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On this page: Carburetor Modification experiment ------------------------------------------------------- Understanding the engine I've been running these engines and providing warranty and repair service for a few years now and I've learned a few things about getting the most out of this engine. I don't claim this is a complete list because I probably can't remember everything off the top of my head. But I should be able to update this over time. If you are reading this to solve a problem with a Mintor or any other 2 stroke, read all of this. I'm sure you'll learn at least one new thing. Most pattern guys know that some effort to create a happy environment for the engine (fuel, cooling, pipe setup, mixture, etc.) is the way to achieve the friendliness and reliability we want. If you are hard on engines or if you expect the engine to tolerate dramatic changes in temperature, altitude, and tank location without adjustment, then go buy electric <G>. Hopefully you recognize the humor in that, but it's true. This engine has the highest power-to-weight ratio of any pattern engine in the world! It is designed for competition and not the average sport flier that might run an engine lean for weeks because he needs "max power". It's important to set it up correctly, and to listen to it when you fly to recognize issues early. Fuels: Stick with all synthetic or 95% synthetic fuel as recommended by the factory. There are some technical reasons for this that I won't get into, but remember that synthetic oils come in many formulations and don't all provide the high temperature protection from an occasional lean run like castor (which creates a lacquer that must later be removed). It's impossible for any person outside the fuel lab to know exactly what's in the oil formulation and the tendency to try and get more power from the fuel can give fuel manufacturers incentive to favor low viscosity oils over thermal breakdown qualities. The fuel used can make a big difference. I recommend CoolPower regular (not the MV, ProPattern or heli fuels) and Magnum, but haven't tried all fuels. Magnum is a castor/syn blend, but it's mostly synthetic. I've used PowerMaster, and the engine ran well, but I noticed more brown buildup than with Magnum. Maybe their castor/syn blend has more castor. In Italy the factory recommends MicroMotul all synthetic, but that is not available in the states. Mostly I use 15% nitro but in the Texas summers (let's say 95 degrees or higher) you can spike the fuel to 17 or 18% to maintain the same power by mixing in a few ounces of 50% or higher fuel from the local hobby shop. High nitro will create too much compression and my experiments resulted in extra head shims, cooler plugs, etc. In other words, too much complexity. Fuel
Oil has gotten a lot of discussion lately. If you visit the Morgan Fuels
site, they say that CoolPower uses an Ultra-High viscosity oil.
PowerMaster
advertises their PowrPak low viscosity oil. Magnum started using a low
viscosity oil in their DZ fuel and then spread it to their entire line because
it gave an rpm boost to everyone. My experience shows that low viscosity oil does not cool the engine as well, reduces compression and leads to erratic idle. Since the engine is already super powerful, using that fuel is not worth it. This includes the CoolPower ProPattern blend, which is a mix of low and higher viscosity oils. The antidote for an oil that can't
protect against a lean run is a richer needle valve and more pump
pressure. Please use a good quality fuel and a filter. Engine Adjustments: Low End: Please turn off the engine when adjusting the low end screw or pump. The low end adjustment range is only about a 1/4 turn or less of the screw. Most engines run with the screw in the center of its range, or a bit on the rich (counter-clockwise) side. Try to set it and forget it. In the right setup the engine may be a bit rich at idle on the ground but perfect in the air. You know that it is a bit rich because it will idle for a minute perfectly but when you accelerate quickly it will choke and bog down a bit. Idling for shorter periods show no significant richness. If the idle is on the lean side of perfect it will give that "rrriiinngg-ding-ding-ding" sound at idle on downlines. It may also backfire when accelerating from idle. Backfires are violent an can damage the connecting rod and crack or misalign pipe baffles. Do not fly if it is backfiring. Land and richen the mixture. One problem I've seen several times on carburetors made before Aug 2004 (inlet is fluted like a trumpet)
is the inside O-ring in the black barrel of the carburetor gets sliced and the
symptoms of inconsistent needle setting begins. I don't really know how
people damage that O-ring but I think it may be from lots of low end adjusting. If
your engine was running great and becomes erratic, this is one of the leading
causes. If you decide to remove the black barrel to inspect this, be very
careful reassembling and use lots of lubricant. The inner O-ring must
squeeze into a tight fit in the carburetor body and it is possible that you can
slice this O-ring during reassembly. The carb bodies made later (straight inlet with a shallow groove around the lip) are a
different design and don't have this problem and I've seen no sliced O-rings in
