Below is a conversation between Ted Bloom <tinablom@netnitco.net> and myself, showing how we came to terms
with the wonderful ill-humored ST G20/23
engine. At the start of the conversation, I had just bought myself a ST.51
and the ST2300
3-okt 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
Date: Tuesday,
October 05, 1999 3:24 PM
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 then 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 okt 99 23:04
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
Wednesday, October 06, 1999 2:33
PM
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 U 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
7 october 1999 0:49
Hello Pe:
Thanks for providing more insight on the workings of this carb. I'm
glad you also think that the carb throat diameter is about correct as it is. I
did some measurements on the spray bar and the venturi and I calculate that the
carb venturi has an equivalent venturi of a round hole of .347 inch in
diameter. Pretty small area for an engine of this size. Anyway, I will try a
choke hole on top of the velocity stack of .300 inch diameter and see what it
does for the midrange mixture. I know it will reduce the power, but I should
have an excess anyway. I will also test the other options later. I have an ASP
1.08 carb and will try that next, followed by the Cline system pressurized from
a muffler tap.
I hate to make irreversible changes to the ST carb barrel, but maybe I
can find one at a swap meet? I sure just wish that Supertigre would make a new
spraybar for the carb that would work properly. I suggested that to AnnMarie in
a polite way. I wonder if Supertigre even knows of the problem.
Regards......Ted
9 october 1999 1:48
Hello again Pe:
Here's my latest update on the ST2300. I tried the carb off the ASP
1.08 (11mm bore with narrow spray bar almond shaped cutaway). I was using an
18x8 APC prop, OS-F plug, and Cool Power 15% nitro fuel. Engine ran fine except
that I could not richen the main needle enough for full throttle. The needle is
almost falling out. The ASP carb spray bar is drilled to a smaller diameter
than the ST. The ST has a .047" diameter hole. Fortunately, I had also
thought about your suggestion to modfiy the ST carb. I drilled two holes in the
aluminum body of the carb to make two progressive air bleeds. One hole is near
the top of the rotating barrel and the other is near the bottom of the rotating
barrel. The holes are on opposite sides of the carb (front and back). The holes
are offset some distance from the centerline bore of the carb. The top hole is
tapped and threaded for a 8-32 size screw, and the bottom hole is tapped and
threaded for a 6-32 screw (if I needed to fill the holes in). As the barrel
rotates at about 1/3 throttle the holes start to get uncovered, at 1/2 throttle
the holes are completely uncovered, and at 2/3 throttle the holes are closed
again. The holes are not drilled into the barrel, but intersect the top and
bottom bores of the barrel on rotation. Look at a carb and you'll understand .
Anyway, it works perfectly. I just got finished running the engine and there is
absolutely no loading anywhere in the throttle range. Absolutely linear
throttle response. Thanks again for your stimulating idea. I will test fly it
tomorrow in a 10 lb EZ Diabolo.
Regards.....Ted
October 09, 1999 3:01 PM
Hi Ted,
What a great idea to combine the air bleed principle with the fuel
metering. You should have it patented. I sure hope it will work out in the air
You said you had these holes off centre, but you did not state what
side you had them offset, nor the size of the holes you used. I have absolutely
no idea about American small screw sizes since they are not inch-related, but I
take it that the hole should be around 3.5mm dia.
Good luck tomorrow!
Regards, Pé
10 october 1999 5:14
Hello again Pe:
In the air today and the engine is much improved, but not perfect. I
enlarged the bottom hole to the same size as the top hole and will try again
tomorrow. Both holes are now drilled to .135 inch diameter or 2.42mm. The top
hole is to the left of the carb bore centerline when standing in front of the
plane looking toward the tail. I can't recall the position of the bottom hole,
but I think it is also on the left side of the carb bore. I think I had a
little trouble with the Cline regulator today because the engine leaned a bit
when I stuck the nose vertical on climbs. It shouldn't do that. I cleaned the
regulator tonight and it seems to check out OK on the bench. I put in 10
flights today, no deadsticks and tremendous power with the 18x8 APC. It turns
8100 rpm, and hauls my 10 lb plane straight up at about 1/3 throttle. Anyway
more tests tomorrow. What kind of motorcycle racing were/are you involved with?
Years ago I used to work for a Norton, Triumph, BSA, and Yamaha dealer. I also
used to ice-race a Triumph B50 MX on a frozen lake in Wisconsin. The B50 was a
single cylinder 580cc 4 stroke with an Amal carb. If I could get that thing to
run in the winter, I should be able to get a Supertigre to run right.
