Informative
Chit-chat
page created: May 19, 2000 last edit oct 17 2000 (header, drawing added)
7-oct-99 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-oct-99 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
9-Oct-99 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-oct-99 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)
12-oct-99 1:15 PM
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-oct-99 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-oct-99 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 does not start. Pumper carbs tend to flood the engine while starting.
Pé
Hello again Pe:
I will try to use muffler pressure on the Cline regulator, but it's going to take me a while to engineer it properly. The vent hole on the regulator is not on a flat surface, so it will be tricky to hook up a pressure tap. I think I will try to get a new regulator plate (maybe Wallbro parts?) so I can modify that one and keep my plane flying in the meanwhile.
Regards....Ted
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