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Atom Minor Mk III

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Clive B04/02/2018 16:38:00
29 forum posts
20 photos

Hi everyone,

After initially resisting all attempts to make it start, my recently completed Atom Minor Mk III finally burst into life; most satisfying! This is my first ic engine. I think the main issue was (and still partially is) the fuel feed which works on a simple suction principle – I have the initial level of the fuel at the same height as the jet in the carb. Anyone know if there is a better way?

The ignition timing could probably be optimised, I am using a Hall sensor and CDi module.

Also, can anyone advise on the best mixture of fuel to oil for this two stroke? I am currently using Aspen two stroke which has a 50:1 ratio; Westbury recommended a ratio with a lot more oil in it – but I guess that was in the days before synthetic oils.

atom minor mk 3 in action.jpg



Old School04/02/2018 17:38:45
323 forum posts
26 photos

Clive, It certainly needs more oil than you are using I would follow whst E Westbury suggests. Castor oil is a good lubricant and is still the required lubricant along with methanol for some model classes where speed is the goal.

Dave Martin05/02/2018 12:04:58
101 forum posts
11 photos
Posted by Old School on 04/02/2018 17:38:45:

.....Castor oil is a good lubricant.....

And you can't beat the smell of Castrol R !

Frederic Frenere21/02/2018 23:39:34
12 forum posts

A fine piece of work. Did you use the castings from Hemingway Kits? How do you find them for working?
Ian S C22/02/2018 10:41:00
7468 forum posts
230 photos

The Castor oil doesn't combine with petrol, and has a great affinity to stick to metal in the engine, where as mineral oil gets washed off.

Ian S C

Andrew Tinsley22/02/2018 13:05:02
1070 forum posts

I run lots of old petrol IC engines. I never use petrol as a fuel, always methanol. As far as model aircraft engines are concerned petrol went out as a fuel during WW2. The old petrol mineral oil fuel mixture runs considerably hotter than methanol castor fuel. The methanol mix is much kinder to the old style metallurgy used in these engines. Castor oil is always the last resort protection if an engine goes severely lean. It will provide some lubrication long after mineral oil is destroyed.


P.S. I always manage to get more rpm on methanol fuel for a given prop.

Richard S222/02/2018 16:24:51
177 forum posts
107 photos

Methanol is the favoured option of fuel for reasons mentioned. I Also ran many old model Aero engines way back in the past.

A good ratio for bedding in a new 2 Stroke engine is/was 75% Methanol and 25 Castor for the first 45 mins.

80/20 thereafter, and to improve power and ease of starting, you can go to 75/20 and 5% Nitro.

Good job there on the engine, have one on my list of 'To do' projects list.

duncan webster22/02/2018 19:39:06
2543 forum posts
49 photos

Having googled castor oil it sounds like just the thing for squirting into steam engine cylinders, stable at over 700F. Might have predictable effects on the driver, but otherwise anyone care to comment?

Edited By duncan webster on 22/02/2018 19:39:33

Andrew Tinsley22/02/2018 19:47:09
1070 forum posts

Castor will break down under high temperature. However the products are still able to lubricate This breakdown continues with higher temperatures, but the residuals are still able to lubricate. This is why castor has always been used in model IC engines when there was a possibility that the mixture might lean out.

Once you have had a runaway and all is cooled back down, then you have some very viscous remnants of the castor oil left. Best to do a good strip down and clean at this stage. so maybe not ideal for lubricating superheated steam!?


Mike Poole22/02/2018 20:29:25
2545 forum posts
60 photos

Castor oil is not really an everyday lubricant, it's outstanding performance makes it very suited to racing engines that are going to be stripped and cleaned regularly but it is not a fit and forget type product.


Ian S C23/02/2018 11:58:42
7468 forum posts
230 photos

Castor oil was the ideal lubricant for the rotary engines of the aircraft of WW1, the down side was that the oiling system was total loss, so surplus oil came out the exhaust covering the pilots gogles, and ingested by the pilot with the usual results.

Ian S C

Clive B23/02/2018 15:23:26
29 forum posts
20 photos

Thank you Fred and Richard for your kind comments. The castings were from Woking Precision just before they were taken over by Hemingway. These castings were fine, although I made the cylinder head from round bar stock and didn’t use the casting for the contact breaker assembly. I believe that there was an article in ME on the design of a higher compression head for this engine so that it could run on glow fuel (methanol), might give this a try! There seem to be pros and cons for castor oil, perhaps a mix of synthetic/castor would be pragmatic if not after the highest levels of performance? Oh, and I’ll try to avoid standing in front of the exhaust! Clive

Andrew Tinsley23/02/2018 15:58:41
1070 forum posts

I would not bother with mixing synthetic and castor oil for such an old design of engine. Just use castor and methanol and don't worry about compression ratios as methanol fuel will run at very low CR without any trouble.

