Friday, September 23, 2011

Finding 8.5 degrees BTDC

This may be found in some shop manual somewhere, or might even be the steps in setting up the ignition timing on some vehicle somewhere in the world. But when I figured out how to re-mark the flywheel in the bike on my 1953 BMW R51/3 after having it lightened, I felt pretty smart.

When I had put my engine back in the frame after having a lightened flywheel put into the engine, it started right up and ran. But in-between 5000 and 6000rpm there was a hesitation, so I took a timing light to see if the timing was advanced or retarded.

For those that have never done this, BMW Motorcycle engines are similar to car engines as there are timing marks on the flywheel. OT is top dead center. S is 8.5% BTDC and the F mark is full advanced. You look through a sight window next to the oil fill and with a timing light, you can see when the spark fires at idle matching the S in the window. As the advance moves, you should see the F mark come into the window. When my mechanic transferred the marks from my heavy flywheel to the lightened one, somehow they did not transfer correctly. At idle the S mark was too far up in the sight window and no amount of adjustment would put it where it needed to be.

Because the advance unit is keyed into the magneto, to make an extreme timing change you have to remove the magneto from the cam shaft nose and rotate it the appropriate direction. This rotates the key way that the advance unit locks into. When I did this, you have to re-set the timing statically. So I set this all up and started kicking away. And Kicking and kicking and kicking. This really pisses you off, because just a half hour early the bike was starting with one kick. Now you are standing there and cursing because you start chasing something simple and now you are off into the woods with a bike that will not start.

After walking away, I was thinking that maybe the timing marks are off. This is the only thing in the whole equation that had changed. To test this I took the spark plug out, got a straw and stuck it in the whole and turned the engine over. And to no real supply while rotating the engine the straw continued to move out as the OT top dead center mark passed through the site window. So if top dead center was not where it was supposed to be, then 8.5 degrees before TDC was not either. And if 8.5 degrees is not where it is supposed to be then no start.

A call to my mechanic suggested remarking top dead center, then remarking 8.5% before TDC. The problem is making marks on a round flywheel which you can only see through a inch round window. My mechanic guessed that the window was about 4% so rotate the engine through 2 window lengths would get me near 8.5%. This didn’t seem like it was going to be too accurate.

It came to me as I was waking up. If the piston went from TDC to BDC in half a rotation of the flywheel, and knowing that the stroke is 68cm, then I know that moving the piston 68cm in the bore will turn the flywheel 180 degrees. So if I need to fine 8.5 degrees before TDC, I need to calculate the corresponding distance the piston needs to be in the bore before TDC. 8.5 degrees is going to be the same percentage of 180 degrees that Xcm from TDC is to the total stroke of 68cm. Calculating X would give me the distance in from TDC I had to measure to get 8.5 degrees before TDC on the flywheel.

Get the Head off, rotate the engine until the piston reaches top dead center. Calculate the % of 68cm which is equal to the percent 8.5 degrees is to 180 degrees and move the piston this measured amount down from top dead center and mark the flywheel.

Set the magneto body back on, advance unit with coil, static time it and one kick (well two/three) later it fires right up.

This was a triumph for me in that I was able to diagnose the problem, find the solution, implement to fix and the satisfaction of the engine firing right up. But this was short lived, as putting the timing cover back on, things did not line up and two parts that use to sync and mesh now grind and whine. This is what owning a vintage motorcycle is all about. Triumph and tragedy all within minutes of each other. So next weekend I will be out in the garage again, looking at the problem, making calls, searching the Internet. Hopefully there will be another triumph, one that is a little longer lasting.

1 comment:

  1. Very clever idea on how to get the timing on the fly wheel, but I don't think you did the math right. The crank rotates and that is translated into the linear motion of the piston, so you have to use some trig. I believe the right equation is
    x = 68cm/2 * (1 - Cos a); a is the angle relative to top dead center.

    So the displacement you should measure to get to 8.5 deg is 0.37 cm. The way you describe it, I think you measured 3.2 cm, which would equate to 25.1 deg. Assuming my equation is correct, I did some error analysis to see how off you would be on the angle give a 0.1 cm error on the length. It was very good, far less than 0.01 degree. So I think the method is good.

    One way to think about why the trig equation is correct, is that from TDC, the big end of the crank is moving up (or down) so there is very little translation in the direction that the piston is moving. So a lot of angle is not much in piston motion