RCU Forums - View Single Post - OS Fs 91spIII W/pump CDI / nitro conversion.
Old 10-13-2021, 02:35 PM
Join Date: Nov 2011
Location: BallaratVictoria, AUSTRALIA
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Did the engine run well with Glow ignition before the CDI Conversion? I have converted a smaller ASP 4S to CDI and running on glow fuel it ran Ok (rich in the midrange, but it was anyway). It certainly helps to start the conversion with a known good engine. My initial tests were also a bit funny, with lots of misfiring and low power, then I found I'd put the cam in one tooth out! fixed that and it was a different engine. Fortunately the valves remained clear of the piston so no longer term damage..

A few things to consider:
- You won't need to run nitro in the fuel blend, it helps a lot with glow ignition but with spark it's not necessary and will make the fuel cheaper and reduce the amount of corrosive products in the exhaust. That said, it won't hurt performance either and may make tuning at the initial stages a bit easier (broadens the mixture range a little).
- Mixture adjustment on spark won't give the same RPM changes that you see with glow engines, as the timing is fixed by the CDI System and magnet placement. I was seeing maybe 500RPM change from rich (but not misfiring) to maximum. It will misfire if very rich. Lean the power will drop off and you risk overheating/knocking. In fact the best indicator of mixture was the amount of smoke in the exhaust, you want a bit to be sure there's enough fuel getting in and it does help to have the plug-in Tacho module.
- Try getting a few more revs up to see if it's better at higher RPM. This will give the engine a chance to catch again if there's a misfire. I was able to get a 2000RPM idle on my 61 once it was tuned in but it will misfire and stop even idling at 3000 if the mixture isn't correct.
- Check the spark quality (take the plug out of the engine and put it in the cap, move the magnet past the hall sensor and check that there's a solid spark). I did have an issue with another engine (EVO 10) where the provided plug didn't give a good spark. I swapped the plug and then had no problems.
- Timing can affect the idle quite a lot. Less advance makes the idle less sensitive but makes less power and more heat at full throttle. Again, with a tacho you can play around a bit, move the timing 1/2-1 each step and test, checking the full throttle RPM to find where it peaks to tune it in. Keep the mix a bit rich and once you find that it's ok, take it back down a 1/2-1 to give some margin on detonation. That said, with methanol and a lower compression there's not too much risk (it's more likely on gas due to the lower octane rating). On my engine the muffler ran very hot with the ignition retarded (instant burn to my finger when I accidentally touched it). Once the timing was dialed in it was possible to touch the muffler briefly without getting burnt..
- Make sure the voltage to the CDI module is within the manufacturers specs - a lot of posts I've read show that as the root cause of poor performance and bad engine behaviour.
Hopefully that's enough to get a bit further with your conversion..