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Changing timing advance via altering rotor nose length?

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Can someone explain the how the timing advance works? I've read that the rotational 'front' of the bump (nose) you see here determines how much advance you'll have-meaning if you weld on material to its front you'll advance the timing; and I've read that ignition happens after the nose passes the sensor (pickup coil)

 

Which is it?

 

005-5.jpg

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Haven't heard of that on until now.

IMO:

An easier way to do the same thing would be to use an offset key in the flywheel.

BTW:

How much lighter (in grams) is your flywheel than a stock flywheel?

I usually take off .040 from the perimeter of the flywheel but yours looked like substantially more was removed?

Edited by adnohguy

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That is not a XR200 two valve motor, unless a TLR200.

 

The way the advance works with a Crank Position Sensor is the CDI module has circuitry to electronically control the advance.  You can obtain more advance by advancing the rotor, or the tab in the direction of rotation, or use an offset key. 

I don't know if Honda senses the leading or trailing edge so using an offset key, or the following, avoids that issue.

Another option that is used on some other Honda motors is to reposition the sensor in a counter rotation direction for more advance.

 

You could also use a CDI box from a different model that had a different advance curve. 

 

What year and model is the engine?

Edited by Chuck.

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Sorry I should have mentioned that this photo is not my engine. I was just showing it for reference to illustrate my subject,  And since the Honda uses a similar ignition I thought to ask it here, though it's not a Honda engine in question (sorry).  It could still be something someone's wants to do to their XR200 if the mechanics behind what's happening down at the rotor are revealed

 

The mod came up on another forum without any photos etc to back it up saying to weld a 3mm extension onto the front of the rotor to pick up 10 deg advance and gain a fistful of hp on a KLX250 as the engine would keep pulling into the upper rpm range to the rev limiter making more hp along the way.

  

But what I have found on timing advance alone on any engine says it beefs up the mid range at the expense of high rpm as the engine is fighting higher cylinder pressures due to the advanced timing.

 

 

So far I haven't seen any general info that says the the timing sensor gets its signal at the front of the nose on any  bike, though info is scarce. I wonder what the length of nose on any rotor has to do with the ignition.?

 

 

I did have my XR200 rotor lightened previously though.. I think it was 180 or 200g

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Ignition timing advance is not something one can

safely generalize about.

The optimum (as much as can be tolerated w/o

being 'too much') is variable for a SPECIFIC

engine depending on a host of conditions, including

atmospheric conditions, engine temperature,

combustion chamber condition, fuel, throttle

position and so on ... The advance that can be

dialed in this morning may be inappropriate

mid-afternoon in July.

Another example of the same model engine will

very likely exhibit moderate differences in timing

because of tolerance variations and wear.

An engine of another design is virtually certain to

have its own advance curve and total advance

needs. Different intake tract, valve inclination,

combustion chamber shape and area, spark

plug position, heat dispersion and so on are

a few of the many design characteristics that

would influence ignition timing requirements

between engine configurations.

Rather than blindly adapt values from other

applications, develop your own values derived

based on the needs of your specific combination

and operating conditions.

This can be accomplished by learning symptoms

of too much advance, variables that influence

advance (fuel, temp, D/A), record keeping and

experimentation. There are engine software

packages that can assist with predictions (provided

they have accurately modeled parameters for

your engine to work from.)

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The mod came up on another forum without any photos etc to back it up saying to weld a 3mm extension onto the front of the rotor to pick up 10 deg advance and gain a fistful of hp on a KLX250 as the engine would keep pulling into the upper rpm range to the rev limiter making more hp along the way.

  

But what I have found on timing advance alone on any engine says it beefs up the mid range at the expense of high rpm as the engine is fighting higher cylinder pressures due to the advanced timing.

 

 

So far I haven't seen any general info that says the the timing sensor gets its signal at the front of the nose on any  bike, though info is scarce. I wonder what the length of nose on any rotor has to do with the ignition.?

 

 

10 degrees is a lot of extra advance, for example the specs for a  XR200 are 10 degrees initial and 30 (+/- 2) degrees total, which is pretty typical for a small hemi shaped combustion chamber.  Add 10 degrees and you have 20/40 degrees, maybe on a large displacement engine with a slow burn wedge chamber but not on a small displacement hemi or pent roof combustion chamber. The goal is to have peak combustion pressure occur at 17 degrees past TDC, any sooner or later and efficiency/power drops off.  From my experiences an extra 2 degrees of ignition timing on a stock XR200 when running premium helps but with a 11:1 compression ratio you can't run more than stock timing without race gas.  So to me 10 degrees is way too much additional advance and the claims made for the KLX250 don't sound rational.

 

As Eddy pointed out a lot of things influence the burn time, which in turn determine when ignition needs to occur. 

Edited by Chuck.

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Thanks for the replies.

 

The OP on the other forum said he was an ex Kawasaki race mechanic in Europe, and did the timing mod on his own recently acquired 07 KLX250 now in Australia. But he hasn't posted one photo or any dyno sheets to support his claim of the jump from low 20's to 29hp just by doing that.  The gain coming just from the engine revving higher and making more power between 8000 and 11,000rpm because of the timing change. These engines have a 10,500rpm rev limiter on them btw.

 

He had our attention for a few days but it's all just not adding up, and though I have an extra KLX flywheel I don't know that I want to risk it if it goes wrong

Edited by ViperGTS

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The goal is to have peak combustion pressure occur at 17 degrees

past TDC,  any sooner or later and efficiency/power drops off.

 

While PCP at 17 degrees past TDC may be appropriate for a given

configuration, this is not a constant.  It is influenced by a number

of engine design parameters.  Gudgeon or wrist pin off-set, rod

length/stroke ratio and others are some of the determinants.

 

I expect that 17 degrees was given above as an example, rather

than a universal truth.  However, this might not be apparent to

those following the thread.

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his claim of the jump from low 20's to 29hp

 

Any claim of an easy 50% HP increase over stock output of a

modern engine deserves energetic scepticism, if not outright

disbelief.

 

Repeating/spreading such claims without disclaimers reflects

poorly.

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Any claim of an easy 50% HP increase over stock output of a

modern engine deserves energetic scepticism, if not outright

disbelief.

 

Repeating/spreading such claims without disclaimers reflects

poorly.

 

 

 

Could be internet HP I'm wondering...............................? ;)   Strange the factory went to as much work as they did to get HP and didn't set timing that way if it works so well............? :excuseme:

 

Old School Al

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Could be internet HP I'm wondering...............................? ;)   Strange the factory went to as much work as they did to get HP and didn't set timing that way if it works so well............? :excuseme:

 

Old School Al

 

I'd as much said that in one of my replies to him-saying that I'm sure Kaw would love to advertise a few more hp ahead of or nearer to their other brand competitors.  

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