Debra
01-08-2007, 07:47 PM
Hi Mr. Lee,
I've been an avid Engine Clinic reader for some time now. I'm writing to you for advice on a conversion I'm considering. First of all, I need to give you some background information. My problem is that I purchased an old (but never seen fuel) Supertigre 3000 from a retiring pilot last year and decided to install it in an old 25% Hangar 9 Chapman Cap 232 I had in storage. The only way to fit it without butchering the cowl was to mount it inverted with a Slimline wrap-around Pitts muffler and the radial mount...It was a nice, clean fit. I was not able to lower the fuel tank to be level with the inverted carb, so I added a 2 oz. header tank at carb level and it seems to work well. Before adding the header tank, fuel would drip out of the open carb when I filled the main tank
Let me say, it's been a rough break-in period for this engine. I started with a glow fuel of 5% nitromethane, 18% castor/synthetic lube. Initially, I ran the engine on the ground without the cowl, using a richer setting and with the lowest recommended prop (18x6), but the engine would load up and quit, especially when throttling down from mid to low throttle...I put up with that for half a gallon fuel. Afterwards, I started leaning the mixture to achieve near peak RPM at full throttle, but the engine pinged and clanked like it was full of loose metal. I solved the pinging by switching to a no-nitro, 18% caster/synthetic lube, FAI fuel. However, after achieving what seemed like a perfect idle, great high RPM (7800 with an 18x10 prop), and smooth transition, the engine would still flame out unexpectedly...just quit like someone flipped the off-switch. I've heard from others that this is a common problem with the big ST engines--especially when mounted inverted. I've also heard all kinds of crazy suggestions for fixes...adapt an OS 7D carb ("if you can still find one")...file a notch in the bottom-center of the spray bar and drill out the fuel inlet and/or add a Perry pump, etc, etc, etc. I haven't attempted any of the latter mods. However, I have tried a few different brands and heat ranges of glow plugs with no real change in performance (well, maybe worse in some cases). I've settled on the OS #8.
After two gallons of fuel were spent on the ground, I finally got the plane in the air by adding a programmable on-board glow driver that I set to come on at 1/4 throttle. It has saved me many times as I've heard the engine flame out, quickly dropped the throttle, then throttled back up. As you can imagine, that kind of flying is very stressful. Furthermore, I'm finding that my glow plugs are deteriorating rapidly with the extra heat....the OS #8s seem more robust and don't go white and crystalline quite as soon as some of the others.
Anyway, on to my idea...The conversion I'm considering is somewhat different from the above suggestions, but not uncommon. Among my collection of parts, I have a walbro carb adapter for the ST that I plan to use with a Walbro WT-158-1 (methanol carb from a Tartan)...this way I can have the reliability of a pumped carb and can do away with the header tank. I also want to add a conventional Capacitive Discharge ignition module with auto-advance and a Hal sensor to fire a sparkplug. Hence, I want to continue using methanol fuels, but have the reliability of a spark-ignition. I'd like to have the cylinder head drilled and re-tapped for the 10mm NGK CM-6 sparkplug (used by 3W and others). I really don't like the price and fragile nature of the tiny 1/4 x 32 sparkplugs. Do you think I'm wasting my time and money? If not, do you still do engine repairs and modifications?...I'd like to send you the ST head and a 10mm sparkplug to be fitted. Can you also tell me, where is the beefiest spot on the crankcase to drill and tap for a pressure fitting to drive the carb pump diaphragm? If, on the other hand, you think this is all unnecessary, can you provide some more suggestions as to other tweaks I can try?
Regards
Dennis
Dear Dennis,
As I have said may times in the Engine Clinic column, any time you invert a 2-stroke engine, you can expect to encounter problems. The glowplug becomes a sump where excess fuel collects. The 4-strokes are less prone to this problem due to the plug being off-set to the front or rear of the cylinder. You might try using an O.S. 4-stroke plug, or Fox Miracle Plug that have an extended nose the raises the element up off the combustion chamber surface. Recently I have been testing the Fuji 4-stroke plug handled by MECOA that also has the extended nose which works extremely well and sells for less money than the O.S. plug.
Any time you lean an engine in and it sounds like it is full of loose metal. the compression ratio is too high. Although eliminating nitro from the fuel helped this, it is also the reason you are having mid-range problems. The addition of 5 - 10% nitro in fuel can make a big difference in an engines idle and accelerat~on qualities. Lower the compression by installing a .015" head gasket.
Using the Walbro carburetor is a move in the right direction, but a capacitive discharge ignition might not be. A sparkplug can drown out the same as a glowplug. In fact, with battery heat the glowplug can burn off the fuel, where as the spark plug remains dead until the excess fuel is removed.
You might want to consider one of the Nelson Hobby Specialties (Jerry Nelson) "Glow Drivers". This unit differs from the other plug lighting units on the market in that it senses the plug cooling off and applies current regardless of throttle position. www.nelsonhobby.com
If you do decide to go with a spark ignition system I can rethread your head for the 10mm sparkplug. See my ad in the classified for address.
The center of the backplate is the most commonly used place to tap for pressure pulses to drive the Walbro carburetor.
