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Keith Miles 215/07/2020 08:40:11
421 forum posts
6 photos
Posted by Denis Watkins on 15/07/2020 07:49:28:

Very accurate rendition Keith, to retune a motor with unknown needle settings.

Thanks, Denis!

I, perhaps, should, or could, have added that to get final adjustment of the idle screw requires checking for smooth pick up from idle to full throttle, so to cover that, just in case......

After the main needle has been set for full throttle, reduce the throttle to idle for about 15-30 seconds, then open the throttle.

If the engine stutters before increasing speed, idle is too rich. If engine slows before increasing speed, idle is too lean. Turn the idle screw in slightly to lean the idle mixture and out slightly to richen it, then re-test.

Final tick over speed should then be set with the throttle trim on the transmitter and, ideally, with zero trim and minimum throttle stick cutting off the fuel and stopping the engine.

Again, a transmitter with a “throttle cut” as well as a “travel adjust” function also makes life very much simpler and avoids  “faffing” about with control linkages to achieve just the right amount of throw at the carb. I remember it well!

Edited By Keith Miles 2 on 15/07/2020 08:49:39

Rogeo198315/07/2020 12:41:59
27 forum posts
18 photos

hi Keith,

I cant thank you enough for the post, a great and very detailed description.

I have new fuel and a new glow plug. So I will tune the carb first then try to start it.

My plan is to get it set up with a long pushrod by hand for the throttle link to the carb, and once i find the settings, I will connect the servo with the throttle at idle but the trim at full. then I can back off the idle screw and use the trim to cut the engine. It worked for me in the past. Its all coming back to me.

I still need to replace the hinges with better hinges with metal pins, and i think i will replace the ailerons, they are wobbling about a lot. a small bit of covering at the aft trailing edge over the fuselage, and fit the new bands.

Once that is done and the engine is working, Its ready for a test flight. Is it still c of g approx 1/3 back from the leading edge?

Im hoping to get it test flown by a local instructor by the end of the week. Amazingly the model is still registered with the same tail number, so once my insurance cert arrives from the club, Im good to go.

I just need to get the cable for my controller to connect to the computer, so i can get my cashes out of the way in a virtual role!!

thanks again for all of the help.


Nigel R15/07/2020 13:06:18
3987 forum posts
721 photos

I'd probably start at 25% of chord for first flights, if I knew nothing else about the model.

Caveman15/07/2020 13:34:17
299 forum posts
242 photos

Rogeo, did you see my message on 13th July on your simulator thread ref the cable for your computer?


Keith Miles 215/07/2020 19:29:39
421 forum posts
6 photos
Posted by Rogeo1983 on 15/07/2020 12:41:59:

hi Keith,


I cant thank you enough for the post, a great and very detailed description.

I have new fuel and a new glow plug. So I will tune the carb first then try to start it.

My plan is to get it set up with a long pushrod by hand for the throttle link to the carb, and once i find the settings, I will connect the servo with the throttle at idle but the trim at full. then I can back off the idle screw and use the trim to cut the engine. It worked for me in the past. Its all coming back to me.

“Back off the idle screw”?

Are you, perhaps, confusing a “throttle stop” screw with an idle screw? 

DO NOT treat the idle screw/ low speed screw as a primary means of setting the final idle speed!

It is there, primarily, for setting the idle (low speed) fuel/air MIXTURE, initially with the throttle barrel at a rough idle position as described (i.e. slightly open) and is then used to fine tune the throttle response from low to high, again, as described.

After the idle MIXTURE screw is set and the throttle operates smoothly from low to high, LEAVE IT ALONE!

Final tick over speed is largely determined by setting the correct amount of throttle barrel opening i.e. by adjusting the cable or pushrod linkage between servo and throttle arm and by using the trim on the transmitter, as required.

(Unless, without rubbing it in, you have a modern radio!)


On the C.G. issue, on that type of aircraft, I would start with it just ahead of the thickest part of the wing which will probably equate to about 25 - 30% back from the leading edge.

Edited By Keith Miles 2 on 15/07/2020 20:15:47

Rogeo198315/07/2020 22:12:42
27 forum posts
18 photos

Hi Caveman,

yes i did, Ive tried to contact that company you mentioned, but i havnt received a reply yet, ill try again tm. I have two other suppliers at the moment checking if it will fit. Thanks for your help.

Hi Keith, perhaps i am. Now that you mentioned it, i was wondering how that screw changed the Idle mix.

So it did actually start, but my glow battery (also 20 years old) failed. New one ordered.

So i have the pictures here, will you tell me which one you are talking about? Now i am thinking that the idle mix is the one at the side, eg straight into the barrel of the carb, the idle stop screw is on top of the housing, and the high speed needle is on the side above.

I will strip it out again tm and start again. Thank you for your paitence, an help!!

sc 40 carb setting adjustments.jpgDo I have this right?

Thanking you in advance

Keith Miles 216/07/2020 01:49:54
421 forum posts
6 photos


Yes, I’m talking about the two mixture screws, idle (low speed) and main (high speed), NOT the throttle/idle stop screw.

