Compression is a function of piston swept displacement and cylinder head volume. Well, valve timing too. But let’s not go into that right now. If you add or increase cylinder head volume, the compression ratio goes down and you make less compression. So, to get accurate pressure numbers the apparatus you use for the test must either add minimal volume to the chamber or have the release valve physically very lose to the spark plug hole, or you can expect lower than book values for your test, as they used an apparatus intended for small engine displacements.
069 a carbs came stock with solid tipped pilot screws and have a different taper angle than the hollow cross drilled screws used on other models. These are either set to factory numbers along with all the other stock components attached to the engine. Or, they are adjusted for correct response from throttle twist under engine load to achieve reliable, if sluggish, pick up in any gear with up to one half throttle twist. These screws adjust idle mixture over-rich just enough so that when the slides are lifted and vacuum is lost, causing less fuel drawn from all carb jets, the motor has enough residual fuel to increase rpm and restore vacuum draw from the jets. The cross drilled pilot screws are a limiter on just how rich you can make the idle mixture. As Honda knew that if you make the mixture too rich, the spark plugs will carbon foul from long idling periods. The 069a solid tipped screws were an attempt by Honda to allow fine tuning of the idle mix to allow passing the EPA demand for low idle emissions required in 1978. This failed, because there is no provision for an accelerator pump to squirt gas into the carb throat to over come the unavoidable leaning caused by raising the mechanical slides. It is also one reason why there are no 1978 F model 550s, as they could not meet 1978 EPA requirements without replacing the carbs. 069a carbs were only used on F model bikes from Honda.
