# A new look at Opposed Twin I.C. Engine



## Brian Rupnow (Sep 12, 2021)

A few years ago, I built my version of an opposed twin i.c. engine. It ran very well, and I was very pleased with it. There was always a problem with the configuration of this engine, because the sparkplugs were at the very bottom of the horizontal cylinders, and if you happened to flood it while starting it, you could crank all day and it wouldn't clear itself. However, it did run, and I made some good videos of it running. The crankshaft was riding on bronze bushings, and the crank wasn't "dead nuts" straight. I was using my standard 1975 Chrysler ignition points, and a cam running on the crankshaft. As time went by, with more and more hours running time accumulated on the engine, the bronze crankshaft bushings began to wear, cause by the crank that wasn't truly perfect. It wasn't a wet sump engine, so that didn't really matter a lot  ---BUT--as it wore, the ignition cam, attached to the crankshaft began to move around with the sloppy crankshaft. Finally, the ignition cam on the less than perfect crankshaft was moving around enough to seriously affect the timing and spark of this engine. This took me the longest time to figure out what was going on, and when I did figure out what was happening the engine went "Up on the shelf". I promised myself at that time that eventually I would redesign this engine, using ball bearings on the crankshaft, a crankshaft that was truly straight, and a configuration which put the sparkplugs at the top of the horizonal cylinders. I would be able to re-use the cylinders, cylinder heads, overhead valve mechanisms and valves and cams, while making a new central crankcase and fan assembly. This is early days yet, and I still have some unanswered questions about the T head engine I have currently been building, but I think this will be the direction I move in next.


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## ShopShoe (Sep 13, 2021)

I'm glad you're revisiting this engine, Brian. I followed the original build and I really liked the whole concept. I'll be looking forward to seeing how it turns out.

--ShopShoe


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## Gordon (Sep 13, 2021)

This is interesting because I built the Upshur version of this engine and never got it to run properly. I just got it back off the shelf last week to try again. I believe that the biggest problem with this version of the engine is the valve timing. It uses only two cams so that the intake of cylinder #1 and the exhaust of cylinder #2 uses one cam and the other cam runs #2 intake and #1 exhaust. The problem is that if you adjust the intake on one cylinder it changes the exhaust on the other cylinder. There is really not enough room to use four cams. I will have to take a closer look at your original design but a quick glance it looks like you used two cam shafts with an idler gear.


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## coulsea (Sep 13, 2021)

Gordon said:


> This is interesting because I built the Upshur version of this engine and never got it to run properly. I just got it back off the shelf last week to try again. I believe that the biggest problem with this version of the engine is the valve timing. It uses only two cams so that the intake of cylinder #1 and the exhaust of cylinder #2 uses one cam and the other cam runs #2 intake and #1 exhaust. The problem is that if you adjust the intake on one cylinder it changes the exhaust on the other cylinder. There is really not enough room to use four cams. I will have to take a closer look at your original design but a quick glance it looks like you used two cam shafts with an idler gear.


My Upshur twin uses one cam lobe for both intake valves and the other lobe for both exhaust. plans are dated 2001.


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## Brian Rupnow (Sep 17, 2021)

Yesterday I was prowling thru the drawings and models of this build, and I discovered something good. I have been thinking all along that I would probably have to build a new crankcase, and probably new cam and crankshaft.---However---Taking a much closer look, I may be able to modify the crankcase to take sealed ball bearings on both the crankshaft and the camshaft. If so, that would be a real bonus.


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## Brian Rupnow (Sep 21, 2021)

This is what some research showed me. At the bottom of this model you see the original crankcase "as is" with a round blue insert on one side to hold the crankshaft bushing and a bushing on the opposite side to hold the the other end of the crankshaft. You can also see a strange shaped red insert on the near side that was the camshaft bushing, with a plain bushing on the far side to hold the other end of the camshaft. The round yellow cylinder represents the outer diameter of the crankshaft bearing, and the green cylinder represents the outer diameter of the camshaft ball bearing. At this point, I believe that I can rework the crankcase to accept ball bearings for the crankshaft and for the camshaft. This would let me keep the original crankshaft and camshaft, which would be a tremendous bonus.


