I understand your logic here but in my experience, it is surprisingly common to see multi-carb engines read differently at higher rpm when tested with a high speed/ high vacuum synchrometer. Many people do not undergo the test with the presumption that if it is synced at idle, it will be fine everywhere. There are many simple mistakes or issues that can cause this, not always major mechanical issues (although this can be true in some instances).Caprimaniac wrote: ↑Sat Oct 05, 2019 5:36 amIf you have unsynced barrels at 3000+ rpm, the shafts or bores or blades need to be faulty. OR, the different cylinders might be different regarding how much it sucks….. based on cam lobe, valves, ignition…… alot of different factors that can vary. Either the carb or engine need to be fixed, at a larger scale...
1) 4 webers synchronised to 6hg at 1000rpm. The throttle screws are holding the blades open just enough to let in a trickle of air. However the linkage on one carb has some very slight slop in the rose / ball joint, which causes it to slightly trail the other 3 carbs. At 3500, this carb will read 3-5hg lower depending o the level of slop.
2) On the same system, one lever has a slightly shorter adjusted travel, which has changed the linkage geometry- the carbs that are actuated by the shorter lever will open at a faster rate and subsequently hold the carb throttle blades in a different position to the other carbs at higher rpm- this would synchronise fine at idle though.
3) Again, 4 weber system- one carb has one leaky butterfly (common). The leaky butterfly bore reads 9hg instead of 6hg like the others. The tuner drops the idle of the affected carb to make the leaky throat read 6hg (like the other carbs) and opens the air bleed on the sister throat to bring the companion throat UP to 6hg (as this would not be 4hg or thereabouts). The issue here, is that you now have the affected carb reading perfect synchronisation at idle, but in fact, its throttle blades are in a more closed position than the other three carbs (one throat has a slight but fairly common mechanical fault and the other has an intentional air-leak introduced). Basically this carb's throttle blades are now trailing the other three carbs, if tested at high rpm, you will see a mismatch between the 3 'normal' carbs and the fourth affected carb.
These are just some quick examples of how mistakes can be made, or slight issues can be overlooked when setting up multi-carbs... all of the examples given are quite easily fixable/ able to be tuned around but its amazingly common how frequently synchronisation at mid-rpm is not done nor checked. Some think I am pedantic when aiming to get every throat on multi-carb setups working 100% in unisyn, as quite often the cars can drive 'ok' even if they're not pulling the same air at mid-rpm, but when corrected they always drive better.
Another tell-tail sign that issues can exist with linkage geometry, or wear in linkage ball joints etc, is to hold the throttles to wide open (engine off) and see if all carbs are hitting their throttle stop at the same time. Quite often you will be surprised at how far a carb can be trailing the others (sometimes this can be due to other issues like slight differences in carb bodies etc but its a fairly good indicator to check everything over before proceeding to sync at idle and again at 3500).