Regards.....Ted
Hi Ted,
I can't stop but wonder what a great medium this internet thing is.
Some years ago I never imagined exchanging ideas with friends around the globe.
I am glad you have Amal experience, because it may help in our queste.
In the Amal or Dell'Orto, you could play around with metering nozzles (STmain
needle), slide needle taper and height (ST low end needle/spray bar combo), and
with slide bottom cut-out relation to the fuel flow outlet (ST Bloom air bleed
or forms of barrel bore entry or exit) . This cut-out is particularly important
in the 1/4 throttle range up to 1/2 throttle.
The last parameter that could be influenced was fluid level in the
carburettor (tank pressure or cline back pressure). All this gave a lot of
parameters to play with.
Our problem is worse, because we have no float. As an alternative, we
have fuel pressure or the pressure regulator (I dislike the word Cline, because
this regulator is as old as chain saw engines), which provides no fuel
injection at all. Probably that is why the ASP unit was way too lean in the
high end. BTW, I modified a YS 11mm carb for use with my 30cc glow engine, also
with pressure regulator, and I had to increase the spray bar inner bore and had
to make a new high speed needle as well, so there is analogy to your
experience.
If you supply exhaust pressure to the regulator, you will have fuel
pressure better in line with fuel demand, and may very well end up closing one
of the two holes, so please go easy on enlarging them.
Because of the inherent complicated gas exchange throughout the engine,
I do not think that two-stroke engines can be made to react completely
proportional to throttle input. That will remain the advantage of four-strokes.
Also, reaction to change in throttle input will take longer in two-strokes,
because of fuel accumulation in the crankcase (centrifugal separation).
As for racing, back in the early 60ies we did a lot of 50cc motocross,
where my job was tuning the engines. My engines received a lot of brass, not
because they were the fastest, but because they finished, while the fastest
didn't quite make it to the chequered flag and became also-rans. We tried
Triumph twins, but being used to small two-strokes, In the heat of battle,
stuck the rods through the crankcase (both mine, as my friends. Both in the
same jump) Exit two fine engines. Then the 50cc class became obsolete and we
grew too old (got married, careers etc.), so I took up model plane flight once
more. But the feel and knowledge about engines is still there, not to be
forgotten.
Pé (é is alt 130 on the num-pad)
Tuesday, October 12, 1999 1:15 PM
Subject: Test
flights G20/23
Hello Ted,
Did the larger second hole pay off? I am extremely curious, and keep
checking my mailbox.
BTW, I measured my carb, and found the barrel to have a bore of 11.6mm
as opposed to 11mm for the housing. Full throttle, the bores did not line up,
so I used some offset in the lathe to both increase housing bore to 11.6mm as
well as to bring both bores in line. Increased bore means faster increase in
air-flow for the same increase in fuel flow, which also should lean out the low
range. Have not test-run yet, but will keep you posted. Will a wooden Menz 18x8
do? it has larger (much) blades than the APC
Do you fly RC contest? I am interested, not because I fly RC
competition(I don't), but because your plane is 10# only. a lot of engine per
pound. How about torque reactions.
Pé
15 october 1999 2:18
Hello Pe:
Sorry for the delay, but I had to go out of town for work reasons.
Anyway, the enlarged hole did not help. It appears to have made things worse. I
had two dead sticks and the engine would go lean on extended vertical climbs at
full throttle. The weather was also much hotter that day, about 25 C. I plugged
the bottom airbleed hole with silicone seal at the field and tried another
flight. It still went dead. On the ground, the Cline regulator did not work
properly. I could set the engine slightly rich when the plane was level, but if
I stuck the nose up, it went lean. Strange! The tank is holding plenty of
pressure and the regulator is within 1 cm of the carb. I also noticed that the
engine went rich at the bottom of loops. Perhaps G-forces are pulling the diaphragm
open at the bottom of a loop? I tried running the engine without the Cline
regulator, but with muffler pressure and it seemed to run OK, except that it
leaned too much when nose high. My fuel tank is about 16 inches behind the
engine and the fuel pressure from the muffler is too small to push it up hill.
I didn't try to fly it that lean. I am confused on the regulator. I do not
understand why it is sensitive to tank position. Perhaps it becomes sensitive
when the carb vacuum becomes too low at full throttle? Anyway, I will soon try
an OS 7M carb about .306 inch diameter venturi along with the Cline regulator.
