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Jan 23, 2017, 09:32 PM
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HillbillyJones's Avatar
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First gas engine - newbie needs the scoop


Hello everyone, I'm trying my hand at fuel power, specifically gas, after a year or so with electric planes. I bought a DLE 30 (for a Phoenix Model Strega), and have built a rudimentary run stand out of plywood and screwed it to my workbench. I have a DuBro fuel tank with the gas stopper and tubing with filter, the engine is installed with the cylinder pointing down, as it will be in the plane. I'm also using a digital tachometer plugged into the ignition box.

Now, for the questions.

I have a steel push rod and clevis for the choke and throttle routed through the firewall. I know the manual states not to use metal for the plane install, but I assume that is to combat radio interference. I don't have an extra Rx to set it up to use my radio for throttle control, so I will operate it manually. What are some ideas for locking the throttle at certain RPMs, instead of holding it for long periods of time?

I will be starting it manually (hopefully) with a chicken stick. What is the proper procedure for priming the carb and getting it to start somewhat quickly?

When breaking in a gas 2-stroke, what is the procedure as far as duration, at a given RPM and total? I've built many car engines, and there are cam and engine break-in procedures for preventing premature wear and ensuring proper piston ring sealing to the cylinder walls. Does the same apply for these engines?

Any other suggestions will be greatly appreciated. Thanks.
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Jan 23, 2017, 11:47 PM
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For locking the throttle I see a few guys have made kind of a makeshift throttle lever on a pivot using a bolt and nut, and just simply tightening it enough to put some tension on it. Everybody has their own methods for starting engines, but in my opinion the easiest way is to turn the choke on, and flip the engine over with the ignition on. When you hear the engine pop, turn the choke off and set throttle to about 1/4 and flip again until engine starts. Dont get in the habit of trying to flip it immediately after the flip before. It will eventually start and if youre doing that you will likely already be on your way to putting your hand into the prop once it starts.

Some people recommend flipping it with the choke on and ignition off and I dont understand why... its very easy to flood an engine this way. But being on a test stand you should be ok, youll be able to see everything. Of course there are various other methods to start an engine, but I believe the way I listed is about the simplest way to start a cowled gas engine, which is what your Strega will be. You will not be able to see the fuel entering the carb once the cowl is on, hence being easy to flood.

Break in... youll get just as many methods if not more for breaking in rc engines as car engines. Kind of gets to the point of people considering it some sort of magical voodoo. For that engine I would personally mix the gas around 32:1 and run it, simple as that. Set your needles to however it runs best and fly it. The only thing you really need to avoid is getting it set too lean and getting it hot. There is a good read at the top of this section on tuning engines if you arent familiar. You will notice as it breaks in some minor tuning will probably be required. And Im talking just small adjustments, nothing drastic.
Jan 24, 2017, 07:40 AM
Oh...that's gonna leave a mark
capt kurt's Avatar
Hi, you can use velcro or zip ties to provide tension to your throttle link. Just be prepared for a sore arm if this is the engines first start. Alot of engines need "help" from a starter on the the first run of the day......let alone, the very first engine start.
Kurt
Jan 24, 2017, 08:40 AM
Registered User
Break in is about heat cycling in my experience. Heat it up, cool it down, heat it up, etc. If you build engines you likely already know the drill. After the first tank or so, if I have steady performance from my needle settings, we go flying. Use your stand long enough to get used to handling your engine, then put it in your plane and go flying.

Don't worry about the metal throttle rod. I would change the metal clevis to a nylon one, or a ball link for your install.

I solder the hole in the choke plate closed for easier priming. While you're at it, confirm the choke plate is sealing all the way around it's perimeter. Doesn't happen often, but on occasion you do see one that's been installed off center.

First start is a pain in the butt because there's no gas in the carb. That means it's internal check valves are working poorly, if at all. If you have something to use for a primer bottle, plan on using that for your initial start. After that, try to leave the carb wet. It will prime and start much easier. Don't run it dry.

Geez, welcome to gas!
Jan 24, 2017, 08:44 AM
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AA5BY's Avatar

Welcome to Gas Engines


The change from electric or glow power to gas presents the modeler with some new issues and I'll cover one of the most important, that being that gas engines have a unique carburetor that incorporates an atmospheric diaphragm that provides a constant for the needle valves to be set against.

On the carb can be noted a plate with a hole in it having four mounting screws. Beneath the plate exist the diaphragm that reacts to the atmospheric pressure to provide the needed constant. As with other engines and motors, air flow is needed off the prop for cooling. A problem can arise when this air flow causes pressure changes at the static port that change with various rpms and ram air.

For this reason, a cowled engine must observe a proper ratio between inlet and outlet with at least twice the outlet area compared to inlet. This is usually achieved by restricting the inlet area, so that air is only directed at the engine jug and head for proper cooling. By restricting the inlet area, it will be insured that the ram air off the prop cannot pressure up the inside of the cowl.

Even if there is no pressuring up of the cowl, air flows off the prop can directly or bounce around and impact the static port and play havoc getting proper needle valve settings. Sometimes the plate can be rotated thus changing the position of the static hole, while other times the solution has been to solder a nipple to the port and run a line to a non influenced position, sometimes doing so within the interior of the plane.

I've personally never had to do this but have had to re-position the plate.

Observing a restricted inlet area has a double value, not only does it prevent pressuring the inside of the cowl but it provides better cooling. The reason why is quite simple. If the inlet area is too large, the exit area area can't handle all of the intake area and thus the the path of air flow will be that which has the least resistance, and that will not be the flow that is impacting the head and jug. The engine will then overheat.

Hope you enjoy your gas engine models.
Jan 24, 2017, 08:46 AM
Multi-Platform Pilot
barracudahockey's Avatar
Starting it is pretty easy. RESTRAIN the plane properly.

