Single cylinder OHV - "Snöffy"

[srobovak]

Hi Modelenginemaker,

After sneaking around for a long time without posting anything, I thought I might also share something.
I have posted this one somewhere else a while ago, so just ignore it if you have seen it already.

I have been working on this one for over 4 years now and it finally came to a successful end...

After researching numerous builds from other people and combining things together I eventually distilled my own engine design.
I wanted to work in the metric system and also wanted to adjust the size of the model and its complexity to my own demands. As most of these models serve only the purpose of pleasing the builder as they come "alive" I also wanted a design that would suit my own aesthetic ideas of how it should look like.

I opted for the looks of an early 20th century motorcycle single. After about 3 years of irregular work, I had my first test run which was a failure. The engine would not run continuously even though I had a few puffs. I re-evaluated a couple of things and decided to change the cams to milder timing characteristics, enlarge the flywheel, improve the valves and their sealing and also improve the sealing of the carburetor. This work took me about 5 more months to complete.
But the second try was an immediate success. The engine now starts really easily and runs quite well. The RPM range is quite narrow though - it idles at around 950 RPM and reaches its maximum at 1800 RPM.
It also needs some time to rev up and down this range which is due to the large flywheel. I am also pleased with the way it sounds which was also one of my goals.

Some engine details:

- the square design of 25 mm stroke to 25 mm bore
- spark plug ignition using NGK CM-6
- home-brewed stand-alone ignition module using automobile coil
- O.S. 1A carburetor
- running on Coleman fuel mixed with 5% 2-stroke oil
- splash lubrication

A short video of the running engine:

<a href="https://www.youtube.com/watch?v=TpfSoePqkJs" target="_blank">http://www.youtube.com/watch?v=TpfSoePqkJs</a>

Till next time. Cheers,

Branislav

[b.lindsey]

That is a gem. Runs and sounds wonderful and I love the slow idle. Well done!!

Bill

[Kim]

Hi Branislav,
Very nice little engine!  And it runs great!
Thanks for posting it.
Kim

[propforward]

Wonderful - love the sound, and it runs so well - very nicely done indeed!

[Art K]

Branislav,
That's a great running engine and isn't hard to look at either. Is the lever with the brass knob on the side of the cam box for adjusting the ignition timing?
Art

[Roger B]

Excellent    A very steady idle and a good sound 

I see you have the fuel level in the tank at carburettor height. Do you see much difference if you lower it a couple of cm?

I also note that you have an insulator between the cylinder head and the carburettor. That was the key change to get my 3cc ohv engine running properly.

[srobovak]

Thank you all for your kind comments.

@ Art K: you're right. That knob is for ignition retard/advance adjustment

@Roger B: I have not actually tried to lower it so much. But around 10-15mm (approaching tank's bottom) I observe no difference. 
I put the polyurethane tube there for fit/sealing reasons. Why did it make a difference for you being an insulator? Can you please elaborate on that?

Thanks and cheers,
Branislav

[Art K]

Branislav,
It could be heat transfer from the engine to the carb. If there is nothing to break heat transfer to the carb it can cause problems. On my own engine it melted the Perry carb I was using.
Art

[Roger B]

Branislav,

As first built my engine had the carb fitted directly to the cylinder head. It would run for a minute or two and then stop but could be restarted with a brief choking. After a lot of experimentation I fitted a Delrin distance piece/insulator and the problem went away.

[Zephyrin]

Well done, nice runner !
what is the part moving near the ignition module in the video? something linked to a Hall device ?

[Brian Rupnow]

Srobovac--That is one of the cleanest, neatest engines I have seen. I particularly like the clear end on the gas tank. I have spent a lot of time trying to get engines to start when the fuel tank had ran dry and I didn't realize it.---Brian

[Roger B]

Me too, even with a transparent plastic fuel tank 

[srobovak]

@Zephyrin:
The magnet is mounted on a piece of brass plate that rotates with the camshaft. and the angular position of the Hall-sensor is adjusted using that brass knob lever. Not very efficient design, a bit too complicated to machine and not so easy to adjust. The next engine should be better...

[AlexS]

Piece of art (nice bronze/aluminium look) and also great running!

For what purpose are the other adjustment buttons used? Like the button on the bottom of the crankcase.

[kvom]

That "ticks" over quite nicely.   

The name sounds like it should be some sort of furniture at Ikea.