I did some adjustments on our automatic altitude mixture carb, so no manual mixture here, with a little help from an O2 lambda sensor. After setting a leaner, more optimised mixture at part throttle we got a bit more mag drop than before and that is reproducible for checking. So this shows that a leaner setting has same effect like a very rich condition.
VicOK, yes, thank you all. I didn’t read that Lyco paper properly. What they are saying is that if the mag drop is excessive when tested as per POH (i.e. full rich) it is ok to do the mag drop test with the engine leaned to peak EGT.
I now understand the reason.
It is certainly true that my current fuel servo is set to a richer mixture (about 26 GPH at full throttle/max rpm) than the original one as supplied by Socata (23 GPH). I had this change done some years ago to help with CHTs during climb. It is a standard thing on TB20s… they all came out of the factory with the RSA5AD1 fuel servo set to the bottom of the allowed range and probably below it, resulting in climb CHTs easily reaching 420F on a hot day. Nowadays I see 370F max.
Hi Peter, I’ve been battling with high takeoff cht on my IO360-m1 for a while and I didn’t do much about it during engine break-in (beside not climbing steep). After break-in I checked magnetos timing (who were slightly advanced) and idle-mixture (who looked slightly lean). The idle-mixture had the largest effect on cht who dropped 20F but not sure if winter OAT played a role too. Takeoff fuel flow has hardly increased (maybe from 15.3 to 15.5 gph). My question: is the idle-mixture “star screw” the only regulation to increase takeoff FF or there are other ways (as you seem to impy in your last post)? I know for carbureted engines people change the “jets” but what is the equivalent in injected engines? If I have a new “degree of freedom” to regulate the takeoff FF, then I’d like to look again on how optimal is my idle-mixture regulation: I’ve read many contraddictory ways on how to regulate the idle-mixture and nobody seems to agree on one method (either you look for 250F egt drop from peak egt to full rich in cruise, or you look for 50Rpm increase at idle just before cutoff, or you seek to maximize takeoff FF to minimize chts, and many others).
I don’t know where it is adjusted on the Bendix fuel servo, but I do know it is done by the overhaul shop. The shop puts the servo on a jig which blows air through it at a given mass flow rate and he tweaks it for the right fuel flow. There is a tolerance band on that, but you can ask for it to be set at the very top.
This has no effect on cruise, because you just set peak-EGT or LOP, so the above adjustment is irrelevant to MPG etc.
The “easy” adjustment on the outside is for the idle fuel flow i.e. (indirectly) the idle rpm. This can be set to what you like; if you always lean on the ground then it should be set to say 600-700rpm under that condition. If too high, you need more runway to land 
A new engine will run high CHTs – perhaps 40F higher than 100hrs later. You can almost see them come down over consecutive flights.
To be sure I understand, if I wanted to increase my takeoff FF from current 15.5GPH to 18GPH (which is reported by others having my same engine), I should just keep turning the idle-mixture “star screw” but I DO NOT need to buy any new hardware part to install/replace on the engine, correct?
I don’t know where the full power FF is set. It is not done with the idle FF adjustment, AFAIK.
It is nothing to do with the engine; it is done wholly in the Bendix fuel servo.
Sorry; that’s all I know
I always get these done by a specialist in the USA (don’t have the name right now but can get it) and have a spare RSA5AD1 one on the shelf at home.
You can check if the fuel pump works as it is supposed to do. I experienced an issue with an airplane where we had similar symptoms and we also saw a lower FF than normal. We changed the pump and then the issue was gone.
Peter wrote:
The magneto is the D3000 single shaft dual type, so nothing to adjust there except the overall timing.
You can actually adjust the timing of one magneto vs the other (by changing the position of the points, if I recall correctly) .It is kind of an art as you are affecting max point gap as well as timing. I am away from home now but can look up the manual procedure for you when back home if interested. In FAA land you can only do it under A&P authority, and it is not an easy one to do for someone that has not changed points in a dual mag before.
Peter wrote:
The “easy” adjustment on the outside is for the idle fuel flow i.e. (indirectly) the idle rpm. This can be set to what you like; if you always lean on the ground then it should be set to say 600-700rpm under that condition. If too high, you need more runway to land
On the RSA5AD1 You can adjust idle RPM (throttle stop) mostly independently from idle mixture (threaded rod linking throttle to idle mixture valve lever). While a very rich or lean mixture have an ompact on idle rpm, they should be adjusted as in the following link. None of these adjustments have any impact at high throttle openings.
Fly310 wrote:
You can check if the fuel pump works as it is supposed to do. I experienced an issue with an airplane where we had similar symptoms and we also saw a lower FF than normal. We changed the pump and then the issue was gone.
Silvaire wrote:
The flame front moves slowly with over rich mixture, as with over lean mixture. With non-optimum mixture, twin plugs is equivalent to advancing ignition timing (the flame front has to travel half the distance) so single plug operation is like retarded spark. With better mixture the combustion does not benefit so much from twin plugs because the flame front speed is high enough to match the bore size and ignition timing.
I like this explanation
Indeed, you can adjust the two halves of the magneto but that adjustment is inside it, and should normally not be required if the mag has been overhauled competently and has not been damaged in transit (they are shipped without the cover which contains the ignition leads).
Rotating the whole magneto changes the overall timing. One just needs to change the gasket underneath it each time one does that
For the full power adjustments of the RSA fuel servo, the info may be here.
Peter, thanks for the link but many people consider rsa manuals half garbage (see post 16 here: http://vansairforce.com/Community/showthread.php?t=117313&page=2).
I followed their technique but depending on the speed of cutoff (and I talk about almost unmeasurable differences) you can get an rpm rise as big or as small as you like, almost. Idle mixture and wot FF regulation are the most controversial issues I found so far in aviation, no mainstream agreement seems to exist! Even my local engineer who worked 40 years on both army pistons (sf260) and various aeroclub trainers seems to agree that is a controversial point.