any of the newer engines. Pump: The Pump is a double flap design that lets fuel flow from the tank to the carburetor, but not back to the tank. This is very similar to the OS140Rx pump so the information that follows here applies to that engine as well. This kind of pump could allow fuel from the tank to drain into the carb when the tank is positioned much higher than the carburetor spraybar (like on top of the wing tube or in the canopy). This can result in a drippy carb. Also, since the pump accelerates the fuel, any tendency for the fuel to drain from the tank to the carb when the plane is sitting on the ground, will increase the fuel pressure to the carb. When you set the needle for proper operation you are naturally taking into account the pump pressure and the "drain" pressure. When the plane is inverted or in an upline, there will be no more "drain" pressure. This seems to result in a situation where the pump pressure and needle setting must be set on the rich side on the ground so they don't go too lean when in an upline. This makes these settings more touchy and sensitive than they need to be. Put the tank behind the firewall as low as possible and back up against the wing tube to reduce this sensitivity. Pump pressure is set at the factory using a manometer to spec for MicroMotul fuel. The factory has found that the newer fuels with low viscosity oils (Magnum, PowerMaster powrpak) may work better if the pump pressure is reduced (screwed out/left) ½ turn. If you are running fuels with normal or high viscosity oil the pump probably doesn't need adjustment, but you'll know if it does because you won't be able to get the 3000-4000 rpm mixture right even after setting your pipe. Don't adjust it for experimentation and start with the factory setting, even if you are using low viscosity oils. If the midrange is rich but idle and high end are OK, the first fix is a longer pipe. If this doesn't work or begins to reduce rpm below spec, then reduced pump pressure can be attempted. The pump adjustment is pretty sensitive and if you go too far it might stop working correctly and can not be returned, so don't take it out! Also, never screw it all the way in and then back it out. You will drive the plunger right through the diaphragm, or at least permanently distort it. If you have to reduce the pump pressure because the low midrange is rich, then unscrew the pump screw about a 1/2 turn and try again. That should do it. Do not adjust while the engine is running. On one plane, I made a hole in my firewall behind the pump adjustment screw that can be easily plugged to give access without removing the engine from the plane. Once my fuel, pipe setup was complete I've moved that setup to new planes with no adjustment. Reducing pump pressure can result in lean running on uplines, so if you do this adjustment, test fly and listen to the engine in the air while flying straight up. If it sags at all, you need more fuel to the engine. If the sag is slight, you can richen the needle mixture. The other alternative is to increase the pump pressure and adjust the midrange with a longer pipe length. You want a nice smoke trail and consistent sound on all uplines or you can do damage rapidly. This information is not Mintor specific and is standard set up procedure for all pumped and pipe boosted 2 stroke engines (Engines designed for pipe boost don't run that well without a pipe. The port timing can allow the scavenging during the boost phase to suck all the fuel oil mixture from the crankcase making a lean setting doubly damaging) The pump pressure screw has a small hole in its center. Do not let crud block this hole. It sets atmospheric pressure behind the pressure control diaphragm. If blocked there will be no pressure control and the pump will flood the engine with fuel. I set my high end needle as rich as possible. There is no reason that an 11 lb (5 kg) pattern plane needs more than 20 lb (10 kg) of static thrust. This can be obtained with a rich running engine, so don't needle for peak rpm. The engine comes with an insulator installed between the pump and
backplate (it is orange). This has
been a real savior when running in hot conditions or high altitudes (or both).
Other engines in this class that are pumped (OS140, Webra 145) have been
known to superheat the fuel in the pump causing it to boil in the needle valve
line in these kinds of conditions. The
insulator prevents heat transfer from the engine to the pump.
However, the 2003-2004 design is such that the pump is screwed into
threaded holes in the plastic insulator. Plastics
are not good materials to thread because after the bolts are tightened (snug
only – not tight!) the plastic will “creep” or “cold flow” and the
bolts will loosen over time. My
recommended fix, is to
cut some Du-Bro 3mm x 30mm bolts down to 24mm to replace the pump mounting
bolts. This length is correct to
mount the pump and insulator directly to the backplate (discard the factory
supplied very short 3mm bolts). I can do this for you if needed. Plugs and Starting: To light both
plugs, start the engine with one and leave the glow driver on.
Let the engine warm up for a few seconds and then run up to full power
for 3-5 seconds. This lights the
other plug. I've never had a
problem using the A5 plugs recommended by the factory. This is a medium
hot plug. I fly in Texas and it gets hot in the summer. Some people
claim that an OS #8 (cold) and an F work very well, but my tests show otherwise.
However, possible alternatives that I’ve tested and appear to work well
are and OSF and A3.
