Ducati 250 single ignition

I have read that the 250 and 350 Ducati singles are very fussy about ignition timing. They need to have about 36 degrees of advance when running at higher engine revs, but about 5 degrees static. The reason that the spark fires so far ahead of TDC is so that the fuel mixture has time to expand at the correct point in the engine, where the maximum pressure from the explosion will do the most work. At low speed it needs to be nearer TDC to allow the engine to be kick started.

The system is an old fashioned points and condenser with a centrifugal advance mechanism that should give aboutĀ 31 degrees of range. There are no timing marks on these engines so a bit of work is involved in setting the timing with the engine static. There are some good explanations of how to do this on the net, notably this oneĀ http://www.liebold.com/timing/timing.htm and I thank the author for sharing it.

I found that after setting the timing correctly, the bike would start but the performance was dreadful, as if the timing was wrong, so I adjusted it by ear and got it running better. It still was not correct. The little bike could only achieve 95 kmh (55 mph). Further investigation was needed.

I took the two bolts out of the points cover and examined the parts behind with a bright light and noticed that the wire on the condenser was attached with a single strand. I removed the points plate to resolder the wire.

The picture above shows the points and advance mechanism housing with everything removed.

And this picture shows the wire that should be attached to the condenser.

Now I have been soldering all of my life, off and on, and it is not difficult to do, but this job was a failure. There was no way that I could get solder to stick to the stub on the end of the condenser, so a new condenser with wire was required.

Behind the points plate is the advance mechanism, it is removed by taking out the small bolt in the middle of the cam. So out it came. I noticed immediately that something was not right with it. There was too much play in the thing. The springs are not stretched but are loose. This means that in the static position it could be either 5 degrees or about 12 to 15 degrees of advance without the engine even running. This could mean that the bike only had a total advance range of about 20 degrees.

The bottom picture shows how there should be no gap (red arrow) but the spring appears loose (blue arrow). This is the position that you would time it static.

The upper picture shows that the gap in the weights (red arrow) would be about 7 or 8 degrees different still with the engine static. The springs (blue arrow) are now touching the post but have no tension on them.

The best solution to this mess is, in my opinion, a nice electronic ignition system that does away with points and condenser, and replaces the advance mechanism with an electronic version. That system has been ordered and will be the subject of the next blog post.