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My Farm Engine

It does not run

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HobbyNut13/12/2010 15:56:12
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How much compression should an engine like this have.
When I turn it over, it does feel like there is some, and this is after I moved the cylinder assy. 3/16 of an inch closer to the crank than the plan said to!!, but I don''t think it is enough.
The original cyl head to top of piston was almost 3/8"..the bore is only 7/8". I have lots of spark, it fires at TDC.
I hope to add some weights and make it a H & M.
 
 
Ian S C17/12/2010 13:28:22
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Hi Hobby Nut, I think the best you can do is go to the IC section , there is a thread running there on Hit & Miss engines, if you don't find what your looking for in the 9 pages they have so far, ask. Compression ratios depend on the fuel you are going to use, as does the spark timing, although its usually before TDC, sometimes 20deg.
                                                              Ian S C
John Wood118/12/2010 12:18:56
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116 forum posts
Hi Hobbynut, you really don't need much compression with this form of low-revving engine, so long as you can feel a noticable resistance at TDC then you should be OK. If it's a new build you might benefit from 'running-in' for a while, I usually hook up a belt to a lathe, take out the spark plug and turn it over on a  medium speed for perhaps an hour or so, this helps bed the rings in and will help compression.  I assume there is an auto inlet valve so you need to make sure that there is enough suction to open this, although (helpfully!) you often can't see this happening, if you suspect a lack of fuel in the cylinder then try using a lighter valve spring.
 
As Ian says compression requirements can vary to some extent depending on the fuel used but I use both petrol and Butane gas without problems.  If it is firing there can't be much wrong. Often these engines go best at around TDC or just before and, in my experience this setting is not that critical. If though you are using  electronic ignition then this may need setting more like the 20 degrees BTDC that Ian mentions.
 
It can help if you put a squirt of WD40 into the cylinder before starting, this helps ensure maximum compression at start-up.  Otherwise do certainly have a look at the IC forum on this site.
 
Hope this helps
 
Regards,  John
HobbyNut18/12/2010 17:35:56
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113 forum posts
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Thanks John for the reply.
I did do just what you suggest as far as running in is concerned. It turns very freely. There are no rings on the piston, this design did not ask for them. I am going to try the WD40 trick to see if it helps.There is a "noticable" suction at the carb/needle valve intake when turning it over, so there is something happening at the intake . I cannot see the intake valve moving, and I have lapped it in good, it shows no form of leakage.
 
I guess I have to try a few other things, or maybe make a new piston c/w rings or 1 ring.

Edited By HobbyNut on 18/12/2010 17:37:58

JasonB18/12/2010 18:33:50
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You need a really weak spring on the inlet valve, just enough for it to return the valve to the closed position.
 
Jason
HobbyNut18/12/2010 21:25:03
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113 forum posts
93 photos
Thanks Jason, I will try a lighter spring.
 
Pat.

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