With the vacuum leak fixed, and with manifold pressure and temperature sensors, I was excited to try another test run. Unfortunately there is no video this time.

The short story is that it runs really well. Static full-throttle RPM is a tad higher than it was with the leaky plenum, about 3230, but more importantly it now idles perfectly fine. Rather than running lean as it was with the leaky plenum, it’s now quite rich.

Let’s look at some data:

This plot is like the one from the first test, showing the exhaust gas temperatures for cylinders 1 and 4 (the #2 EGT gauge is having a bad day and need to be replaced, so I’m using #1 instead) along with the fuel flow. The x-axis is time in seconds where zero is approximately onset of leaning, so negative times is full rich.

The solid lines show the current, non-leaky plenum, and the dashed lines the leaky one. It’s obvious that the current run has much higher fuel flow at full rich mixture than the leaky one, and correspondingly lower exhaust gas temperatures.

When leaned, both runs end up at similar fuel flows and EGTs. This isn’t surprising since you lean until the engine starts running rough and then go a bit richer, so you should end up at the same fuel flows. (Those fuel flows happen at quite different mixture settings in the two runs, however.)

The instructions for setting the carburetor say that full rich should have about 90-100F (i.e. 55-60C) colder EGTs than the lean letting. In the present case, the difference is more like 70-100C, with the back #4 actually having a higher difference, so appearing to run richer than the front cylinder now. In any case, the engine is quite rich. At idle, it also had to be leaned almost to cutoff mixture to run cleanly.

The full throttle manifold pressure was about 97kPa, within a few percent of atmospheric pressure, so the air filter can’t be a significant restriction. After warming up, it would idle at a manifold pressure around 37-40kPa, which is quite low compared to the motorcycle which idles at about 60kPa. This is presumably the difference between having a nice big plenum to even out the pressure.

The manifold temperature was kind of interesting. When idling, it showed 28C, pretty much ambient temperature, but at full throttle the temperature dropped to 17C, more than 10C below ambient. This must be due to the evaporation of the fuel droplets in the plenum. This makes sense, since I had already noted that the stock intake pipe had condensation on the outside after running the engine. I’m just surprised it makes that much of a difference.

So this is very promising. The carb needle needs to be adjusted to lean the mixture a bit and then we’ll see if we can make a more thorough check of the mixture distribution.