YZ 250 long rod ignition timing

FWYT
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FWYT
Edited Date/Time 7/8/2018 12:59am
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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FWYT
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FWYT 7/5/2018 11:17pm
7/5/2018 11:17pm
Paw Paw 271 wrote:
It should be the same.

Paw Paw
Thank you, sir!

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Mr. Robinson 7/6/2018 6:02am
7/6/2018 6:02am
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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FWYT 7/6/2018 7:28am
7/6/2018 7:28am Edited Date/Time 7/6/2018 7:38am
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 piston...
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.
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?
Mr. Robinson 7/6/2018 7:46am
7/6/2018 7:46am Edited Date/Time 7/6/2018 7:48am
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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FWYT 7/6/2018 8:16am
7/6/2018 8:16am
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. You will...
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.


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 . . .
Mr. Robinson 7/6/2018 10:09am
7/6/2018 10:09am Edited Date/Time 7/6/2018 12:34pm
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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FWYT 7/7/2018 11:25am
7/7/2018 11:25am Edited Date/Time 7/7/2018 11:38am
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 changed.








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?

Mr. Robinson 7/7/2018 4:42pm
7/7/2018 4:42pm
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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FWYT 7/8/2018 12:58am
7/8/2018 12:58am
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 5mm long...
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.
Thanks!

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