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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #51  
Old 2 Weeks Ago
JackPrice53's Avatar
JackPrice53 JackPrice53 is offline
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Originally Posted by GA_Ironhead View Post
I've often wondered what dictates ignition advance, but not enough to actually research it. So why is an IH set to 40 or 45° of ignition advance and not say 30 or 60°?
This is a great technical question and probably deserves its own thread. I've always assumed when designing a motor, it's a trade off of meeting your product goal, creating ideal TQ/HP vs. motor longevity and of course cost/profit is very much part of the equation. I've also assumed that setting the timing is a function of the compression your after which is effected by the shape of the combustion chamber, the piston dome shape and trying to maximize the burn. I'm sure it's a VERY complicated trade-off of mathematical, physical, theoretical and practical design including lots a empirical experimental trail and error.

As to why the IH "needs" a little more advance, ie fire further from TDC compared to the K model, must have to do with the increase compression of the IH which gets back to the combustion chamber shape/size and the piston dome shape. OK, I've stuck my neck out, now ya'll can tell me how full of sh*t I am.

Last edited by JackPrice53; 2 Weeks Ago at 23:52.. Reason: spelling
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  #52  
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The only criteria that determines optimum advance is the flame propagation speed so that engine reaches max pressure from burn when piston is at TDC.

It then derives that the slowest the propagation speed is, the more advance you'll need...

It also derives that high dome and orange peel combustion chamber shape will slow down burn... So more advance...

Don't worry, there are worse things!!!

Like some italian race engines with : O ( 120 deg valve angle that needs up to 55 deg advance!!!

Also worth noting, since your piston rises against expanding gases, there a trade off in power to advance needs, and then, twin sparks start to be attractive despite complications.

Patrick
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  #53  
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Originally Posted by Tator View Post

... So my crude way of measuring this is to stick a wooden dowl down in the spark plug hole and I marked it with a magic marker. Then I measured it, it came out 1 3/4". So my confusion is this is supposed to fire 11/16" BTC. There is where I was confused.
Glad you got her going in the end. But just for in case you lie awake at night wondering:

What you need to do is set the piston at TDC, then stick your dowel down the hole and mark it. Then rotate the flywheel backwards until the 45 degree BTDC mark appears in the timing hole. Then stick your dowel down the hole again and mark it again. The difference between the TDC mark on the dowel and the 45 BTDC mark on the dowel will be 11/16".

The actual distance (1-3/4") from the top of the plug hole to the piston is irrelevant. Its the difference (11/16") between the readings in the two positions that counts.

Glad you got her going in the end though. Yes, old magnetos can be a special challenge all of their own. Good when they are properly working though. Often it takes more than just a remagnetize job though. Old insulation breaks down. Old wires crack and wear thin. Condensers go south. Plug wire connections fail to connect. And sometimes the juju just ain't in there.
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