Brad The Bike Boy: Keihin FCR main air jet test

Wednesday, December 12, 2012

Keihin FCR main air jet test

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Today was Minnies last day of rego, and at this point of time I'm not planning to re-register her for the minimal use that she gets. But I had one test I wanted to try with the FCR, and that was larger main air jets.

The idea is that the main air jet sizing wil influence the shape of the fuel curve. Read Patick Burn's tuning guide for a much better explanation. I have found that with the FCR on Ducati engines you often get a mixture that tapers rich as rpm climbs. Specifcally it seems to be with larger carb size to engine capacity ratios. Or, more simply, 41 on a 900 as compared to 39 on a 900, or 39 on a 750 compared to 39 on a 900. Comparitively, the 900SS I fitted FCR39 to gave a relatively flat curve.

The FCR for Ducatis come with a 200 main air jet, the largest Keihin make. So the assumption was I could get some 200 and drill them out. As you do. With my jet drill kit the 200 was between 1.90 and 2.00mm, and I drilled them to 2.10mm. I also went up to 165 main jets from the 158 that I had fitted previously on the assumption that I'd need more fuel. Turns out, perhaps, that the reason the biggest main air jet available is 200 is because that's as big as the carb will respond to. That's the theory I'm running with at this point, based on tody's runs.

If I told you the graph below was for 158 mains in red and 165 in blue you'd probably believe me. I would. Same shape, just richer. So, no joy in main air jet land, but at least now I know. And that has to be worth the effort.

Now I have to put the 158 mains back in, and probably some undrilled 200 main air jets, just to be sure.

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