then slowly turn in the main until it just starts breaking into a clean two stroke sound then back out a fraction from here 2 drop back to an idle and sit for a few seconds
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Old Oct 04, 2009, 12:24 AM
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Now as the guys have said above turn in your idle just a little and then try opening the throttle, if it dies move the idle in just a little more and try again you should end up with it having a clean transition from idle to full if the idle is too lean you will see the motor hesitate then speed up to full throttle
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Old Oct 04, 2009, 12:27 AM
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NOW LAST THING if you have moved all your settings way off the standard spec see if anyone has a manual
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Old Oct 04, 2009, 12:35 AM
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SK seem to be the same as JBA look at page 3 rh side at the top idle starting is 1.5 turns out so it seems
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Old Oct 04, 2009, 02:59 AM
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Hey thanks for all the useful information!

i did as suggested with the low speed needle and pinched the fuel line to see what would happen. as you'd expect the engine revved higher then eventually stopped.

i turned the needle in and repeated the process until it would just die off nicely and by this time there was a nice smooth transition from idle to full throttle.

i had to turn the low speed needle in quite alot to get this (2-3 turns) which is a bit weird considering it said on the manual that the low speed needle is factory set.

anyway it all works fine now and thanks again for the help!
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Old Oct 04, 2009, 02:59 AM
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Tokoroa
Joined Mar 2004
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Welcome to the world of cheap Chinese engines ;-)
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Old Oct 04, 2009, 03:36 AM
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haha yea i guess you get what you pay for.

but i have to admit seeing as i payed only $85 for it, it runs really well
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Old Oct 04, 2009, 08:38 AM
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Quote:
Originally Posted by DeeLuxx
Im guessing it loads up because i had the leaning out problem before.

but in both cases it doesnt even get to full throttle before dying, it just sputters and stops as soon as the throttle is opened quickly

Opening the throttle quickly is part of the problem.

Go to a real airport, watch the planes taxiing around. Notice that no one is quickly jamming the throttle to full at any time. Modeling tries to emulate full scale, or used to, practices and has not pursued the use of accelerator pumps in their carbs. You would need an accelerator pump in order to get away with jamming the throttle instantaneously to full without the engine bogging or quitting. Cars have them - but model airplane engines do not. Instead, smooth and easy throttle advance is the norm in modeling. This type of operator behavior has been incorporated into model engines for so long that no one remembers it, except old fuddy-duddies like me.

An engine that will not instantaneously respond to throttle is not considered defective by many manufacturers since that isn't part of the model airplane culture any way.

Now, there are engines that do respond well to quick throttle movements. YS is one that comes to mind, so it is possible to attain. But first you have to realize your engine's ranking in the pecking order of model engines. It is a cheap engine. Cheap engines sometimes have good throttling ability, but most of the time they are just passable.

Then figure in that the engine is brand new (I'm assuming). Engines need break-in time before low speed adjustments can even be considered worthwhile doing. How much running time do you have on the engine in total?

I can tell you right now that 15% nitro with a hot glow plug in a Chinese engine (high compression) is just begging for trouble. You are advancing the glow engine's ignition timing a terrific amount by using either one of these ingredients. Using both at once is really asking for it.

The last factor in the equation is prop size. What are you running for a prop? Very important. Especially with the fuel and glow plug that you are using.


Ed Cregger
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Old Oct 04, 2009, 06:35 PM
The reviewer
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Tokoroa
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Quote:
Originally Posted by Ed Cregger
Opening the throttle quickly is part of the problem.

Go to a real airport, watch the planes taxiing around. Notice that no one is quickly jamming the throttle to full at any time. Modeling tries to emulate full scale, or used to, practices and has not pursued the use of accelerator pumps in their carbs. You would need an accelerator pump in order to get away with jamming the throttle instantaneously to full without the engine bogging or quitting.
BZZZT... wrong answer Ed :-)

Sorry but anyone who flies 3D will tell you that very rapid throttle movements are a part of normal flight.

What's more, a *good* engine can handle these rapid transitions without any problems at all. No carby pump is required -- a good engine will respond instantly to rapid opening of the throttle without hesitation.

The fact that a few manufacturers can do this (OS, TT, Webra, etc) means that it *can* be done -- it's just that the others either can't be bothered investing the time/money to develop a product capable of it or don't have the expertise to do so.

Quote:
Cars have them - but model airplane engines do not. Instead, smooth and easy throttle advance is the norm in modeling.
Hmmm... only if you spend your day flying circles ;-)

Quote:
An engine that will not instantaneously respond to throttle is not considered defective by many manufacturers since that isn't part of the model airplane culture any way.
Sorry but 3D flying is becoming *very* popular these days and is invading the mainstream model-flying culture rather aggressively, just look at all the 3D models you can buy now.

Quote:
Cheap engines sometimes have good throttling ability, but most of the time they are just passable.
Correct!

Quote:
I can tell you right now that 15% nitro with a hot glow plug in a Chinese engine (high compression) is just begging for trouble. You are advancing the glow engine's ignition timing a terrific amount by using either one of these ingredients. Using both at once is really asking for it.
Correct again!

See, we can agree on *some* things
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