Regards,
Clarence Lee
(to write to Clarence Lee, email man@airage.com)
I've been an avid Engine Clinic reader for some time now. I'm writing to you for advice on a conversion I'm considering. First of all, I need to give you some background information. My problem is that I purchased an old (but never seen fuel) Supertigre 3000 from a retiring pilot last year and decided to install it in an old 25% Hangar 9 Chapman Cap 232 I had in storage. The only way to fit it without butchering the cowl was to mount it inverted with a Slimline wrap-around Pitts muffler and the radial mount...It was a nice, clean fit. I was not able to lower the fuel tank to be level with the inverted carb, so I added a 2 oz. header tank at carb level and it seems to work well. Before adding the header tank, fuel would drip out of the open carb when I filled the main tank
Let me say, it's been a rough break-in period for this engine. I started with a glow fuel of 5% nitromethane, 18% castor/synthetic lube. Initially, I ran the engine on the ground without the cowl, using a richer setting and with the lowest recommended prop (18x6), but the engine would load up and quit, especially when throttling down from mid to low throttle...I put up with that for half a gallon fuel. Afterwards, I started leaning the mixture to achieve near peak RPM at full throttle, but the engine pinged and clanked like it was full of loose metal. I solved the pinging by switching to a no-nitro, 18% caster/synthetic lube, FAI fuel. However, after achieving what seemed like a perfect idle, great high RPM (7800 with an 18x10 prop), and smooth transition, the engine would still flame out unexpectedly...just quit like someone flipped the off-switch. I've heard from others that this is a common problem with the big ST engines--especially when mounted inverted. I've also heard all kinds of crazy suggestions for fixes...adapt an OS 7D carb ("if you can still find one")...file a notch in the bottom-center of the spray bar and drill out the fuel inlet and/or add a Perry pump, etc, etc, etc. I haven't attempted any of the latter mods. However, I have tried a few different brands and heat ranges of glow plugs with no real change in performance (well, maybe worse in some cases). I've settled on the OS #8.
After two gallons of fuel were spent on the ground, I finally got the plane in the air by adding a programmable on-board glow driver that I set to come on at 1/4 throttle. It has saved me many times as I've heard the engine flame out, quickly dropped the throttle, then throttled back up. As you can imagine, that kind of flying is very stressful. Furthermore, I'm finding that my glow plugs are deteriorating rapidly with the extra heat....the OS #8s seem more robust and don't go white and crystalline quite as soon as some of the others.
Anyway, on to my idea...The conversion I'm considering is somewhat different from the above suggestions, but not uncommon. Among my collection of parts, I have a walbro carb adapter for the ST that I plan to use with a Walbro WT-158-1 (methanol carb from a Tartan)...this way I can have the reliability of a pumped carb and can do away with the header tank. I also want to add a conventional Capacitive Discharge ignition module with auto-advance and a Hal sensor to fire a sparkplug. Hence, I want to continue using methanol fuels, but have the reliability of a spark-ignition. I'd like to have the cylinder head drilled and re-tapped for the 10mm NGK CM-6 sparkplug (used by 3W and others). I really don't like the price and fragile nature of the tiny 1/4 x 32 sparkplugs. Do you think I'm wasting my time and money? If not, do you still do engine repairs and modifications?...I'd like to send you the ST head and a 10mm sparkplug to be fitted. Can you also tell me, where is the beefiest spot on the crankcase to drill and tap for a pressure fitting to drive the carb pump diaphragm? If, on the other hand, you think this is all unnecessary, can you provide some more suggestions as to other tweaks I can try?
Regards
Dennis
Dear Dennis,
As I have said may times in the Engine Clinic column, any time you invert a 2-stroke engine, you can expect to encounter problems. The glowplug becomes a sump where excess fuel collects. The 4-strokes are less prone to this problem due to the plug being off-set to the front or rear of the cylinder. You might try using an O.S. 4-stroke plug, or Fox Miracle Plug that have an extended nose the raises the element up off the combustion chamber surface. Recently I have been testing the Fuji 4-stroke plug handled by MECOA that also has the extended nose which works extremely well and sells for less money than the O.S. plug.
Any time you lean an engine in and it sounds like it is full of loose metal. the compression ratio is too high. Although eliminating nitro from the fuel helped this, it is also the reason you are having mid-range problems. The addition of 5 - 10% nitro in fuel can make a big difference in an engines idle and accelerat~on qualities. Lower the compression by installing a .015" head gasket.
Using the Walbro carburetor is a move in the right direction, but a capacitive discharge ignition might not be. A sparkplug can drown out the same as a glowplug. In fact, with battery heat the glowplug can burn off the fuel, where as the spark plug remains dead until the excess fuel is removed.
You might want to consider one of the Nelson Hobby Specialties (Jerry Nelson) "Glow Drivers". This unit differs from the other plug lighting units on the market in that it senses the plug cooling off and applies current regardless of throttle position. www.nelsonhobby.com
If you do decide to go with a spark ignition system I can rethread your head for the 10mm sparkplug. See my ad in the classified for address.
The center of the backplate is the most commonly used place to tap for pressure pulses to drive the Walbro carburetor.
Regards,
Clarence Lee
(to write to Clarence Lee, email man@airage.com)