Question on setting timing for a '75 DT250

I'm getting a 1975 Yamaha DT250B back on the road and want to check the timing and point gap. I know the basics and have set points on old bikes in the past but I'm hoping there's some DT experts that can help with a couple of questions. I have an off brand service manual for it that might be confusing more than it's helping.

The DT250 has a plate that carries the points, ignition, and lighting coils with an external rotor/flywheel that fits over it on a keyway.

1. The factory timing spec is 3.2mm BTDC. I have a dial indicator and based on a visual check of the points, the timing is retarded quite a bit, probably around .8 to 1.0mm BTDC right now. This makes sense as to where the backing plate is oriented on it's screws. It's shifted pretty much all the way one way that would retard the timing. What's confusing is that the manual calls for loosening the set screw on the points and moving the points to adjust timing. It seems to me I need to turn the entire backing plate to adjust the timing and the screw for the points is primarily for adjusting the point gap. Pain in the butt since you have to pull the flywheel each time you want to get to the screws to move the backing plate. Is my assumption that I set the actual timing by moving the backing plate correct?

2. When setting points, I like to use my volt/ohm meter to check continuity and know exactly when the points are opening. The manual I have describes this same method but calls it a "point checker". I've unplugged everything coming from the ignition at the wiring harness (4 wires in 1 plug) and I put my red lead on the points wire (black in this case) and ground my black lead. I'm expecting the circuit to show grounded until the points open then break the continuity. I can see the points open and close but I never break continuity. I know I've had this issue before but can't remember what I had to do to isolate the points so I get a good reading. Any ideas? I've got a good spark at the plug so assume that everything is wired correctly. If I can't get my meter to work, I'll just have to set them visually.

Thanks for any help you can provide!

The old Chilton's manual I have isn't very model specific,but does state that some Yamaha Enduro models require the point gap to be adjusted to set the timing. It also says not to connect the positive clip of the ohmmeter to the black/white wire,but to disconnect at the plastic connector at the magneto and connect the positive clip to the black wire.

I hope this helps.

You're not alone, I have a 71 RT1-B and found it very hard to understand the timing procedure with both the clymers and the original factory manual. It did appear that the adjustment of the timing is done with the points.

You probably need to disconnect the wire from the points/condenser to the coil. That will isolate the points so you can get a clear reading. If memory serves me, you should set the points at .012 and rotate the backing plate till you get it as close as possible. Then do your final adjustments by the point gap. You should not have a finished point gap any closer than .008 and no wider than .018, or you need to read adjust your backing plate

I ended up unhooking the points lead wire although I've since figured out I could have done it by just using a higher range on my ohm meter and not expecting the ground to be lost, just the resistance would change. It took a lot of trial and error and was surprised that moving the backing plate changed the timing relatively little compared to what the smallest movement of the points would change.

In the end I think I've got it pretty much spot on. I fired the old girl up for the first time since I purchased it in non-running condition in January. It runs like a top. Very cool old bike.

The correct method is to set the plate in the approximately correct position, final timing adjustment is made via the point gap. If, you find the gap excessively large or small, then pull the flywheel, move the plate a little, and redo the gap. Once the plate is in a good spot, it whould never have to be moved.

Also, visual is a very inaccurate method. The correct method is to use a analog 'needle' type ohm meter. When the points are just barely opening, the needle will drop a little. If it drops all the way, you went too far.

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