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That changing the main jet doesn't alter midrange acceleration much is no surprise to me. One of the functions of emulsion tubes is that their diameter combined with the fuel level forms a reserve of fuel that has already gone past the main jet.
Paul, you've lost me. The only way fuel can enter the airstream in the main circuit is through the aux venturi, and the passage to that is solidly connected to the main jet. Nothing gets there without going through the main jet, and the main jet leads nowhere else. How do the e-tubes get fuel that's already gone through the jet?
It's this reserve of fuel that controls richness during the early part of acceleration & the main jet has no effect until the reserve has been spent.
Yes, I do understand that the fuel already in the tubes is a reserve until the aux vents start drawing, and that emulsification doesn't take place until the reserve is at least partially expended. Tube diameter and hole configuration should have a large impact when the throttle is first opened. Our first course of action to correct the midrange was indeed to try all the different tubes we had... and it had surprisingly little effect. Nothing much did on that day.
To determine the main jet size, load it up on the dyno & hold it at say 4000rpm & pick the jet that gives the correct mixture, repreat this process at 6000 for the air correcton size. Hold it at 4700rpm & adjust the timing. You now have the correct main jet, air corrector & ignition timing for the worst conditions. Any richness, leaness, backfiring, etc is the result of the wrong choice elsewhere.
This was not our actual approach, but in a sense we did just that. Note that these were long runs against a heavy load. While we didn't load to produce steady rpm, the climb was slow -- we had lots of time to observe what was going on around any particular rpm. The tube's fuel reserve was surely out of the picture for 95% of each run.
Andrew's emulsion tube selection guide makes no sense at all, selection of emulsion tubes is based on the level of modifications to an engine not it's capacity.
Agreed. I mentioned that site with a sense of irony that I ended up with exactly what he suggests. His formulas don't work at all for the much milder motor in our 122S.
As you would know the main jet & air corrector determine the mixture during full load/no acceleration. The emulsion tube determines richness during acceleration.
No, I don't know that, and it's not in accordance with all the literature I've read (all of which was borrowed and since returned, so I can't cite specific passages). The rough summary is to select 1) the main jet that produces the quickest acceleration, 2) following that, the AC that produces the fastest top speed, and 3) the tube that pulls cleanly as the throttle is first opened.
If you are overly rich at low to midling speed, changing the main jet is not what you would do, you should first change to a tube that has no low holes.
That makes perfect sense to me in theory, but the fact is that the F7, which has a ring of midway-down holes and a low hole, produced a leaner midrange than the F16, which has only a ring of high holes.
If you still have excess richness you should then go to thicker tubes.
Thicker as in having greater wall thickness and less fuel reserve? Again that makes sense. Unfortunately I have no chart of tube dimensions. If you could point me at one, I'd appreciate it.
If F16's are giving you grief by being too lean higher up, you should probably reduce the size of chokes. 38mm would be better for your requirements & still outflow anything that a 45mm DCOE can do.
We did put ~161 corrected HP on the ground (without attempting to tune for max HP), and I don't think we'd do that with 38mm chokes.
The fact is that the F2 tubes produce the best drivability (these are also what the Des Hammill book recommends for engines of a similar displacement and state of tune as the MPPE). With much larger ACs than we had, it's finally extremely well behaved. I ran it up over 6500 several times yesterday and it certainly doesn't feel like it lost any power up there, while the midrange is clearly much stronger, and throttle response is infinitely improved.
It may not be logical, but I'm going with it for now.
8^)
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