Hi All,

Quick question - I've had a quick hunt through the forum but couldn't find a definitive answer.

Just about to convert a Saito 125 four stroke to CDI using the RCCDI unit. The instructions suggest a timing of 28deg BTDC, however this is for gas/petrol as the fuel. I'm going to be running glow fuel (my normal 5% nitro, 20% oil mix) and was wondering if this means a slightly different timing would be better*.

Cheers,

HS.

* I can envisage that, for example, burn rate differences between the two fuels might mean a slightly different timing.

With you using a better fuel than gas you can bump up the timing to around 30 or more degrees.You might want to play with the timing to see what works best.

Yes you can run more timing even on gasoline than the recommended 28 but there is no power gain. Alky burns just slightly faster than gas but you can certainly try a little more timing lead to see if it helps. I've moved my Saito 91 on gas to as high as 38 degs with absolutely no problems. It idles better though at 30 so that's where I run now. (I can't tell the difference between 28 and 30 for that matter) Any slower than 28 to 30 and it starts losing top end power. Definitely experiment with yours to find the best tune, just remember these cylinders are very tiny and it doesn't take long for the flame front to propagate across the chamber which is why they don't need very much advance. Good luck with your Saito conversion.

Thanks guys, that's really helpful as usual.

I ran my YS170 last night which I've just put the YS CDI conversion on. This CDI is the same as the RCCDI unit.

Tuning proved interesting, since the spark ignition removes the timing advance you usually get with a lean mixture and which gives you a warning courtesy of the engine sounding "hard". As a result I leaned it and leaned it, watching the tacho for any sign of sag and listening carefully. Then BANG the most enormous detonation I've ever had, even with a YS. Shattered TruTurn spinner, wrecked prop and a chunk out of the chin cowl (its soft mounted). Doh! I can only assume that cylinder temps went up and up to the point where something (end of the plug?) got hot enough to ignite the mixture early.

Anyway, lesson learned and hopefully I won't subject the Saito to the same torture.

:-)

WOW, I can lean my Saito sparker until it dies without anything weird like that happening. Strange magic man.

Well, nothing detonates like a healthy YS. But then, nothing drags a pattern model round the sky like a healthy YS*. I just wish I would learn the lesson of removing the spinner before doing any significant "playing"!

Of course, now I have the problem that it still feels very rich on the pump (i.e., mid range) but I'm petrified of leaning the thing. A few flights on the current richer settings I think to dull the memory of that horrible bang [sm=what_smile.gif]

Is your Saito on gas or glow fuel?

HS

* Only in my opinion of course

I've run it on both but currently it's on gas (petrol). Things that go bang make me nervous too. Good luck with it.

Hi Guys,

Well I ran the CDI on the Saito at the weekend (picture of the sensor mounting attached). Here's the results, running on Southern ModelCraft straight synthetic and doing a direct comparison with an OS type F glow plug on an APC 16x6:

: tickover was stable down to about 1900rpm on CDI. Below that it would hunt around and the slow running needle was hyper-sensitive*. On glow it would also tickover at 1900rpm, but wasn't as confidence inspiring or steady.
: With the slow running needle set right for tickover/transition, the mid range was very rich especially around 5000rpm. Typical glow engine carburation, but interestingly it was more noticeable on the CDI than the glow ignition.
: Top end performance was identical, about 8300rpm on a safe-to-fly tune.
: No quantitative results, but it was definitely using less fuel on CDI even though the difference in slow running needle and main needle was only 1/4 and 1/8 turn respectively.

To be fair this engine only has about 40 mins running time on it, so I expect things to change a bit over next hour or so of running.

All in all, the CDI just works as it should. The rich mid-range is just a result of the specific carburation on the 125Fa I think. I might look at filling the carb barrel slot and milling a new slot profile to lean the mid range a bit. But before that my father's going to run it in a YT Cessna to see how it behaves in the air.

A

* I have my doubts about the carb on this engine not having a carb barrel spring in it. There is definitely some end-float in the carb barrel as a result and I wonder whether this was causing the hunting around by causing small changes in mixture.

Hi Jezmo!

Could you please tel me what kind of CDI do you use for the conversion? May it also run on glow fuel?
any product links will be very appreciated.

Thx, Nikolay

Hi Nikolay,

Ignition is the RCCDI unit (which is often sold under the name RCEXL CDI), with a plug cap for use with glow plug thread spark plugs. I bought mine in the UK from http://www.justengines.unseen.org/

The unit is the same physical unit as used by YS engines in their YS170CDI, which runs on glow fuel. The ignition itself is suitable for use with either glow fuel engines or petrol fuel engines.

A

I saw in the product description that it's have a max 8000 RMP, but my OS FS52 can run till 10000 RMP. How it will works in that case? Any experience?

It'll work easily up to anything your OS52 can manage :-)

I've turned it over 15,000 on a converted ST51 without a single hiccup.

RCEXL has developed a CDI test kit. Also a sensor test device. Plugs into sensor connector and with spark plug attached on plug cap, if you connect the battery you can test from 400 rpm to 20.000 (as instructions says) . I've tested the CDI device and the result is A LOT OF SPARKS. It can be used to check suspected bad CDI, batt consumption and more.
Look and ear this video (sorry for the quality) . The sound is from spark only , and it sound much more than 10.000 rpm
http://www.youtube.com/watch?v=8Cw3p-Sz2GE

Regards