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Thompson Engine from Scratch

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Ron Laden19/01/2021 08:05:09
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2134 forum posts
429 photos

Very nice indeed, impressed with the fabricated frame and base.

I to look forward to the video I should imagine it will sound good.

Ron

JasonB21/01/2021 16:37:23
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19965 forum posts
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Thanks for all the appreciative comments

But onward to the important bit before anyone thinks Unlike a few engines this one seemed to run better after stripping and painting and starts first time after a pull of the cord which I can get 5-6 winds around the starter pully. It's still running quite hot particularly when I lean it out which also ups the revs and makes the throttle a bit more responsive. Richening the mixture a bit slows it down to a more pleasing (to me) speed but then the throttle has little effect.

I did not play with the timing on the video as it is hard to do up the adjuster one handed and did not want to risk things coming loose as during initial testing that did happen and the brass ignition contact went through the open gears and was spat out the other side in several pieces blush

The exposed gears and crank make it a bit noisy but it does make for a more interesting engine when running rather than having everything hidden away inside a crankcase and the dirty oil seems to wipe off Ok smiley

John Dean 221/01/2021 17:15:18
18 forum posts
3 photos

Looks good Jason. Fuel looks to be a clear liquid, what is it? Could you possibly describe your ignition system as it seems to work very well.

Thans, John

JasonB21/01/2021 18:53:28
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19965 forum posts
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For the liquid fueled IC engines I tend to use Colmans fuel which is available from most camping and outdoors type retailers as it is used as a camp stove fuel.

The Americans sometimes refer to it as "White gas" as it is almost colourless unlike pump gas (petrol) but the main reason I use it is it smells far less so you don't end up with a workshop smelling of petrol fuels and if you bring the engine into the house for display/storage there is no lingering smell.

As this is an open crank 4-stroke I am running a very small amount of Husqvarna 2-stroke oil and after about a min into the running you can see I've filled up with a slightly stronger mix as there is more of a green tinge to the fuel. Some people add WD40 or the Americans like their marvel Mystery Oil.

As far as the ignition goes I tend to use the CDI ignitions from S/S in the States this being their single spark unit, you can just see the outer cover of it laying over the battery holder though they are winding the business down and retiring. The unit can be tripped by a hall sensor but also has the option for a pair of simple contacts just by using 2 of the three wires that would have gone to the hall sensor and I much prefer to do it that way. You don't need anything fancy for contacts like you would with a traditional car coil, just two metal contacts, on this engine I have a fixed brass stud in the side of the larger cam gear and an insulated flat brass strip contact on an adjustable are so that when the two meet the circuit is completed and you get your spark.

Power comes from four AA batteries and I'm using a standard length 1/4 x 32 Rimfire sparkplug with one of S/S's silicon boots as the plug is a bit too close to the exhaust rocker you a more traditional looking brass clip causing the spark to take the easy route and not happen inside the engine. There is a ground wire that completes the circuit back to the CDI which I've just got hooked up to one of the cylinder head screws for now.

Nearest equivalent from a UK source now that S/S are winding up would be the MCL-1 unit done my Minimag

DiogenesII21/01/2021 19:10:37
197 forum posts
88 photos

Great to see & hear it, it's an interesting one to have seen built, thanks.

John Dean 221/01/2021 21:44:03
18 forum posts
3 photos

Thanks Jason, same system as the RCexcel unit that I had when model flying with petrol engines.

Ron Laden22/01/2021 08:24:13
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2134 forum posts
429 photos

Great Jason, I do like the open workings rather than tucked away.

Sounds nice especially at the lower speeds there is a nice period towards the end of the video where the speed is lower with a nice put put sound.

Very nice.

Ron

Ron Laden23/01/2021 07:17:50
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2134 forum posts
429 photos

Morning Jason

Not knowing much about it I wondered why an engine running rich loses rpm and has less throttle control is it simply a case of too much fuel.?

Ron

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