72 Yamaha AT Timing Help

Ok.  Issues with timing.  No dial gauge available.  First, gapped the points to .015 per Clymers.  Then wired the advancer weights fully out.  Stuck a pencil in the spark plug hole and used a mm ruler to find TDC.  Turned cam clockwise (BTDC) for just under 2mm (1.8 called for in book).  The points are touching all the way through the adjustable area of the points, so the spark is going way to early.  By continuing to turn the cam, it looks like the spark is firing about 1/2 - 3/4 inch before TDC.  It backfires then runs backwards for a short time.  I need help.  It's almost like the cam is 180 degrees off.  I have adjusted the points plate all the way through the available slide space.

Update: I pulled the spark plug today and hand turned the motor to see when the spark plug fired.  It appears to be firing about 180 degrees off.  I have not split the crankcase on this bike.  I'm wondering if it's possible that the previous owner rebuilt the motor at some point in the past and installed something wrong.  Any ideas where to start?

I'm trying to attach photos to see if I'm on the right track.  I think someone in the past pulled the armature and installed it incorrectly.  The pencil in the pictures show the postion of high point of the governor lobe at TDC.  It's on the opposite side as the points base (part that touches the crankshaft).


I'm thinking I need to pull the armature and reinstall.  Thoughts?



Edited by k5wdj

the only way the armature can be out 180 is if there is no key or the key is broken/sheared.

the only way the armature can be out 180 is if there is no key or the key is broken/sheared.

I agree.  I had a DT250 with a sheared off key when I bought it, even though the systems are different.   The only other thing I can think of is I had an AT1 that had main bearings so bad the spark was at such a random timing, it would BARELY run.  See if you can move the rotor/flywheel in any direction.  Free play should be pretty much ZERO.

Woodruff key was sheared.  Pulled and replaced... timed it again... started right up!!  It's alive!  Now on to the rest. 

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now