Today I was going to do more running in of the engine, trying to bed in the new rings a bit more. But I am confused….

Last time I ran the engine I cleaned the spark plugs, as some of them weren’t firing properly. Some were very dirty, and I assumed that that was as because I had oil on top of the pistons in earlier runs.

After cleaning them, the engine ran fine, and the spark plugs stayed nice and clean. A bit to clean, very clean in fact!!!! Also the engine was getting hot and had issues stopping because of it. So yes running to lean 🙁

So today I dropped the needle in the SU carburetor about 2mm, to get a richer mixture, and yes the engine is running cooler now, but, and here is the confusing bit: Two of the 5 cylinders have very dirty spark plugs, the same ones I cleaned last time (this makes the engine run effectively on 4 or even 3 cylinders).

The spark plugs in the other 3 cylinders are clean, with a bit of a brown “healthy” color. So the mixture they get is right, but its these 2 cylinders that get a richer mixture, or that’s what I think.

I was talking with Nick, who has a plane next to me. and I think he know a lot of engines, think he works a lot with cars.

He was looking at the engine with me for quite a while and there are 3 points he was “worried” about:

  1. The easy one is that the oil I am using in the damper inside the SU piston damper is to thin. This means that the mixture ratio might not be constant because of vibrations of the carburetor while the engine is running. I need to put SAE30 oil in there.
  2. The 2″SU carburetor I am using, if I remember well comes out of a 4L Jaguar engine. So same engine volume, but that engine is running up to 2.5 times faster, so would be drawing over twice as much air (faster air flow). But also there are two carburetors on that engine so the overall mixture speed would be similar, so maybe nothing wrong here…
  3. The pipe in between the SU carburetor and the engine is 2″ as that’s the size of the carburetor. But because the engine is only 1.5″ we had to reduce the pipe size. This is done just underneath the engine. Nick’s concern is that the mixture might be swirling a lot, going through that pipe and through the reduction, and as a result when the mixture enters the engine, more mixture might be getting into one half of the engine and less into the other half. Not due of this is correct, but sounds feasible.

Hope someone has some interesting comments on this !!!

1 thought on “Confused

  1. Hi Gert. This is way out of my league and pardon my ignorance —-but if I’m correct in thinking the air/fuel mixture travels to each pot via the collector ring, is it possible the cylinders furtherest away from the induction point might tend to run leaner? Would suggest two induction points to balance things out. On the other hand, radial engines have been around for so long you’d assume such issues had been accommodated….. Good luck, folk are rucking for you!! Rgds, Neville.

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