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## Brian Rupnow (Sep 21, 2021)

So yes, it can be done. I have been holding off on this project, wanting to completely finish the T-Head engine before wading into this. The T-Head is essentially finished, and my new bandsaw is installed and working, so I think I will be making chips with this rework of my opposed twin engine.


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## Brian Rupnow (Sep 22, 2021)

I'm having a quiet day, tearing down the old engine. I gave myself a terrible flash yesterday with my old A.C. welder, and my eyes are howling about it. The new helmet I got when I bought my tig welder has a sensitivity dial on the side of the helmet, and somehow the dial had been turned to '0", so when I started to weld the lens didn't automatically darken. As I begin removing pieces from this engine, I am impressed by how many machined parts there are, and beginning t wonder if I should just buy some angle of the correct size and make a new crankcase.---Not sure yet.


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## Brian Rupnow (Sep 22, 2021)

So, here we are, right down to the nitty gritty. I guess the most surprising thing I have seen is how incredibly dirty everything was. Lots of grease and oil and sludge. I don't know how long ago it was that I designed and built this engine, but it's been a few years. I see a few simple things that I would change a bit if I was starting to build this engine from scratch now, but really not much. I can bore straight thru the large top-hat that supports one end of the crankshaft and out thru the other side of the engine in one set-up. The engine is 2.75" out to out, and the stroke on my mill quill is 3.00" The cam bore will be a little more difficult because the tappet guides are loctited in place and there is only 0.632" between the inner ends. The ball bearings for the camshaft are 0.750" o.d.---I can probably put a bit of heat on the tappet bushings to release the loctite and take them out until the camshaft bearing bore is completed and then loctite the bushings back into place. Actually. the bore for the camshaft bushings is only 0.75" so I would probably just run a 3/4" reamer thru there.


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## Brian Rupnow (Sep 23, 2021)

All four ball bearings have been installed, two on the camshaft and two on the crankshaft. I still have some spacers to make and will put up a picture after they are installed.


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## tornitore45 (Sep 24, 2021)

I built the Upshur twin and my biggest problem to getting it run was to discover the spark jump to the push rods.
Once the plugs were booted it run with no problem.


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## Gordon (Sep 24, 2021)

tornitore45 said:


> I built the Upshur twin and my biggest problem to getting it run was to discover the spark jump to the push rods.
> Once the plugs were booted it run with no problem.


I built the Upshur twin several months ago and cannot get it to run at all. I put it aside a few months ago and got it back out last week and I am at a loss to find the problem. I have checked everything I can think of but nothing seems to work.


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## Brian Rupnow (Sep 24, 2021)

So, here's a picture of the crankcase with all ball bearings installed. There isn't that much that you really can see. I had to open up a few counterbores and make some bearing spacers, but it's all done without having had to change the overall shape and dimensions of anything. The camshaft bearings set about 0.020" proud of the top surface, but that isn't a problem, as there is a magnetically attached plate over that top surface when everything is put back together. I am so happy that I didn't have to make a new crankshaft o camshaft.


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## Brian Rupnow (Sep 24, 2021)

I have never been happy with the base I made for this engine. I am going to build a new, much shorter base because I think the proportions are better.


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## Brian Rupnow (Sep 25, 2021)

Today I shortened the plate directly below the crankcase, and machined two new "feet" that are much shorter than the original ones. I stuck that evil looking flywheel/fan on just to make a good picture.


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## Brian Rupnow (Sep 26, 2021)

Today I reworked the flywheel/fan guard to fit the shorter engine base. Also took the opportunity to J.B. Weld shut a bunch of holes I had drilled in the guard to mount a governor. The governor kinda sorta worked, but I could never get the engine to run consistently long enough to get the governor tuned properly. I may still try a governor to control this engine, but if so it will be mounted on a separate base and connected with an o-ring drive belt.


