Hsr48 tuning

[txchop]

Hi Mike, love the site.

Tuning a 117ci motor right now. It has a ported heads, 11.4 compression, tw9fg cams, Mik48 with S+S "d" manifold. Also running a boarzilla. I just swapped cams from tw9bg's and now am running the f's. With the 9bs the bike tuned in really nice.130/136 sae 4th gear. Flat and very controllable afr line and responsive to jet changes. Now that i am running the 9fg the carb seems less responsive to jet/needle changes and is running a bit lean down low, and goes really rich after 4k rpms. Tried jet changes and moving needle clip and it seems to remain the same afr wise.

Any thoughts on tuning tips? Maybe the bagger boarzilla and the 9f's dont get along? Maybe the mik isnt the right carb for the job anymore?

Thanks Mike!

[Mike]

The Mikuni would be my choice for your combo for street work, maybe the S&S D for track. I'm surprised the added lift of the F cam has made that big of a change in carburetion. Were the heads modified in some way to accept the F cam? Raising the valve seats to accomodate the high TDC's can often have adverse affects on port flow. This is why we've actually had better results with the B cams.

Changing needle position should not have a large affect on WOT runs but the main jet should. Set the needle based on part throttle dyno step tests, then use WOT runs to get the main to come in. Make sure your dyno O2 sensor and pump are clean and working correctly. We've been bit a number of times by this. Also check the accelerator pump rod to make sure it's moving freely. Sticking is common for Mikuni's.

I would stay with the BZ, should work well with the F or even more cam. You may have to go back to the B cam, bigger is not always better.