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Scale RCM 1/3 Scale Champ Build


More Trials and Tribulations with the Champ. Put new plugs in last Monday, flew four flights on Monday and 3 flights on Saturday, started to miss again on Saturday. Sunday morning first flight starts missing again. Land and replace the left plug that is black with the plug I removed from the right side on Monday. Sky was bad so I waited about 2 hours for clearing skies. Ran fine for a while, missed a few times, land and notice it is idling ruff, never did this before. Pack up and go home. Wake up this morning and decide I need to remove the left head and clean the exhaust valve. All goes as planned and reinstall the engine. As I install the prop I notice one cylinder has no compression. Take the plug out of the left cylinder and now there is no compression! OK remove the right exhaust pipe and plug and with compressed air determine that the right exhaust is leaking. Take the engine back off and clean the right cylinder exhaust valve. OK I'm going to 50 to 1 ratio oil using Sthil Ultra pre Roto Website. Get to the field about 1330 hours, set the high speed needle, turning 5850, more than ever before, feeling real good. Take off and about 8 minutes in the plane starts to miss. Must be a bad ignition. Land and feel the ignition box, it's hot and mounted in the cabin, everything is cool.


Call CH Ignition, speak with Adrain. He says it's getting hot and move it to the other side of the firewall, I said it's already there and the ignition box is the only thing inside the cockpit that is hot. New ignition is on order. Hopefully it will be here before the weekend.


640cc Uber Pimp
Since you said the left cylinder is getting more oil than the right, the left cylinder has no compression, and the hot ignition box makes me think of a few scenarios.
1. Ignition failing due to temperature and lack of airflow cooling. Install a blast air hose from the cowl inlet baffle for cooling ram air. Can you swap ignition leads to see if the problem follows the lead?
2. Left cylinder ring never seated or ring gap is inadequate. If it were a full scale plane I'd say the oil scraper ring is backwards. If the ring has any sort of taper to it - that would pull oil into the compression chamber like a check valve

Hopefully the new box fixes it, but I would make some sort of fresh airflow to it so it's not stagnant


It is cool inside the cabin where the ignition module is located. Flew the plane yesterday with the new ignition, first flight about 8 minutes in picked up a miss. New plugs, new ignition module, what's left? Hall sensor or the Aerotech IBEC. Before flying I had installed a voltage sensing wire on the ignition side of the power wire. The IBEC output seems to get a little noisy after 6 minutes or so however I am only collect data at 1 second intervals, I have reduced this to .1 second. As a second test yesterday I installed a 7.4 volt 2200ma Lipo on the ignition. flew the plane for 8 minutes without a miss. I plan to repeat these tests today while collecting data at a higher rate. The ignition module is 6.0 - 12.0 volt input, the IBEC installed is the lower voltage unit with a maximum setting of 6.6 volts, the output is less than 6.6 volt when connected to the module and not running. Somehow the scaling got messed up yesterday so my absolute values are off about 25%, but I suspect that the voltage may be dropping below 6 volts during the transients. Tech Aero does offer a HV IBEC that goes up to 8.0 volt.
I don't know how these Rcexl ignitions are effected by input voltage, is 12 volt better than 6 volt? I know that Bill Carpenter recommended running his ignitions on 4.8 volt and using 6 volt was no benefit. I never experienced a failed Hall sensor so I don't know how they fail, I don't have the little box to set the timing on these newer ignitions nor a piston stop so that will the last thing to replace.
Thank you for your thoughts on the matter.

I just finished checking the voltage sensor setting and it appears as something is failing inside the IBEC. What I thought were incorrect scaling was actually correct. On Friday when I installed the ignition and voltage sensor, the IBEC was putting something less than 6.6 volts. This morning it is putting out full lipo voltage same as my measurements yesterday. Don't plan to try the IBEC and will order a new HV unit.
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OK the first flight was about eight minutes long, not a single miss, even the fellow fliers commented. Need a new HV IBEC.
The second flight was not so good. Been practicing scale like takeoffs as I hope to fly in a scale contest someday. Taxi out and stop on the centerline, advance the slowly accelerate the plane while the tail is flying, then allow the plane to gain speed and liftoff will very little elevator input. The right wing normally drops slightly after takeoff, make the correction and slowly gain altitude as the plane wangs it's tail . Most of the flight is with throttle at half, just a slight increase will gain altitude. As the plane took off it when left which caught me off guard, over corrected and began a pilot induced oscillation ultimately resulting a stall. The plane was only 30' high. Broke the left wing bow, no damage on the fuselage or right wing. I had previously planned to recover the wings since I didn't care for the screws I had used and already have replacement screws here so I'm not feeling too bad, more embarrassed I made such a stupid mistake.


Not a problem, and yes all mu future builds will incorporate voltage monitoring downstream of the IBEC. This is my actual second failure, last year I had an IBEC that provided expected open circuit voltage however dropped to 1 volt when plugged into the ignition module.

AJ Sun

New to GSN!
The later, one side fires, the second fires 1 rev later. Not sure how the original ignition worked, the pickup was on the cam gear. I assume there are two magnets. When I sent it to CH Ignition for conversion, a magnet and hall sensor were installed on the crank nose. So it is working in a wasted spark mode. On another site I ask Adrian why it was done this way and his reply is the timing is set different with the stock pickup.