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## Brian Rupnow (Sep 27, 2021)

Well, that's a day out of my life, but it's been a good day. There was no way I could rework the coil mounting and switch bracket to make them work with the new engine frame, so I built new ones---and modified the guard for the wicked flywheel/fan to fit the much shorter engine base. Everything looks a bit rough right now, but I haven't done my cosmetic magic yet. Tomorrow I will tidy things up a bit, and see what's involved with mounting the gas tank and cylinders---They should just be direct bolt ons.


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## Brian Rupnow (Sep 28, 2021)

Today seen the cylinders cleaned up and reinstalled, along with the pistons and connecting rods. I could have used one of Chris's elves to get down into the crankcase and tighten the rod bolts. I had forgotten just how inaccessible they were when the bottom plate was installed on the crankcase. I had to take everything apart to get at the rod bolts to tighten them. Before I go any farther now, I have to time my valves before my gear cover goes back in place.


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## Brian Rupnow (Sep 28, 2021)

--I just spent an hour reading thru all the posts made on the forum when I originally built this engine, and I discovered something. On the original build, that diamond mesh on the flywheel guard was between the flywheel and the engine. Now it's on the other side.  Ah well, I didn't plan on sticking my fingers in there anyways.


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## Brian Rupnow (Sep 29, 2021)

So, here we are, all reassembled. Crankshaft and camshaft are both riding in sealed ball bearings. That lever at the very front of the engine has my ignition points mounted on it, so I can adjust the timing by lifting the lever up or down. The plate will rotate to the limits of the curved slot, letting me fine tune the timing while the engine is running. That knurled pulley and key setting in front of the engine  go on last, as it has part of my starter which I engage with my variable speed drill. I still have to remove the fan guard and give it a bit of cosmetic work and a new paint job. I might machine a swoopy looking air horn to fit on the intake side of the carburetor. Tomorrow I have some real, paying design work, so I probably won't post anymore progress for a couple of days.


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## Brian Rupnow (Sep 30, 2021)

Will it start or not? I don't really know. The engine is wearing the newest edition of cylinder heads, with the sparkplug situated more or less in the center of the cylinder. This of course means that it has new valves and valve cages, and I'm never sure if the valves are going to seal properly or not until I actually try to run the engine. With brand new ball bearings on both shafts, the engine is "stiff" to turn by hand. Of course, if it does take off and run on it's own, this stiffness goes away after the first fifteen minutes of running. I will probably be finished with my design work by tomorrow afternoon, then I will try the engine and see what happens.


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## Brian Rupnow (Oct 3, 2021)

Yesterday I added 1" extensions to the sparkplugs for this engine to get the plug wires up out of the valley they sat in between the rocker arm towers. I don't know for sure that they were shorting out to the towers, but I think they may have been. I was fortunate enough to buy a couple of extra deep sockets as seen in the foreground that let me install these longer sparkplugs with no difficulty. The socket required a little machine work, but they are perfect for this job. I started a separate thread about the lengthening of the sparkplugs. I set the ignition timing yesterday, and sorted out the wiring required for the snowmobile coil which supplies spark to the plugs. Near as I can tell, the engine is ready to run.---This new camera sees far too much, but this picture shows the extended sparkplugs tightened into place in the cylinder heads.


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## Brian Rupnow (Oct 3, 2021)

I didn't want to start pulling things apart, but I had to check and see if my valves were sealing or not. That required that I make a new "blow yer guts out" tool to screw into my engines sparkplug holes. Of course nothing is easy--The tool I have for this wouldn't fit down between the rocker towers, so I had to make a new longer tool. By watching the rocker arms and turning the engine by hand, you can tell when the cylinder you are testing is up on compression stroke. If both valves are sealing properly, you won't be able to blow at all. If you can blow, then you can determine if the air is coming out the intake or exhaust valves. If you can blow but air isn't coming out the carburetor or the exhaust stack, then your rings are bad.  One cylinder is sealed tight.--No air escaping at all. The other cylinder is leaking a very small amount from the intake valve, but not enough to keep the engine from firing. I tried to start the engine this morning, but it wouldn't fire at all.  My "stretched" sparkplugs aren't shorting out to the rocker towers, but they are working---I checked this out early this morning with the sparkplugs laying out on top of the cylinders----lot's of spark, where it should be.


