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When you add a shim or decompress the head, you will have to turn the needle in...leaner... >
Randy, why would the fuel/air ratio change?
I'm not Randy, but you lose power by reducing the compression, which will require you to run closer to the mixture closer to the ideal to compensate, hence, leaner.
Most of what I hear people talking about with head shims is backwards, and this sort of stuff is becoming a lost art to some extent, since 4-2 break motors are rarely used for serious competition any more. You can still use the compression to adjust a piped engine, but you don't necessarily have to, so people have sort of lost the bubble.
more compression: more overall power, bigger power difference between 4 and 2, faster transition from 4 to 2 and back, cooler (until you go over the top...)
less compression: less overall power, reduced power difference between 4 and 2, and slower or delayed transition from 4 to 2 and back, hotter. In particular, it will tend to hang in a 2 longer due to thermal effects of running hotter in the first place. The ultimate expression of this is putting in so many head gaskets that it never transitions back to a 4, hence, runaway
I have found most of the piped engines I have used to lose power rapidly with compression changes (leaving everything else the same) but don't gain much in terms of 4-2 power difference, basically, you are only reducing the 2-stroke power only slightly faster than the 4-stroke power. That has made me try to control the 4-2 power break with the pipe and the prop, or, once we got 61s and up, run it in a 4-stroke all the time, pump it to the gills on compression, and not worry about the transition effects.
That's not what generally happened with many 4-2 break motors. Reducing the compression tended to reduce the 2-stroke power and either do nothing, or increase the 4-stroke power, so slight compression changes would significantly effect the 4 to 2 difference but not hurt the overall power very much.
Brett