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Basic Tuning

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GrumpyGnome24/07/2008 19:02:00
512 forum posts
145 photos

I need some very basic advice ................

I have a JEN '57 which, whilst very powerful, does not pick up from idle at all well.  I'd like to fix this but I'm not sure where to start ................ and I'm too impatient to wait for a club 'expert'

I think there are three 'controls':

1) Main needle - I have set this so the engine gives just shy of max revs at full throttle.  This seems OK

2) Allen bolt in top of carb - I have no idea what this does .............

3) A screw in the carb barrel (horizontally 'along' the needle, by the throttle arm) - ditto.

As I say, it runs fine at ful throttle (pulling my SIG Ultimate Fun Fly vertically quite easily), and idles away merrily - albeit a bit fast.  On opening the throttle, it splutters, then burbles a few times before gradually getting up to full revs - takes between 1 and 3 seconds and produces a lot of smoke whilst doing this. 

It's second hand so between me and the previous owner(s), I reckon we've meddled with the latter two things .................. I could contact Just Engines but I tiught I'd try here first.



simon barr24/07/2008 20:01:00
1035 forum posts
16 photos

Hello GG,

If the engine splutters and burbles when you open the throttle, it is too rich at the bottom end. That screw in the carb barrel is what you need to adjust. You need to lean the mixture slightly so turn this screw in about 1/4 turn (clockwise) and try again. You may need to do this a few times until the engine picks up smoothly. If the engine fails to pick up at all,and just dies, then the screw has gone in too far and needs to be wound out a touch.

All the best,


GrumpyGnome25/07/2008 10:12:00
512 forum posts
145 photos

Cheers Simon,

I'll give it a whirl.  Any idea what the Allen bolt on top of the carb is for ?


simon barr25/07/2008 14:51:00
1035 forum posts
16 photos

It is probably just a guide screw for the barrel to rotate correctly, have to see it to be sure.

By the way, as you follow my previous instructions, as the engine starts to sound more healthy, start to make smaller adjustments to the idle needle as when you are close to the optimum setting you will only need to screw it in a smidgen at a time or else you will go too far too quickly....if that makes sense.


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