Brad The Bike Boy: Keihin FCR main air jet test: post 2

Friday, December 14, 2012

Keihin FCR main air jet test: post 2

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I was going to put more info in the other night, but it was late and I had work to do.

The graph below shows three different bikes all running the same 152 mains (just can't get rid of them) and how the curves taper (or don't). Blue is M750 with FCR39, Red 900SS with FCR41 and green 900SS with FCR39. The green curve is by far the most consistant.

It also seems that the richer my M750 gets, the less consistant the WOT mixture becomes. In particular, the more prominent the (comparatively) lean peak at 7,200 becomes. Blue is 152 mains, red is 158 mains and green is 165 mains. Also, the three step richer change from 158 to 165 has a much bigger impact on mixture than the two step 152 to 158 change. I have done a few FCR main jet changes that have struck me as having less effect than I anticipated.

Although, the Mikuni 600 and later 900 carbs I had previously been running on Minnie could also show a lean peak at similar rpm depending on inlet and exhaust configuration.

I also ran some part throttle checks. Initially when I fitted the FCR I tried 52 pilots, then went down to 48 after the first ride and down to 45 before the last dyno runs to try to cure the 1/8 or so throttle richness. It's possibly still a little on the rich side. I have left the idle mixture (slow fuel) and slow air screws at their spec settings, just because they seem to be a bit more stable that way in my limited experience. And some playing with them didn't seem to help.

The next graph shows 1/4 throttle, which has remained surprisingly stable. I say that because both pilot and main jets have been changed, and I'm told that the main does have an impact on part throttle. So I'm not sure if the return to smaller mains will have an impact on the 1/4 throttle mixture. Hopefully not. Blue is 52, red 45.

1/2 throttle shows a richer mixture with the larger mains, blue 158, red 165.

Similarly, 3/4 throttle also shows a richer mixture with the larger mains, blue 158, red 165. The sudden step at 4,000 rpm is a bit wacky too.

Given Minnie is now unregistered, I won't get to try out any more changes. But I think I'll set the FCR up with 160 mains instead of the 158 and leave all else as was. Hopefully the jetting will work ok in an SS, as that's where the engine is theorectically going.

She was running very nicely, although the 1/2 throttle wheelies when taking off from the lights were getting a little too easy and frequent. Started well too with the "two pumps, wind in idle speed screw, press button" procedure.
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