Modern ABC and AAC engines are said to run best on synthetic oil and I don't argue with that. If being used for RC, the engine can always be shut down if it goes lean. I still fly control line and you don't have the luxury of shutting down if this happens. So I always add 5% castor to the fuel just in case!


Jeff Jorgensen06/03/2018 11:23:36
5 forum posts

Methanol........................... 4 parts

Castor oil (castrol M)......... 1 part

Clive B08/03/2018 06:59:33
29 forum posts
20 photos

Thanks Andrew and Jeff. Please forgive me a dumb question (I am new to building ic engines), but can I run the methanol mix using the existing spark ignition or will it be better off with a glowplug? Clive

ChrisH08/03/2018 17:01:21
857 forum posts
29 photos

Hi Clive, can't answer your question re spark ignition and glow plugs, but was interested to hear how, after seemingly many attempts to start your engine without success, you managed to get it to run; what did you do different?

I ask as I am in the same position, with my first engine which is a glow plug sleeve valve engine refusing to start. I was going to try some Easy Start and give it another go when the weather allows, but the engine seems to have little compression, a pressure gauge in the glowplug hole shows about 15-20lb/ when turned over by electric drill which doesn't seem enough, even though it is made exactly to the drawings with all the correct clearances everywhere, but am looking for other hints, hence the query, what did you do different?


Trevor Crossman 108/03/2018 19:22:53
131 forum posts
17 photos

Hi Chris, definitely not enough compression for a glow, should be above 115psi. so the low compression is either due to leakage which you would probably hear as a hissing sound when turned over whether it is past the piston or through the junkhead seals, or excess head volume or valve timing. Can I ask the capacity and method of sleeve drive?


Trevor Crossman 108/03/2018 19:40:08
131 forum posts
17 photos

Nice piece of work Clive B​, well done! It's always really gratifying when a new build or repaired engine springs to life, even more so when it is one's first! You are more or less right with your fuel tank placement, though the old general rule for non pressurised tanks was that the outlet​ should be at the same level as the spraybar/needlevalve holes. More oil , and as others have said, castor, would be advisable for a this 'old style' engine which is perhaps a lapped c.i. cylinder and piston set up?


ChrisH08/03/2018 20:28:28
857 forum posts
29 photos

Trevor, from memory the capacity is about 0.3 The EN24 sleeve is a sliding fit inside a CI liner, with a aluminium piston a sliding fit in the liner. The piston has 2 CI rings, the junk head has 1. The timing has been checked and is OK, but I cannot hear any hissing as the piston goes over TDC compression. The sleeve is driven off the end of the crankshaft via a 2:1 gear reduction. The CR I calculated at being about 10 or 11:1, a bit difficult to do accurately by calculation as the junk head has a small inverted dome in way of the glowplug to ensure the end of the glowplug is standing proud (4 stroke engine), but I thought that OK for a glow engine. Bump clearances were taken and was to the drawing. Should add a friend 'Down Under' has also failed to start this engine, and his has been double checked against the drawing too. We've both run out of ideas to start it, other than a try with Easy Start.

Clive, sorry if I am hijacking your thread, was just answering Trevor's question - I can ask the moderators to move this to another thread if you wish, but info on starting tricky engines is worth having whatever it is I think!.


Clive B09/03/2018 09:38:02
29 forum posts
20 photos

Hi Chris,

Initially I noticed a slight leakage in the crankcase - some sealant cured that (I used hylomar blue - perhaps someone could suggest a better sealant?)

Fuel supply - as stated on my first post, the fuel feed needed some fettling to avoid flooding (thanks for comments Trevor).

Other than that, very patient adjustment of the carb needle and air shutter to find the "working envelope". I did make a small mod to the carb re the position of the needle seating in the venturi; not sure if this really helped but this mod will be specific to this design of carb. If you're interested I can send you a sketch. Incidently the needle in the original drawing is not tapered enough.

I tried Easy Start but this didn't make any difference.

I tried measuring the compression with a car type gauge but I think that the volume of air inside the gauge itself led to misleadingly low readings for an engine of this small size (6cc).

A cold garage probably didn't help matters.

The cylinder and piston are both cast iron.

Best of luck!


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