Intro
never
trust a ST-lover, there must be something fishy
Supertigre Woes
Below is a conversation between Ted Bloom <tinablom@netnitco.net> and myself, showing how we came to terms with
that wonderful but ill-equipped ST G20/23 engine.
At the start of the conversation, I had just bought myself a ST.51 and the
infamous ST2300 (G20/23)
3-oct-99
Hello Pe:
I'm wondering if you have any experience with the Supertigre 2300. I am having problems with mine running rich in the midrange. I'm using an OS-F plug, a Cline regulator operated by crankcase pressure, and a Bisson Pitts muffler. When I first ran the motor on a test stand, I was using the stock muffler and without the Cline regulator. It also ran rich in the midrange. I can set the motor for an excellent idle and top end power. The motor will idle reliable for several minutes and give almost immediate transition to full power when I slam the throttle open. However, if I open the throttle to one third or one half, the motor runs very rich. It does not die in the air when flying at half throttle, but the power is not linear and controllable for aerobatics. I have tried adjusting the spray bar and it still runs rich in all positions including the factory setting (about 4 o' clock) with the cat's eye facing straight down the venturi. The cat's eye slot in the spray bar is extremely wide compared to other carbs. It is about twice as wide as an ASP 1.08 carb. I will try the ASP carb as soon as I can talk someone into making a sleeve to fit the Tigre. Do you know if anyone is making after-market spraybars for the Supertigre? I have been running a 15x10 and a 16x8 APC props. Next I will try an 18x8 to see if the heavier prop makes a difference. The motor makes super power and I don't want to give up yet. Any advice from your end?
Regards....Ted Bloom
5-oct-99
Hi Ted,
I do have a ST G20, but have only bench run it at no more than 1/2 throttle. I bought it together with a .51, which had this problem to the extreme. It quit at 1/3rd throttle because of an overly rich mixture. Upon checking things I found, that the L-shaped groove in the barrel displaced the barrel too much in the first 1/3rd. It also had a rather large almond spray bar milling. Having an old .61 carb handy, I replaced both spray bar and barrel. (they are interchangable). Hey presto! The nicest engine of the world, and a great value for money.
I contacted the dealer, who is an RC1 aerobatic champ, to talk about this problem. He checked the new Tigres in his shop, and noticed they all had the L-shaped displacement groove. He also complained about the throttle response not being linear, so we discussed the possibility of rotating the barrel 180 degrees and milling in a new, straight displacement groove.
I then checked all well running .51 engines in our club (4 of). They all had a straight groove milled in the barrel! as well as a smaller milled almond in the spray bar.
I asked for response in the newsgroup, but got little to go by.
To Do:
This being quite a nice engine, my best guess is to replace the 11mm unit of the 23cc engine by a similar one from OS. I am also thinking of fitting the MVVS pumper carb.
As for the Cline unit, It basically is a pumper carb with a fuel line output where the spray nozzles are. So I converted such a carb by screwing the needles tight and fitting small air hose nipples. I used this assy on a 30cc glow engine, and it works like a charm, for a fraction of the cost of the Cline unit (any scrap Walbro will do)
I think, that with the Cline, you have to open the main needle more than with exhaust pressure, because essentially, now you have zero fuel pressure under all flight conditions. This aggravates the 1/3rd richness. I have heard people solve the problem by blocking off one of the two stinger pipes in the exhaust. This also makes the engine much more silent. The main needle needs not be opened so much, which provides more restriction in the fuel path, so a leaner mid-range.
I hope all this helps, please keep me informed, because I do not plan to run mine in the near future. It is meant for a 17lbs Spit that I am designing right now (scale 1 to 5.5 and 2.03 meters span) Before fitting it to the Spit, it will have to run in as a piggy-back haul plane next spring.
Regards,
Pé
5-oct-99
Hello again Pe:
I received a reply from AnnMarie Cross (representing Great Planes the US distributor). It is their feeling that the carb on the G2300 has too large a venturi, creating poor fuel draw at full throttle and making you open the main needle wide. When you throttle back to 1/3 or 1/2, the fuel draw increases and now it is rich in the midrange. They say the cure is to block off one exhaust opening and run muffler pressure to the tank so you also lean out the main needle. Another cure is to use a venturi restrictor in the carb, which will also improve fuel draw at full throttle. I am going to try to fit an ASP 1.08 carb to the engine, which has the same size venturi as the G2300. The ASP has an almond slit in the spray bar only about half as wide as the G2300. If that doesn't work, I think I will try to fit a venturi restrictor inside the rotor barrel of the G2300 carb. My third option is to modifiy the cline regulator so that instead of sensing atmospheric pressure on one side of the diaphragm, I will install a vent nipple and connect it to the muffler pressure nipple. At full throttle, it should richen the mixture, thus allowing the main needle to be turned in leaner. At part throttle, less muffler pressure and less richening. I will keep you informed. As it is, the motor is pretty useless for aerobatics. The power control is like a light switch.
Regards....Ted
6-oct-99
Hi Ted,
I think AnnMarie has it all wrong. Just because the carbs works better with one exhaust stack blocked off does not mean that the carb is too large. It means, that the carb set-up with spray nozzle settings are dead wrong! Carb diameter is a function of the max torque RPM and the engine displacement. This ST is well known of delivery down to 6000 RPM, so the torque band is OK, and so is the diameter of the carb. It might even be a bit bigger! If I calculate the dia in relation to 50cc gas engines for standard road use, it is...11.3 mm! (16mm for the 50cc engine) For racing in the 13.000 rpm range (motocross) we went up to 24mm carbs (17mm for the ST23) and 28mm carbs for road racing (no torque at lower RPM, special, narrow gear ratios.)
If you should restrict the carb throat, torque suffers over the total RPM range. Also max power is generated at much reduced RPM, and will be lower than the original value at that same RPM.
If you plan to buy an ASP unit of 11mm throat, you might try to ruin the original ST carb by grinding (Dremel) material off the outer bore of the rotating drum. The reasoning behind this is as follows.
In fact, the rotating drum affairs are twin valve devices. One valve before (upstream of) the spray bar, and one valve below the bar. These valves operate in unison. The pressure at the spray bar is the ''mean'' between atmospheric and the crankcase suction. Opening up the upstream valve a bit, reduces vacuum (and fuel draw) at partial throttle, and that might fill the bill for you (and me?), as well as save some money. Changing the shape of the upstream closing part from round to squarish might also help increase upstream airflow for the first 1/3rd partial throttle. This way, the required closed overlap of the barrel is not reduced so much. This is the action I plan to do on the .51.
It seems a great idea to change the cline to exhaust reference pressure. That way, you have that well proved system right next to the carb. Great thinking! Take care of oil accumulating in the set-up though (small bleeder hole?)
BTW, adding lots of nitro is another way to go. (20 - 30%?) Nitro has a much wider combustion range, and thus is less sensitive to the right mixture, as long as it is rich!
Will keep in touch,
Pé
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