3 Dot timimng mark on '73 Merc 850

[TheGeek]

Hello, I have a 1973 850 with serial number: 3744577. I am currently in the process of replacing a stretched timing belt. I have the old one off as well as the pulley and am realigning the motor with the distributor. The timing decal has been completely removed from the flywheel as well the white dot. The only marks on the flywheel are the 3 punch marks and an X on top of the flywheel. So my question is: what is the position of cylinder number 1 relative to the arrow on the distributor pulley while the arrow aligns with the 3 dot on the flywheel? Furthermore, the .464 measurement is that measured from the outside of the cylinder head e.g. where the spark plug flange, the bit with the washer, mates with the engine, or is it from the inside of the cylinder head?

Sorry for the wall of text, but any help would be appreciated.

Thanks, TheGeek.

 

[Texasmark]

I had a 70's version...74-76 area as I recall, of that engine and don't recall if it had the magneto or electronic. I think electronic as I don't remember the belt driven magneto.

I don't have a clue but since your post isn't answered, I'll stir things up somewhat.

Magneto says points to me. Points close and allow current to flow into your ignition coil building up energy in it. At the correct time, the points open and the energy is transferred to the spark plug. On my last engine, 2000 90 hp, the ignition timing was 4 ATDC at idle to 20-22 BTDC at WOT.

Turning the flywheel CW and looking at #1, with the throttle set to wide open, the points should open 22 degrees of flywheel rotation BEFORE you get the #1 piston crown to the top. I'd say get in that area, continuously turning CW; don't back up to get your measurement. If the dots are relevant, you may be sitting on one of the dots to answer your question.

360-22 = 338. 338/360 = 0.939. Take a tape measure and measure the circumference (distance around the perimeter) of your flywheel and multiply by 0.939. Rotate the flywheel till #1 is at TDC. make a mark on the flywheel and turn it CW until your dot has traveled that far around the circle. That should put you at 22 BTDC. Check the dot you referenced and note if the points just opened. If not, tweak the pulley on the mag till they do. Repeat the process until as you come by the point the points just open. Install the belt. Where is the dot?

Best I can do and not knowing any better that's how I'd do it.

 

[Chris1956]

No points in that motor. It is battery driven CDI ignition system. Without a timing decal the timing will be hard to set.

When installing new timing belt, set the distributor cast arrow to point to the punch marks on the flywheel. Cylinder #1 will be at approximate TDC. I would look up the part number for the timing decal and see if fleabay has one. If you get one, the dot on the timing decal must align with the punch on the flywheel.

Also, the .464 measurement is from TDC of #1 piston. So you need to measure from cylinder head to top of piston at TDC and then add .464 inches. I had thought the '73 models had the timing marks on the cowling support, rather than a separate timing pointer.