The factory wants you to stick to A5’s but
these are my results. Don't experiment unless you have to. I personally
don't like adjusting engine timing with plugs. In the very hot (>100
degrees F) flying days I put an extra head shim under the head to prevent
detonation instead of using a colder plug. It works well for me.
The factory says that if the A5 is not working start by trying a higher
quality fuel. The plug is a symptom of the real issue. I have found
that the wrong plug combination (for example, two #8s) can make the engine
hardly even start. Headers: The 170 has a lot more "kick" on a soft engine mount than other engines so you need a stiffer engine mount. If you go with a soft mount and the engine shakes all over, it must drag the header and pipe with it. The stress on a fixed header will be just behind the exhaust port and the header may break. O-ring headers won't break but you'll only get a few flights before they start to leak or fall off. After a lot of experimentation I've found that all exhaust systems perform best when their are two mounting supports: a firm one at the rear of the pipe near the stinger and a soft and flexible one around the coupler. The firm mount at the rear should not be hard. As the engine shakes it swings the exhaust port side to side. The distance from the exhaust port to the rear pipe mount increases slightly at full deflection, so the rear mount must be able to flex fore and aft a bit. Lord mounts, well-nuts, or Dave Brown mounts are good ways to mount the pipe in the rear. The purpose of the front mount is to dampen vibration. I've seen the PAC 1" pipe clamp mounted to a piece of 4-40 threaded rod used effectively. Here's a picture of my Symphony setup which has a piece of throttle cable inside fuel tubing wrapped around the header for stability and a little flex.
The
O-ring headers are more tolerant of a soft mounted engine than a fixed header
because of the flex provided. But all 2 stroke pipes are better off with
these mounting considerations. Bearings:
The rear bearings are expensive but will pay for themselves if
cared for properly. People have different mileage but I really believe
that the variability comes from the way they care for the bearings. The
bearings in the 170 are a dual row, angular contact bearing that has almost
twice the thrust load capacity as single row bearings.
Two stroke engines are much harder on
bearings that 4 strokers. This is partly because the cam gear in 4 strokes
helps hold the crankshaft in place and the fact that the corrosive exhaust gases
have minimal contact with the bearings.
Our model engines are
designed so that the pulling propeller puts a thrust (side) load on the bearing
and the piston puts a radial (spinning) load on the bearings. The
170 bearings should last 200 flights from a load perspective. At around
that time it seems the non-metallic races seem to loosen up a bit and its worth
replacing them. The thing that results in early replacement is corrosion.
Pipe boosted engines have a significant blowdown interval in their timing and
this allows combustion gases to blow down onto the top of the bearing.
Since nitro turns into nitric acid in combustion, this is very corrosive to the
bearing. Protecting the bearing from this acid puts everyone's care
regimen to the test. I have tried several end of the day care options,
from just running the engine dry, leaving it wet, storing it inverted, etc.
I was hoping to find a short cut that was simple and easy. No such luck.
My best results come from the factory. Run the engine out, then start the
engine again on FAI (no nitro) fuel and let it run for 30 seconds. I keep
a small 6 oz fuel tank in my flight box for this. Let this run out and put
a few drops of after run in the carb. then flip it hard or hit it with the
starter. Turn the prop so that the exhaust port is blocked. This
prevents nasty exhaust residue from running back into the cylinder, down the
ports, and onto the bearings. Using this method I've gotten over 200 flights repeatedly. This is a high performance engine and will give you amazing results if you pay attention to these details. When mounted inverted I've found the engine settings to be different than when upright. My experience has been that the factory pump pressure should be reduced by 1/2 turn and the idle mixture set to a richer setting. If the engine quits without the glow driver on at idle, but runs with the glow driver, richen the low end. If the engine runs well at idle, or the high end, but over heats and smokes at the other extreme the engine is running lean at the smoky end. This situation can be an indication that the pump pressure is wrong (usually too high) because the needles can't compensate over the entire range. If the engine backfires when accelerating, or makes a ring-a-ding-ding sound on downlines, the idle should be richer. In my experience the proper idle setting in the air results in it being a bit rich on the ground and it will load up. Then when you accelerate to take off, it will not come up. I've asked others about this and they don't agree but this has been true for me on several planes. Obviously, if you are not having this issue do not consider the information that follows. I modified a test carburetor to allow more air at idle. This appears to have fixed this problem on the test stand,but more testing is necessary in the air. To do this the round hole in the carb barrel is filed with a small round file to allow more air at idle. The round groove is made about 1/8" deep. I filed off center slightly so the file did not touch the spray bar. |
