the motor i a newer 250 so i am sure it is a chain. I have the vaccum advance plugged off when doing the timing. The mark on the harmonic balacer stay consistent when i hold the throtle at a constent rpm. Its just really eratic when its ideling. The vaccum advance is new. I have replaced the the distrubutor and carburetor during this troubleshooting. I have been trying to figue this problem out for about 2 months.