Tuesday, May 18, 2010

Farmboy Hit and Miss Theory??

Here's an email I received May 18, 2010:


Hi all! I am new to the hobby, but have about 20yrs machining experience, (mainly CNC) and am very keen on building a farm boy. Before i get the plans there is a few queries i have. Firstly can someone please explain the HIT AND MISS theory to me? Secondly would this engine be a good first project?
I like the idea of working with bar stock materials, had enough of castings over the years! LOL!
Thanks in advance to all!!!


Dad ranked the Farmboy the third easiest of the 6 internal combustion engines (see http://www.model-engine-plans.com/plansinfo.htm#Experience Levels ).  It could be done as a first project - it just all depends on your skill level.

As far as theory, I assume you mean the 'hit and miss' gearing, and not general internal combustion theory.  In a nutshell, a centrifugal ball governor (see http://en.wikipedia.org/wiki/Centrifugal_governor ) will hold the exhaust valve open until the engine RPM's slow to a set point.  When this set point is reached, the govenor will release it's hold on the exhaust valve, thus allowing the next IC cycle to fully complete and the engine will fire.  More information is available at http://en.wikipedia.org/wiki/Hit-and-miss_engine

Hope this helps!