Choke on, throttle stick at about 30 percent. Ignition on.

If you're smart you drilled your prop so its about 2/8 oclock. Flip the prop following through to get your hand or stick out of the way. A few flips and it should pop.

Choke off, grab the spinner and rock the prop back and forth against the compression a few times.

Flip the prop briskly, it should start on the 2nd or 3rd flip
Jan 24, 2017, 12:05 PM
Right on the Edge!
Quote:
Originally Posted by AA5BY
For this reason, a cowled engine must observe a proper ratio between inlet and outlet with at least twice the outlet area compared to inlet. This is usually achieved by restricting the inlet area, so that air is only directed at the engine jug and head for proper cooling. By restricting the inlet area, it will be insured that the ram air off the prop cannot pressure up the inside of the cowl.
I saw you mentioned this also in my other thread. I'm not 100% sure I get what you mean - or maybe I did get it but I am not sure how to actually check if the air moved by the prop is creating pressure inside of the cowl.

All cowls I've seen have some form of hole right behind the prop, and I guess that that's the part you call the inlet. Am I right? English skill not helping here...

Now I guess that all that air must find a way out from the cowl to avoid pressure-rise inside of it. In my cowl I just made a hole on the bottom just to let the engine jug & muffler phisically out of the cowl... I don't know if this is enough. Also, I have a hole in the frame, below the firewall, that leads to the inside of the fuselage, but no other holes are there, so it's kinda of a dead end. Maybe it's normal, I don't know. All my electrics does have holes there, but then also other holes on the bottom of the fuselage, to let air circulate - I guess.

I know this cannot be the source of my issue, because the engine behaves the same way even without the cowl, but you know...
Jan 24, 2017, 01:14 PM
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AA5BY's Avatar
You are correct, if an issue exist without the cowl, your problem isn't relative to the atmospheric metering circuit of the carb unless someone has been into the carb and re-assembled it wrongly. It is critical to have the metering circuit atmospheric diaphragm and its gasket assembled properly or the metering circuit will not function properly. The engine will run, but it will display a couple of different issues depending. If the gasket is put on the cover plate side of the diaphragm, the engine will tend to flood and will require the HS needle to be much too far in. If the diaphragm is reversed, the engine will struggle for fuel and opening the HS needle fully will not richen the engine adequately.

As well, the diaphragm must be supple. It is not unusual to require a new every 2-3 years. If it is not, significant changes in atmospheric pressure are not compensated for and require needle valve changes for altitude variations and weather conditions.
Jan 24, 2017, 02:11 PM
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kimchiyuk's Avatar
Quote:
Originally Posted by barracudahockey

Choke off, grab the spinner and rock the prop back and forth against the compression a few times.
What is the purpose of this? I've seen people do it, but never knew why. I've been flying gasses for years and never saw the purpose.
Jan 24, 2017, 02:22 PM
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exf3bguy's Avatar
Quote:
Originally Posted by kimchiyuk
What is the purpose of this? I've seen people do it, but never knew why. I've been flying gasses for years and never saw the purpose.
It activates the pump and brings fuel up into the carb but not into the engine. Saves a few full flips when starting.
Jan 24, 2017, 02:25 PM
Registered User
When preparing to flip start an engine, I'll do that - with the ign. turned off and the choke on. Each one of those bounces against compression counts as a flip as far as the fuel pump is concerned. You do need to be careful because too many of those "bumps" can absolutely flood an engine. I would advise against the practice until you get to know the engine pretty good, and it's very consistent regarding starting.

For first start of the day, ign. off, choke on, and bump 10 times (no more!). Ign. turned on, choke off, and a couple flips generally start them. If not running by 3-4 flips, leave the ign. on, engage the choke, and flip until it pops. Choke off, flip a couple of times to start. Do not "bump" beyond those first 10 or so. It really is easy to flood if you do. -Al
Jan 24, 2017, 02:26 PM
Multi-Platform Pilot
barracudahockey's Avatar
Different techniques, hopefully same result.

I know mine works consistently. And I fly a lot all year round and deal with more gas engine installations than most casual modelers
Jan 24, 2017, 05:42 PM
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HillbillyJones's Avatar
Thread OP
Thanks for all of the good info, guys.

What is the proper operating temperature range for 2-stroke gassers?

I've seen some videos of people breaking in engines by letting them run for a few minutes at different RPM settings, and revving to full throttle a time or two. Is that the correct procedure? I know it is when breaking in a camshaft on an automotive engine.
Jan 24, 2017, 06:30 PM
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exf3bguy's Avatar
Temps will range from 180 to 300 while in flight. Like others, I like to run an engine just enough to verify/adjust to make sure it is reliable and then fly it while working the throttle. Final tune according to what you see/hear in flight. Seldom does the perfect ground tune remain perfect on the air.
Jan 31, 2017, 08:26 PM
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HillbillyJones's Avatar
Thread OP

First runs


So, I finally mounted this DLE 30 on a stand and hooked everything up. I quickly realized my fuel tank being below the carb makes priming difficult. Otherwise, it fired pretty easily but wouldn't idle on the factory setting. After 3/4 turn on the idle speed screw it idles between 155 and 175 RPM. I haven't noticed any hesitation or bogging when I give it throttle. Lots of black residue on the stand, behind the muffler outlet (32:1 mix), though. After half a tank (4 of 8 ounces) the head was barely 140*F.

The only concern I have is with the prop. It's a XOAR 18x8. It looked beautiful and balanced right away, but after a couple of restarts I noticed the trailing edge of one blade is splintering. I'm using a mass produced, rubber coated starting stick, so I have no idea why it did that.


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