All that the throttle/idle stop screw does is stop the throttle barrel falling out and it also limits the amount by which the throttle barrel can rotate towards the closed position, hence the “stop”. In other words it is purely a mechanical function and nothing to do with mixture setting.

So all you really need to do is screw it in enough to hold the barrel in place and to allow the barrel to rotate to a closed position without binding the servo. The more that you screw it in, the less the barrel will be able to rotate in the closing direction. Try it and you’ll see what I mean.

Bear in mind that the idea of the 1mm drill and “ hiss” technique is to initially achieve a rough normal idle position for the throttle barrel and a corresponding rough simulation of required fuel flow at that position.

So, what you can possibly do is screw in the throttle/idle stop screw to help you to set the 1mm gap whilst you adjust the idle screw during the “hiss” test. Once you have an initial setting for the idle mixture screw, you can then unscrew the throttle/idle stop screw sufficiently to allow the barrel to rotate to the closed position or even beyond closed, but not so much that the barrel is no longer held in place! The spring on the screw is to hold its position and prevent the screw from working loose due to vibration!

Obviously, if you leave the throttle/idle stop screw set for idle when you have the engine running, you won’t be able to stop it by closing the throttle manually or on the transmitter and you might also put strain on the servo if the link has been set up for full closure! Other methods of stopping an engine are best avoided!

This is also why modern radios are so useful for setting up throttles because they allow you to not only fine “trim” the initial servo output arm position but also to fully adjust the amount of servo movement in each direction either side of centre. It allows you to much more easily get exactly the amount of movement required to achieve full and closed throttle or full, idle and a button or switch for throttle cut-off. No need to faff about experimenting with different holes on the servo and throttle arms, adjusting clevises etc. etc.

So, to recap, the “stop” screw has nothing to do with adjusting the fuel/air mixture. It merely positions the throttle barrel.

The idle screw sets idle/low speed fuel flow.

The main screw sets high speed fuel flow.

Air flow is determined by barrel position and fuel/air mix is achieved by a combination of the rotational (for air) and sideways (for fuel) movement of the throttle barrel as the throttle is operated.

Don’t be afraid to ask if you need any further clarification or help!


Edited By Keith Miles 2 on 16/07/2020 02:15:40

Nigel R16/07/2020 09:05:54
3987 forum posts
721 photos


The OS engines always come with an excellent guide to setting up a twin needle carb from scratch.


OS engine manual

Page 17 onward.

There is one main difference from your engine, the OS motor in that manual has the full speed needle mounted at the back of the engine, but, it operates the same way as your main needle, ie clockwise to make lean.

Page 20 / 21 has the main needle adjustment routine.

Page 22 has an excellent idle adjustment chart. Note OS call the idle mixture screw the "mixture control valve".

Keith's posts above are also excellent info.

Keith Miles 216/07/2020 11:13:20
421 forum posts
6 photos


Excellent tip from Nigel but bear in mind that the advice given for re-setting the initial position for the idle/low speed mixture screw applies specifically to the OS engines whereas the initial set up method I described will work with ANY “twin needle” type of carburettor.

It is worth repeating that once you have the idle mixture screw set to give you a smooth throttle response from idle to full throttle without the engine “stuttering” (too rich) or slowing (too lean) as you open the throttle, leave it alone! Any subsequent adjustment should only need to be made with the main needle and with the engine at full throttle and then, preferably, checked by raising the nose of the aircraft to ensure that it is not set too lean. Obviously, it is safest, especially with a large model, if you have another pair of hands to assist here, one pair for the model and one for the transmitter!

Don’t forget, of course, all of the usual safety considerations and precautions to avoid injury!

If you go to the top of this page and click on “Features/Engines/Go Glow”, you will also find an excellent article by Alex Whitaker. Well worth an additional read if your memory needs refreshing!

Edited By Keith Miles 2 on 16/07/2020 11:37:06

Rogeo198317/07/2020 09:22:04
27 forum posts
18 photos

Hi Nigel,

yes i have had to replace the carb due to mine not working. The original rear mounted needle is bypassed, and the fuel line goes straight to the front carb mounted high speed mixture screw. I didnt have shorted srews to replace the rear mounted bracket. Thanks for the link to the manual, mine went walkabouts years ago.

Hi Keith,

Thanks again for your help. I did run the engine yesterday for a few mins, it ran really well, i had forgotten how much of a mess that these little engines make!!

I am going to strip the cowl again today and tune the carb as per your instructions, and start again. I am still waiting for a new glow battery, so ill crack on with that.

Ill report back once i have some progress.

Cheers, and thanks again

Keith Miles 218/07/2020 01:58:06
421 forum posts
6 photos


Strip the cowl?

Looks to me, from the photo, that you can get to all the adjustment screws without the need to do that unless you have another reason for doing so.

Oh, and use some clean fuel tube for the “blow and listen” technique. You don’t want fuel residue in your mouth or your ear!


If you have any problems, do ask!

Caveman11/08/2020 13:17:40
299 forum posts
242 photos

Rogeo, I've just sent you a PM


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