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## Brian Rupnow (Oct 4, 2021)

I pulled the one cylinder head off, and relapped the offending valve so it was air tight when closed. I have a clue as to what is happening. There is a transparent fuel line between the carburetor and the gas tank. On an engine which has proper valve timing, if you crank the engine while holding your finger over the carburetor intake, you can see the fuel in the transparent line go rushing up to the carburetor. This is not happening for me. In fact it is chasing the fuel back from the carburetor back into the tank. The only thing that can do that is incorrect valve timing. Time to get out my degree wheel and do some timing magic.


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## animal12 (Oct 4, 2021)

you made your own " leak down tester " . A very valuable tool for a mechanic .
animal


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## Brian Rupnow (Oct 6, 2021)

I finally had to resort to using the old cylinder heads with the sparkplugs located right at the very bottom of the cylinders. There was just no way I could get the engine to run with my reworked cylinder heads which moved the sparkplug up to the center of the cylinders. Finally, today, after a lot of fuss and bother, the engine ran for it's first time on it's own. It isn't running good, but it's running. My next labour will be to get it fine tuned and idling slower. There has been more work to this than I thought there would be.---Brian


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## Brian Rupnow (Oct 9, 2021)

The engine is running good and starting very easily. The only issue is that I can't coax it into a slow idle. Research shows that a bit of advance on the valve timing will lead to a steadier, more stable idle, but top end rpm will suffer. I am not that interested in reaching a higher rpm, but I do like the slow idle very much. The cam design I am using asks that the cam be set to just start opening the exhaust valve at 40 degrees before the piston reaches bottom dead center. I set everything up on spec, but needed to verify that my valve timing was correct. 40 degrees before bottom dead center is 140 degrees after top dead center. So---accurate measurement should show the valve beginning to open at 140 degrees after top dead center. However, when set up with a timing disc and the end of the dial indicator resting on the end of the rocker arm to clearly identify when the exhaust valve begins to open, it is reading movement beginning at 125 degrees after top dead center. That is too much advance. It makes for a very interesting set-up, so I thought I would post a picture.---Brian


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## Steamchick (Oct 10, 2021)

Hi Brian, having tinkered with valve timing on Triumph twins, of 1950s vintage, you must be very careful to set the tappet clearance correctly. Basically, a few thou extra - or less - clearance at the tappet can move the contact point for valve lift 10 degrees or so earlier or later. The key is to measure the point of start of lift and also the point of valve closure (start of lift in the opposite direction?) in degrees from your datum = TDC.
To make an engine "more suitable" for slow running, the valve opening timing shall be later (bigger angle after TDC - when everything is defined 0 - 360 degrees) than for high revs. All related to TIME, so the gases move at their optimum TIME through the open valves. So I think you should retard the valve timing 5 degrees or so and see if the idle is better? 
I assume you have a cam-shaft with both inlet and exhaust cam? The Triumph engines I played with had separate cams for inlet and exhaust. Modern cars have twin cams and usually have an advance device on the INLET cam for 3500rpm and above (varies with engine design, but that is typical). Basically, the cam is timed for low speed intake, then advanced 5~10 degrees for higher revs. Rarely is the exhaust cam altered... as the pressure differences across the valve port when pumping gases out at the end of stroke are much higher than those on the inlet, so the exhaust is less sensitive to timing. 
Thinking of your statements about "advancing timing of the exhaust" for a smoother idle, I don't really understand "how" it does that? Considering at idle you have the least fuel = least power developed, I would have expected to need to maximise the expansion of the gases before opening the exhaust valve, so as to extract maximum energy from the small amount available..? - I.E. a later opening exhaust valve than for higher revs. Anyway, 125 degrees after TDC for the exhaust opening sounds too far advanced -  from where I am sitting!
Have fun and let us know the result,
Regards,
K2


