Starting troubles

Ever since I have installed the Ms2 board I have had some problems starting the engine.


The characteristics are that if it doesn't start straight off, then it requires a bit of cranking. Sometime it hicups a bit. Once it kicks then heavy application of the throttle gets it going, but it isn't so nice. The battery is excellent, so that isn't the problem.

Not sure why, but the Ms2 has loads of nice tools for investigating this stuff. It seems (from the msextra forum) that the Ms2 is cleverer but consequently more temperamental.

I logged the trigger wheel which was interesting. Basically it is a 11 tooth wheel, one tooth every 30deg and one missing (a so called 12-1). This runs from the crankshaft (not the camshaft, which is why I run wasted spark ignition).

When running the time between teeth look like this:



This is exactly right, as the long gap is after the missing tooth, and all the other teeth are the same length. This works perfectly with the MS as it is looking for a gap that is more than 1.5x the length of the previous gaps, and then the gap after this long (missing tooth) gap must be 75% of the long gap. All syncs up nicely and everyone is happy.


But, what does it look like whilst cranking?



As you can see when it is cranking the gap between teeth varies through time. This is all to do with the forces required to compress the gas in the cylinders. Note there are a couple of things, firstly you can see the 4-stroke nature of the engine as alternate crank rotations have different profiles. I think this is because I have different compression on the frontmost cylinder, which results in the engine running a bit faster at that point.

Note also later on how the sync is lost (sync is the blue curve along the bottom, 1=synced, 0=no sync) after the engine fires. In this case the long gap is still seen but the gap after it is not.

The problems are due to the low inertia of this engine, the small number of teeth, and the high and variable compression. I already knew that I needed an engine rebuild, but here is more evidence.


Incidentally I ran a compression test.
12 16 10 12 (where 12 is the one that was previously down at 8). I don't know what is going on but I was already planning an engine rebuild so its not a big deal.