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Mills S75 starting problems

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Help! Re: Mills S75 starting problems

Post  Oldenginerod on Fri Jun 02, 2017 5:30 pm

Welcome Flight-Risk, from a fellow Aussie. Yes, I've suffered the old Mills rotating liner syndrome myself. I have the damaged con-rod to prove it.
Yes Fred, many of us Aussies grew up with diesels, generally British, although local Taipan engines were predominantly diesel and got many of us our start in powered modeling.
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Help! Re: Mills S75 starting problems

Post  Oldenginerod on Fri Jun 02, 2017 5:33 pm

By the way Risk, is that Snake Valley near Ballarat, or elsewhere?

Rod
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Help! Re: Mills S75 starting problems

Post  ffkiwi on Tue Jul 10, 2018 8:40 pm

OK-time for some input [and frankly I 'm staggered that anyone would have starting problems with a Mills 75-which has to be the easiest starting diesel ever produced! ..along with the MP Jet Classic 040] -but easy starting does not mean that the engine design is not without its irritating flaws-some of which have been touched on here.

First-we haven't had confirmation yet-unless I missed it that the engine is now actually running-andv as I own a heap of the things-from originals, Doonside, Indian Irvine and Chinese ones-I'll give you the benefit of 50+ years operating them:

first issue: is the liner in the right way? It is very easy to get it 180 degrees out-the exhaust ports are obvious-but the inlet and transfer ports are simply a pair of small diameter holes close together-like a figure '8' on it side [sometimes they are so close together they actually form a figure 8, other times they may be slightly out of line]-they are NOT at the same level-relative to the exhaust ports, the top and the base of the liner. The pair that are the highest (if you stand the liner upright and view it) are the transfers-and these go to the FRONT-the lower pair go to the rear and are the INTAKE ports. If you are out by 180 degrees-and the engine is put together in this state-the timing will be all to hell-and while the engine may fire-it will usually be most reluctant to keep running-and if it does it will sound most unhappy

second issue: is the piston in the right way? The 75 piston has a step machined in it-about 3/64" deep in both directions-this step acts as a deflector-exactly the way a baffle does in a crossflow glow engine-and since the transfer ports are at the front-that is where the step should be as well-just as with the liner, the piston can be assembled 180 degrees out-except in this situation the engine definitely won't run!

third issue: where is the contra piston set? I had this exact problem come up last week with an out of town friend who had a Mills 75 'binding at the top'-after explaining by email the various possibilities-it turned out to be relatively minor-the contra was too far down the bore. I took the time to check half a dozen of my own examples-and I can conform that in all of them-the contra sits flush with the top of the liner,...if yours doesn't then it is well off a suitable running position!

fourth issue is the fuel system unblocked? The Mills uses a surface jet-and has a hefty amount of suction (as do most sideports, BTW) -but it would pay to check that the jet hole is clear of any blockage and congealed oil. You can do this without removing the tank bowl, simply by taking the needle right out and probing the jet with some fusewire-and ensuring it will reach down into the tank and strike bottom. Whilst you're doing this-check the needle itself-that the end is not burred and the stem not bent. If its an original needle it will be a gramophone needle inserted into the threaded brass stem-it should have a tensioning spring which fits on the threaded shaft-but if it is missing a piece of fuel tubing can be used as ac substitute-but you need a bit of trial and error to get the length right so that it is slightly compressed when the needle valve is screwed home. Finally-for this aspect-the needle should not need to be more than two turns open for starting-and the engine may well only need about one turn open for running-most of mine run at 1-1/4 to1-1/2 turns open.

fifth issue Props-if you're trying to start it on a typical 1/2A prop-you're setting yourself up for failure...they get quite unhappy and snappy on a 6'' prop-and I wouldn't even attempt to try one on a 5 inch! The Mills runs easiest on an 8x4 prop (preferably a flexible nylon one) and puts out maximum power on a 7x4. These things are only turning about 7-9000rpm-but turning a decent prop in doing so-and that translates into thrust-and plenty of it. Powerwise they are only producing 0.05-0.06 BHP-about the same as a Cox Babe Bee-but at about half the rpm! Try and prop and ruin them like an 049 glow-and they will simply not deliver.