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## Brian Rupnow (Oct 10, 2021)

So, this is the final final. I made a nice video, and forgot to mention that I had not only changed the base, but had also put sealed ball bearings on both the crankshaft and camshaft. Strangely enough, I couldn't get it to run the way I wanted with my Traxxas 4033 carb, so at the last moment I used a carburetor that was originally designed by Malcolm Stride, and that seemed to do the trick. A person could tune away forever trying to get a slower idle, and yes, it will idle much slower than seen in the video, but it gets a bit unstable at very low idle and will quit right in the middle of making a video. I'm happy to have it up and running again, and yes, I will sell a complete, up to date set of plans for $25 Canadian funds paid to Paypal to [email protected]   This engine uses a single set of Dodge ignition points with a dual output two cylinder snowmobile coil. I can take pictures of the ignition set-up I used but really, if you build this engine chances are you will be using a different coil set-up than I used anyways. Thanks for following the build.---Brian


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## ShopShoe (Oct 11, 2021)

Brian,

I like the way that runs now. I have to say again that I admire your stick-to-it-iveness.

I like this engine from a design standpoint and I think you have done an outstanding job. I also like your choice of nice idle over screaming speed.

Thank You for posting, as always.

--ShopShoe

P.S.: Have you ever considered a design with more cylinders? Just asking. At least one curious mind wants to mull over the possibilities.

--SS


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## CFLBob (Oct 11, 2021)

ShopShoe said:


> P.S.: Have you ever considered a design with more cylinders? Just asking. At least one curious mind wants to mull over the possibilities.



I'll second that.  My plate's kinda full now, but always thinking about what's next.


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## Jasonb (Oct 11, 2021)

Well this engine does have some of it's parentage back in Malcom Strides' Bobcat and Lynx engines. He had also started to draw up a V6 and straight 4 that were to use the same cylinder & head parts so there is a starting point if you were tempted Brian. I think I'm probably the only one with his initial drawings for these which are little more than a basic block and Schillings type crankshafts..


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## Brian Rupnow (Oct 11, 2021)

I have thought of making multiple cylinder engines, but that's not really my thing. I'm happy with the one or two cylinder engines. All of my machines are manual, no CNC . -----Brian


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## Nerd1000 (Oct 11, 2021)

Steamchick said:


> Hi Brian, having tinkered with valve timing on Triumph twins, of 1950s vintage, you must be very careful to set the tappet clearance correctly. Basically, a few thou extra - or less - clearance at the tappet can move the contact point for valve lift 10 degrees or so earlier or later. The key is to measure the point of start of lift and also the point of valve closure (start of lift in the opposite direction?) in degrees from your datum = TDC.
> To make an engine "more suitable" for slow running, the valve opening timing shall be later (bigger angle after TDC - when everything is defined 0 - 360 degrees) than for high revs. All related to TIME, so the gases move at their optimum TIME through the open valves. So I think you should retard the valve timing 5 degrees or so and see if the idle is better?
> I assume you have a cam-shaft with both inlet and exhaust cam? The Triumph engines I played with had separate cams for inlet and exhaust. Modern cars have twin cams and usually have an advance device on the INLET cam for 3500rpm and above (varies with engine design, but that is typical). Basically, the cam is timed for low speed intake, then advanced 5~10 degrees for higher revs. Rarely is the exhaust cam altered... as the pressure differences across the valve port when pumping gases out at the end of stroke are much higher than those on the inlet, so the exhaust is less sensitive to timing.
> Thinking of your statements about "advancing timing of the exhaust" for a smoother idle, I don't really understand "how" it does that? Considering at idle you have the least fuel = least power developed, I would have expected to need to maximise the expansion of the gases before opening the exhaust valve, so as to extract maximum energy from the small amount available..? - I.E. a later opening exhaust valve than for higher revs. Anyway, 125 degrees after TDC for the exhaust opening sounds too far advanced -  from where I am sitting!
> ...


