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Old 04-23-2009, 01:46 PM   #1
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Default Hit and Miss IGN Timing

Hi Guys,
The engine I'm building has no info on ignition timing. I assumed TDC would be the place but the engine seem to run better as I retard the timing. Does anybody have a general rule of thumb on this? This is my first H/M. I'm more use to 8 deg BTC engines rather than the ATDC timing. Using Coleman wd40 mix for fuel
Thanks
Tony



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Old 04-23-2009, 05:44 PM   #2
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Default Re: Hit and Miss IGN Timing

I have 4 H&M engines and they all run best ATDC a couple degrees. I think it is because they are so low on power. I also have an Atkinson and a Webster that are carburetor controlled and they also run best slightly retarded.



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Old 04-23-2009, 07:08 PM   #3
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Default Re: Hit and Miss IGN Timing

Thanks Putputman,
I just needed a verification of my thoughts on the timing. My needle valve is very sensitive and I think I need to cut a smaller tapper into it. My engine runs at ~1000rpm and I'd like to get that down to 500rpm. My thought were to retard the timing to do this because when I richen her up she stops. So, I still need to refine things a bit.  I even tried to put a choke/butterfly on it. It works a little but not the best throttle.

But thanks again for the confirmation.
Tony

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Old 04-23-2009, 07:34 PM   #4
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Default Re: Hit and Miss IGN Timing

The physics behind ignition timing is that with certain givens, octane, compression, fuel mixture and combustion chamber design, fuel will burn at a constant rate. Optimally you want the ignition to occur around the time that peak cylinder pressure builds (compression). This can be possibly 20 crank degrees before and after TDC. With an automobile engine idling at anywhere from 600-900 rpm the timing is generally about 10 degrees BTDC. This allows all the fuel to burn and create maximum cylinder pressure (combustion) just as the piston goes past TDC. As the rpm increases the timing advance can go up to 35 degrees BTDC for the same reason. On a model hit
& miss engine that runs quite slowly and the flame front taking less time because of the small displacement it would stand to reason that the timing could be later than conventional full sized engines. Exactly where is a good question. Probably the best answer is wherever your engine runs the best. The one thing that would change this is the type of fuel being used. I don't know the octane of Coleman fuel but the lower the octane the quicker the fuel will burn given a specific compression ratio. With new engines hearing an engine ping because of preignition is almost a thing of the past but if you are familiar with this the cure for it was to go to a higher octane gasoline which burns slower, therefore eliminating the ping. I have a moveable set of points on my Holt engine and when I get the engine set to a certain rpm I move the point lever back and forth and the change in engine performance is quite noticeable, at high and low speeds. That is why these forums are so helpful, someone, somewhere has run into a problem and solved it and with luck is able to pass along their hard earned knowledge.
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Old 04-23-2009, 07:52 PM   #5
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Default Re: Hit and Miss IGN Timing

Tony, to slow your engine down you would have to either reduce the venturi area of the carburetor or adjust your valve lockout mechanism so that the engine coasts longer between firing cycles. Changing the timing or adjusting the fuel mixture isn't the correct way to go about it. The amount of air being drawn into the engine determines the amount of fuel that is needed for proper combustion (14.7:1) The only way to slow the engine down by way of the fuel system is to reduce the amount of air/fuel going into it. Changing timing to slow the engine down isn't the right way either. The timing should be set to where the engine runs the best. This will mean that the fuel is burning efficiently and thereby creating the most power. Yes if you retard the timing it will slow the engine down but then you will foul the spark plug because of poor burning. If you think of the combustion in the cylinder to an oxycetylene torch, when you first light the acetylene it burns dirty with soot coming off the flame, as you add oxygen the flame cleans up and becomes blue and hot. If you add too much oxygen the mixture becomes lean and the torch will pop and go out. The same thing applies to an engine. You want the very best mixture for the most effeciency and cleanest burn.
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Old 04-23-2009, 08:04 PM   #6
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Default Re: Hit and Miss IGN Timing

Thanks gb,
The Coleman fuel is about 50 octane and the addition of WD40 probably lowers it to 45 octane. I have some 93 oct for my giant scale rc model airplane engines. It's a two cycle mix 100/1 Amsoil and 93 oct. but I can give it a try for experimental purposes! After all, flame propagation is what its about. I can bring the timing to TDC and not get a knock and work my way back from there. I'll play tonight
Tony

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Old 04-23-2009, 08:19 PM   #7
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Default Re: Hit and Miss IGN Timing

Hey gb,
I was replying as you were replying. Yes, I know the engine should have the right timing and mixture for the best performance. I think I'm being swayed by lack of knowledge on the H/M technology. I understand the idea behind it and you are absolutely right. Let the engine run right and play with the "miss" mechanics. I don't have the gov hooked up because I wanted to get the "hit" part right first. I think I got a little nervous with this eng. because it cruses like a son of a gun!! I guess that's a good thing. Time to work on the "miss"
Tony

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Old 04-23-2009, 08:34 PM   #8
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Default Re: Hit and Miss IGN Timing

This is how I want to get my engine to run!
http://www.ronsmodelengines.com/Galloway.html
Tony

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Old 04-24-2009, 12:36 AM   #9
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Default Re: Hit and Miss IGN Timing

You have to understand that this engine has 2 large flywheels, 12 inches, and the centifugal weight sure helps to let the engine runs slow because it can overcome the compression pressure.
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Old 04-24-2009, 11:59 AM   #10
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Default Re: Hit and Miss IGN Timing

Hi gb,
That engine is a beauty though Thanks again for the advice it is well taken
Tony



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