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-   -   OS Fs 91spIII W/pump CDI / nitro conversion. (https://www.rcuniverse.com/forum/engine-conversions-92/11688441-os-fs-91spiii-w-pump-cdi-nitro-conversion.html)

Steve1971 03-10-2021 12:36 PM

OS Fs 91spIII W/pump CDI / nitro conversion.
 
So, I have put an rcexcel CDI system on my OS FS 91 spIII W/pump on nitro using a 1/4 32 plug and 10% nitro? Itís timed at 30* and runs for a short time and die. The engine is inverted and the low speed needle seems to be in the zone of good idle but I havenít done a full run up. Please help?

Steve1971 03-10-2021 12:37 PM

Morris mini motors
 
Iím also using the sensor and ring magnet from the above company made specifically for this motor.

jakobsladderz 10-13-2021 02:35 PM

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 1/2-1ļ 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 1/2-1ļ 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..

1967brutus 10-20-2021 06:11 AM

A few things to consider: Glow engines use their mixture strength to "control" ignition timing, and the difference between a good running glow, and a poor one, are in the carburation, plain and simple.
It does not work that way when you convert to spark, not even when running methanol as fuel. The entire characteristics of an engine change.
Because of that, glow carbs tend to have a fairly rich idle mixture and a fuel linearity that creates the required ignition advance. The bit of fuel "loaded up" at idle also serves a purpose as "acceleration booster" (a bit like an acceleration pump).

On spark, best advise I can give is to run a timing of appr 32~34 degrees BTDC, and set idle at the absolute leanest it will continue running. Forget the pinch tests as you knew them for glow engines (a few seconds of running, while speeding up) and forget throttle response. They are a thing of glow, they do not belong to spark.
Idle mixture is correct, when a pinchtest results in an immediate stop without any increase in RPM.

Throttle response will now be apparently very bad, and this can be solved by reducing the servo speed on the "up-stroke" (down stroke not necessary). Done properly, you won't even notice the reduced servo speed, the engine will more or less spool up like always.
2~5% Nitro help to make it easier to find these settings, more is unnecesary, unless you need the power.


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