My understanding is that the point where the inlet valve closes (some time after BDC) has the biggest impact on running characteristics, so (on a pushrod or SOHC engine) within reason you can move all timing events forward or back to fine tune it with an existing cam. Advancing? (unsure what the correct way of saying it is) the cam will make the inlet close later in the cycle, favouring higher RPM. Of course this messes with the other timing events at the same time so it's nowhere near as good as having DOHC and adjusting the inlet timing independently.


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## Brian Rupnow (Oct 12, 2021)

Graham Vickery---Email me at [email protected]  I've got your drawings but don't know your email to send them to.---Brian


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## IC-man (Oct 13, 2021)

Brian Rupnow said:


> Graham Vickery---Email me at [email protected]  I've got your drawings but don't know your email to send them to.---Brian


Hi Brian,
Email address is:-
[email protected]
Thanks Brian.


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## Brian Rupnow (Oct 13, 2021)

The first set of 60 .pdf drawings has been sent out to Graham Vickery. This is a fairly complex engine, and I expect Mr. Vickery to have a few questions which I will attempt to answer for him.  This engine is a bit more complex than what I usually send out, because it was first designed 5 or 6 years ago, then there were some design changes made about two years ago when I was trying out different cylinder heads, and then the final design changes this year when I made a much shorter base and added ball bearings to the crankshaft and camshaft. Of course the trick for me is to send out only drawings that refer to the current engine as it sets. Good luck Graham, may the force be with you!!!---Brian


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## Bill Lawson (Oct 13, 2021)

Brian, what is your next project? Have you considered an opposed single cylinder dual piston engine similar to a Fairbanks Morse marine engine?
Thanks Bill.


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## Brian Rupnow (Oct 14, 2021)

Bill---I already have a dual opposed piston engine that I built many years ago. I'll try and find a link for you.---Brian





						A different opposed piston engine---
					

I have been fascinated with a post on another forum about an opposed piston engine. I do not want to plagiarize someone else's post, so I have been surfing around the internet looking at "other" types of opposed piston engines. I found one today, and spent some time doing some preliminary...




					www.homemodelenginemachinist.com


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## Brian Rupnow (Oct 21, 2021)

Today I was getting pretty bored with my self imposed "Do Nothingness", so I snuck down to my machine shop and made an "Air Intake Horn" for the carburetor. This really adds nothing to the performance of the engine, it just looks neat. Many years ago I actually bought one of these for a Sno-Jet snowmobile I owned, and was sucked in by the salesman who told me about the performance enhansing properties of such an item. It didn't do a damn thing for performance, but if you sat to close to the engine it would suck in your snowmobile suit and stall the engine.---But----It did look neat.


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## MRA (Oct 22, 2021)

That's the way to do it - sit and do nothing until boredom forces inspiration to strike.  But there are things which make boredom leak away before it has built up enough pressure to make anything happen - like a crankcase leak in a two-stroke.  I never was much into TV, but the internet is a terrible waster of boredom.  So I need to get off it and feel the boredom pressure rise


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## Brian Rupnow (Jan 17, 2022)

Today we have a blizzard going on outside, and I wanted to add a throttle control to my twin cylinder engine. So, almost a full day later, I have added one of my Casey Jones throttle levers to it. The throttle lever has a couple of Belleville washers (spring washers)  down at the pivot point, and they provide enough friction to hold the throttle arm in place. this way the engine doesn't get away on me and rev up to the point where it's screaming and I'm having a heart attack trying to hit the ignition kill switch.


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## ShopShoe (Jan 17, 2022)

Love it.

--ShopShoe


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