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Mills S75 questions


CorradoMatt
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Evening all,

 

I recently bought an old Mills .75 (photo attached) that I'm planning on building a Veron Cardinal for. From what I can find on the web (https://modelenginenews.org/cardfile/m75.html), the engine appears to be a Mills S75 with a throttle cut fitted. I have test run the engine but the lever above the fuel tank appears to have no effect on the engine speed what so ever! The lever is joined to a pretty loose fitting collar that rotates above the tank and has 2 holes at 180 degrees to each other. This rotates on a tube which also has 2 holes at 180 degrees. The lever is missing the small tension spring which should hold it in the cut position (according to the website mentioned above). Does anyone have any more information on this type of Mills? I'd be interested to understand how it is supposed to work.

 

Cheers, Matt

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I recon what the cut out arm does is line up the hole's in the arm and venturi, this then should allow air in and should result in a lean cut. it is in no way a throttle. Could be vibration has worn the arm to such a poor effect that it lets air in most of the time and operation compensates by having needle screw further open

  Pics of my standard P75 Cardinal that I built for the engine some 3 years ago, a non modeling mate found it in the metal skip at the local dump. [recycling center] He saw it on top of the junk just as he was about to add to it and knowing my interest climbed in and rescued the Mills.

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The fuel cut off is just that for timed  free flight engine runs. When the holes in the lever line up with the holes in the fuel pipe the suction is lost and engine cuts. They used a clockwork timer back in the day to stop flyaways. Very simple but with two channel light weight radio its a bit obsolete. 

Looks like a nice little Mills .

The S may denote the cut out but the engine was designated P-75

All as JD8 says he beat me to it 🙂

Edited by Engine Doctor
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18 minutes ago, Jonathan said:

Hi JD8 may I enquire what size prop your using please. Jonathan 

On my Mills powered Mamselle I used a 7x4 wooden prop. Wood or nylon are preffered as the cranks have been knowen to snap with composite props.

Edited by Engine Doctor
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Thanks for the replies JD8 and Engine Doctor. This would make sense but what confuses me is that when the spring pulls the lever to the cut position, the holes are not aligned. The holes are inline with the venturi.

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Ok, thanks. So presumably i either need to make a new cut lever which is a better fit or remove it completely and blank the holes by sliding on a piece of tube. Hopefully this will improve the engine performance as well.

 

I'm also using a 7x4 nylon Kavan prop.

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 It is just part of the charm with my Cardinal to fly until fuel gone and glide back to base. Some times I let it go with the compression backed off some so motor gives less power and model pootles around at low level engine giving the classic diesel brrp brrp noise. 

 

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My Mills had a cut out and yes with time it came a bit loose. I never used it so left it in place with the spring. Any air leakage was compensated for by needle valve adjustment.

The Mills has an wide torque range so can handle bigger than might be expected props. The modest drop in engine revs can be countered by the prop generating more thrust.

For a scale effect my Mills 75 happily turned a 7x6 four blade wood prop.

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Just an update on this. I stripped the engine down as i noticed that the liner was slightly out of alignment with the exhaust ports. The sides of the conrod were slightly worn down but i don't think this is a problem. When i stripped the carb i found that the 2 holes in the fuel nipple were not aligned with the holes in the housing (out by 90 degrees). I used a thin washer to pack it out so that the holes were perfectly aligned. Test run this morning and the throttle cut is now working perfectly. It won't be used but it's nice to have everything working properly.

 

The engine drains the tank in about 1 minute - is this about right? I wanted to check rpm but my tacho battery is dead🙄.

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  Once in the Cardinal and in the air you may well find that as the engine unwinds as there is less load on the prop when it is moving it will run for longer than a ground run.

Bane of the free flight modeler as model disappears in the distance engine still going.

 I have an extended tank on mine, about double the standard size pot that fits in the usual way and motor runs for about 5 mins

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My own (Outerzone plan) Cardinal features a Mills .75, but fitted with an RC carb from an Indian Mills 1.3, which works quite effectively.

 

Obviously I had to fit an external fuel tank - in my case a free flight item.  Filled to the brim, I get 4-5 minutes of relaxed pottering, which is exactly what was required!

 

Tim

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