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ASP FS61AR CDI Conversion - Q regarding Rich Midrange.

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ASP FS61AR CDI Conversion - Q regarding Rich Midrange.

Old 09-08-2021, 06:50 AM
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Default ASP FS61AR CDI Conversion - Q regarding Rich Midrange.


I've been playing around with an ASP FS61AR of late. It was sitting for near on 10 years and needed a new set of bearings through it (and a couple of other parts) to make it new again given the rust and gum from me forgetting after-run oil on the last run. The liner, valves and rod bearings were all in perfect condition It was otherwise barely run-in, probably 8 flights, so it's pretty much a new engine again.

During the bearing swap I couldn't bear to see the rough edges in the intake manifold and the step from the bore used to make the intake port into the head, so it got a bit of a cleanup with a ball end carbide bit in a die grinder. I'm now pretty happy with the intake air passages.

I also drilled a couple of 0.8mm holes in the crankcase from above the cam followers into the cam case to let the oil mist flow up the pushrod tubes and a 0.7mm hole from the intake valve bowl into the rocker area to let the engine draw the mist back into the engine, that way the breather hole can be plugged and the blowby oil ends up back through the engine.

After flying an Evolution 10GX for a while I was really keen to try spark ignition on the engine, so picked up an RCXEL ignition module, 1/4-32 plug and the hall sensor.

For the trigger magnet, I found a 1mm diameter, 5mm long Neodynium magnet on Ebay and drilled a 1mm hole in the prop driver, gluing the magnet in place with epoxy. It's barely visible on the prop driver and works quite ok - the range to the hall sensor is up to about 3mm with an axial range of the sensor (front-back) of about 2mm. Not nearly as much as with a larger magnet but plenty enough to get a realiable spark without being too precise.

While the bearings were out I drilled and tapped a couple of M2.5 threads on the front bearing housing to hold the hall sensor braket, which I make from a section cut out of some 25mm thin-walled aluminium tubing, with the sensor glued to that with acrylic 2-part adhesive.

I've been experimenting with running the engine on E85/oil blend (currently at the usual 20%/1:4 ratio).

I have no issues at full throttle, it's spinning a 13 x 6 Schimitar Master Airscrew at 9800-10000 RPM static and mixture is more sensitive than methanol, maybe 1/5 turn from losing RPM Rich to losing RPM lean.

Idle is also able to be adjusted nicely, it's happy to tick along at 1950-2050 RPM for minutes on end and then pick up happily after that.

What I am finding though is that the mid-range is all over the shop. Moving up from idle the RPM rises comfortably to about 3.6-4krpm. Opening the throttle further, the mixture gets progressively richer as the throttle is opened further, until the engine is sputtering rich at about 3.6krpm (yes, the revs drop as the throttle is opened at that part of the travel). further opening the throttle the mixture stays rich and sputtery but with the revs increasing until the throttle is opened enough to reach maybe 7.5krpm, where it starts to lean again until it's perfect at full throttle.

I'm not running exhaust pressure at the moment as I found it was worse with it connected and would wander around more (perhaps as the exhaust pressure was breathing in and out of the tank at different load points).

I did think it might have been that the carby-manifold O-Ring had lost its seal but that was replaced without any change to the behaviour.

Given that this engine runs a low-speed needle running in a metering tube with a metering slot cut into it, it seem that an option would be to massage the metering hole profile to tame the midrange. I'm tempted to give this a go but thought I'd see if anyone out there has tried this before and if so what method was used and whether it was successful? It seems like one of those things that could go either way really easily given the small size of the fuel passages.

Any other suggestions on ways to reduce fuel in the midrange? Another thought I have is to have a mixture control servo and map that with the throttle setting, that was I could have different maps for different fuels as well.

Given that the engine is not a particularly expensive unit, I don't know it's worth to pick up another caby for it (the NGH 9cc carby looks the goods but I haven't seen one under USD$85 which is probably what the engine is worth if I'm lucky)..

Just as a final note, the engine did run Ok on 4:1 Methanol with spark ignition (and with Exhaust pressure connected), the midrange was still rich but not nearly so much as on E85, So it seems that it's tied in with the different mixture requirements for the ethanol/gas blend as compared to methanol.


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