sixth issue Fuel-and this has been covered fairly well already. Mills 75s are not too fussy on what you feed them-certainly not to the extent of some other diesels. Oil-wise I would stick to castor-or mineral oil, at least 25%, and at least 30% ether with the rest can be kerosene-with about 1% of the ignition smoother additive of your choice-Amsoil, DII or whatever you can lay your hands on. The key point-as you have discovered I think-is that 'engine start' can vary widely in its ether content. The 'John Deere' brand is usually around 80%-but some others can be as low as 25% ether-and at that level you are simply wasting your time-as the final ether % in the fuel is only 20% or less-even with only 20% lube added-and that is simply too low for effective running.
The other problem with using engine start as an ether source-is that every can I've ever examined does not state an absolute % for ether (or any of the other components) but gives a range-something like 'diethyl ether' 60-78%,paraffinic hydrocarbons 15-23% upper cylinder lubricant 0-3% etc etc [these figures are indicative not from any actual product]-so you can expect to find quite a variation from batch to batch of the same brand product-which makes mixing fuel a bit of a lottery....! The old fashioned 1/3-1/3-1/3 brew for diesel at least had the merit of being easy to mix-if a bit overgenerous in the lube department.....but too much lube never did our engines any harm...whereas too little does...

Hopefully if you take these points on board and check your Mills 75 against the various issues I mentioned you should be rewarded with successful running. Mills 75s were made from very high quality materials-and provided they have not suffered abuse-will run for a modelling lifetime-the only weak areas are the rod and the tank retention-both areas in which some of the replicas -mainly the Doonside and Irvine-improve upon the originals.

One final point-and it concerns parts-there are several replicas of the Mills 75s-few-if any of the parts are interchangeable, other than between the Indian made ones and the originals. The Irvine, Russian and Chinese replicas are metric, so not dimensionally identical to the English and Indian ones.

ChrisM
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Help! Re: Mills S75 starting problems

Post  flight-risk on Wed Jul 11, 2018 2:11 am

Oldenginerod wrote:By the way Risk, is that Snake Valley near Ballarat, or elsewhere?

Rod
Sorry for late reply, been other things for a while.
Sure is the one near Ballarat !
cheers
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Help! Re: Mills S75 starting problems

Post  Oldenginerod on Wed Jul 11, 2018 5:08 am

Nice to see the Mills thread re-activated. Makes me want to dig mine out and relive 40 years ago when the Mills was my first engine experience.

Hey Chris, just when you think someone has disappeared again, you're back. Good to hear from you again. I value your knowledge of diesels among other things. All good info there.

Flight-risk, I'll be up your way for the weekend in just over a week. I'll pack the winter-woolies as I know it can get really cold up there. I have a cousin in Wendouree. Just a quick visit to Sovereign Hill with the wife.

Rod.

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Help! Re: Mills S75 starting problems

Post  ffkiwi on Wed Jul 11, 2018 5:14 am

feel free to drop by for a coffee-and a lot of engine talk then....

ChrisM
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Help! Re: Mills S75 starting problems

Post  Oldenginerod on Wed Jul 11, 2018 6:15 am

ffkiwi wrote:feel free to drop by for a coffee-and a lot of engine talk then....

ChrisM
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Sorry, I'm not dropping by NZ. Going to Ballarat where Flight-risk (I hate using these titles rather than names) lives. Two message in one post. Sorry for the confusion.
Mind you, I may end up there some day as my wife keeps making noises about doing a major bicycle ride over there. I figured I could tour around looking at the things I want (Peter Jackson's air museum, Warbirds over Wanaka etc) while she rides all day, then rendezvouz at the end of the day. Got to get to Invercargill where Burt Munroe came from, just to say I'd been there.
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