YZ 250 long rod ignition timing

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7/5/2018 11:40 AM

Hey all!
Did some searching and didn't seem to come up with anything.

I'm rebuilding a '95 YZ250 long rod motor. Is the timing spec changed
due to the longer rod? If so, anyone have an idea what it should be?

Thanks!

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7/5/2018 4:54 PM

It should be the same.

Paw Paw

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7/5/2018 11:17 PM

Paw Paw 271 wrote:

It should be the same.

Paw Paw

Thank you, sir!

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7/6/2018 5:03 AM

That is not correct. The whole reason for the longer rod is to change the dwell of the piston. Run your numbers here and see. http://www.torqsoft.net/piston-position.html

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7/6/2018 5:20 AM

Mr. Robinson wrote:

That is not correct. The whole reason for the longer rod is to change the dwell of the piston. Run your numbers here and see. ...more

I agree that this will show the port timing changes but not the ignition timing changes.

Paw Paw

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7/6/2018 6:02 AM

Timing is timing, be it ignition or port it is still effected by the rod length.

20 degrees BTDC is no longer at the same piston height. If he sets the timing to the mark it will be slightly advanced from the standard timing.

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7/6/2018 7:28 AM
Edited Date/Time: 7/6/2018 7:38 AM

Mr. Robinson wrote:

Timing is timing, be it ignition or port it is still effected by the rod length.

20 degrees BTDC is no longer at the same ...more

If I set the timing according to piston height using a dial indicator, as opposed degrees BTDC, then would the stock spec still be correct?

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7/6/2018 7:46 AM
Edited Date/Time: 7/6/2018 7:48 AM

As I mentioned, if you set the timing with a dial indicator to the stock spec for piston height you will be slightly advanced.

You will have to calculate the stock height spec to find the degrees, then factor in the longer rod at those same degrees to get the new proper height for the piston at those degrees.

If you post the numbers I can do that for you.


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7/6/2018 8:16 AM

Mr. Robinson wrote:

As I mentioned, if you set the timing with a dial indicator to the stock spec for piston height you will be slightly advanced. ...more

Ah, got it. I thought you meant the piston height would be the constant, but it's the degrees BTDC?
Let me grab the manual next time I go to the shop . . .

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7/6/2018 10:09 AM
Edited Date/Time: 7/6/2018 12:34 PM

Yes, at a given crank degree the piston will be in a slightly different spot in the bore if the rod length or stroke are changed.

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7/7/2018 11:25 AM
Edited Date/Time: 7/7/2018 11:38 AM

Mr. Robinson wrote:

Yes, at a given crank degree the piston will be in a slightly different spot in the bore if the rod length or stroke are ...more

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Not sure what I'm doing wrong, but using the stock rod length of 125mm, and plugging the numbers into
the calculator; it shows Piston Position (mm) = 0.539.

It is not until I put in 11 degrees that I get a Piston Position (mm) = 0.804

Any idea of where my mistake is? Something with the spec listed being advanced vs. static
or something like that?

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7/7/2018 4:42 PM

There may be some offset in the crank. For some reason it's not letting me log in, so I can't confirm that.

Regardless, the 5mm long rod is not making that much difference on that configuration. So you should be able to just go ahead and use the stock settings without issue.

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7/8/2018 12:58 AM

Mr. Robinson wrote:

There may be some offset in the crank. For some reason it's not letting me log in, so I can't confirm that.

Regardless, the ...more

Thanks!

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