If it still goes lean, when nose high, then I will contact Jim Cline to find
out what's wrong. When the engine is running, the plane is very enjoyable to
fly. With the 18x8APC prop it pulls like crazy even at 1/3rd throttle. A nice
slow constant speed, level or going straight up. The EZ Diablo was designed for
a 1.08 2 cycle or 1.20 4 stroke, and it's about the same size as a Goldberg
Extra 300, but a little heavier. The plane picked up about 6 ounces when I
switched from a YS 120 to the ST2300, due to the weight of the Bisson muffler.
With the YS, I only used a short exhaust pipe without muffler. I have a friend
who has the same plane with the ST2300 and Bisson muffler. He's running muffler
pressure to the tank and not having as much problems as I am having. He also
has his tank closer to the engine. His
engine still goes rich in the midrange. I passed the word to him that
AnnMarie says to block off one of the exhaust outlets for more muffler
pressure, and he will try that this weekend. We will keep you informed.
Regards....Ted
15 october 1999 2:30
Hello again Pe:
I forgot to answer all of your questions. I don't have experience with
a Menz 18x8. If you have it available, give it a try. I read that the pattern
guys are using 16x12 APC and other props that pull the rpm down to 7200. I
would think if you load the engine with a prop so it turns above 7000 rpm it
should be OK. I fly some aerobatic competitions in IMAC and pattern. The
strange thing is there is less torque reaction with the ST2300 than with the
YS120. I don't understand it, but the airplane is much easier to steer in the
verticals and not so much heading change when I throttle up. The reason I went
to the ST2300 is because my YS blew up (fragile pistons, rings and liners).
When mechanically sound, the YS is a great engine (wonderful throttle response)
and very good power. The ST has a lot more power than the YS, which I wanted
for aerobatics, but the carburetor is very unsophisticated compared to the YS.
I also have a YS pump carb for a ST blue-head .60 engine. I might try to adapt
that to the ST if all else fails.
Regards...Ted
16 october 1999 23:31
Hello again Pe:
I tried the ST2300 with the stock carb again, but with the
throttle limited to about 2/3 of max via the servo throw. Then I could lean the
main needle. At 2/3 throttle I found that the Cline regulator does it's job and
the engine does not go lean when nose high. Apparently if the fuel draw is low,
the Cline regulator does not work very well. I still wasn't happy with the
midrange and low end on the carb, so I switched to an OS 7M carb (.306 inch
venturi). This carb has 3 adjustable mixture screws for idle, midrange, and
high speed. The engine ran very well without even touching the idle and
midrange mixture controls. The Cline regulator also worked very well with no
change in mixture with attitude. The only fault I can find now is that the
engine is down a little on power because of the smaller venturi. This engine
really needs a big carb with the YS type pressure feed.
Regards....Ted
17-10
Hello Ted,
I read your mail, and was pondering the possible reasons for your
set-up problem.
- you have your tank 16" behind the pump
- you have the regulator next to the carb
- you have different fuel flow when the nose is up
- full throttle, both air bleeds are closed
- combustion range for methanol is large enough to compensate for about
4" fuel pressure head difference.
It seems that somehow, the Cline pump does not provide enough fuel
under pressure to the regulating part, or is too much influenced by tank supply
pressure, so the regulating part does not receive adequate fuel head under all
circumstances.
It definitely looks to me, that the Cline unit is at fault when all
else is OK. Something beside the carb mods must have changed!
I take it that you already checked:
Is the fuel line inside the tank, and between tank and Cline unit
sound? (no air leaks)
Is the crankcase connection to the Cline pump ok?
Do you have muffler pressure on the tank?
Is the carb clean?
If all is well, I would suggest you check the Cline unit for pump
action, and the regulator for first flow level and flow cut-off level.
I would do it by supplying fuel under pressure to the unit, and raising
or lowering a loose outlet fuel line to check when the flow starts, and when it
stops again. On my modified walbro unit, it is exactly at level, alongside the
regulating membrane.
I have a YS pumper carb modified for regulated zero fuel pressure, and
it took me a lot of time to set it up. Bored out the fuel flow path, reshaped
the low end regulating parts, made a new needle.
I Dremeled the L-slit in my .51 carb to straight using the cut-off
blade. If my pupil leaves me alone long enough, might try to set up the carb
tomorrow (Sunday)
Will let you know, good luck with yours,
Pé
Hi Ted,
While sending the previous mail, I received yours.
You seem to be very close to solving your set-up. Congratulations!
Maybe retro-step to your idea of adding muffler pressure to the Cline? That sounded like a very good solution to start with. It has the added advantage of zero fuel pressure when the engine do