Gas Engine FAQ

[PaulH]

In the year or so that I have been a member here, I have seen many people ask similar questions about setting up and maintaining their gas engine. I asked Will if I could start a Gasser tuning FAQ. He was gracious enough to allow me that opportunity. This thread will be a living discussion. I will modify this first post as people post their questions and answers, similar to how Finless Bob has been handling some of his threads.

I am not the final authority on much of this. There may be innaccuracies. Please let me know what they are so that I can fix them. The hope is that this first post will become a "living document" with one-stop information for new and experienced gas engine owners alike.

A. FUEL SYSTEM

A1. What type of fuel should I use?
Many people have good luck with Coleman camper fuel and its counterparts. However, due to the inconsistancies in its octane rating, most of the vendors and tuners recommend plain-vanilla 87 octane gasoline from the pump.

A2. What type of oil should I use?
Again, most people have good luck with a wide variety of oils, from Mobil-1 to Amsol. The engine vendors and tuners recommend Blue Marble even though it is not a synthetic. You can go to their web site to find a dealer near you. At the time of this writing, Hanson and Bergen R/C resell Blue Marble. That being said, these oils will prolong your break-in period. You should use an ashless dino oil, like Lawn Boy Ashless for the first gallon or two before switching over to Blue Marble.

A3. What gas/oil ratio should I use?
25:1 Seems to be common. This translates into 5 ounces of oil per gallon of fuel. People use more and less oil than 5 oz with success, but this is a good starting point.

B. ENGINE TUNING

B1. What should my initial needle settings be?
This depends on the carburator used. The Walbro 64x series are the most common on our G23 and G26 engines. For this model, both needles should be closed all the way, then opened to 1-3/8 turns.

B2. I'm used to tuning nitro engines. How do I tune my gasser?
You can't tune by the amount of smoke coming out as with a nitro engine. Gas engines burn quite clean once they're up to temperature. Two years ago, John Garst made a wonderfully comprehensive post in this topic that explains tuning about as well as can be done with mere words. Here is what he said:

To initially set up the low needle, make it purposely rich (the tail will kick as you apply power). Then take the low end in a bit at a time listening for the motor to clean up and the tail to stop kicking. The low needle commands the mixtures through hover-range where the high needle picks up nearer wide open throttle. Listen and watch the machine. A burble accompanied by a tail twitch is still rich. As you adjust leaner and this goes away, stop. A lean bottom end condition will show up first in the skids as a vibration. Once set, you can then work on the high end needle.

High needle: Once again, start from rich and do powered climbs and listen for a rich condition at wide open throttle. A sag or bog could be taken as either rich or lean (this is why you make it purposely rich). Richen up the top and see if it gets better, if worse go leaner and then try again. What you are looking for is a good hard "pull" without a sag and as always err to the rich side (take in mind the gas needles have a much narrower operating window). Do not mistake the motor "loading" for a bog (this is where the ear comes in). The high needle is hardest to get right so take this in small steps and develop an ear for it.

C. TRANSMITTER PROGRAMMING

C1. I have heard that gas engines do not work with a linear throttle curve. What should my throttle curve look like?

A gas engine uses a butterfly for the intake, which means it does not have a linear response like a nitro engine. Half throttle (hovering speed) is around 25% or so on your throttle curve. Assuming that you hover at 3/4 stick, you curves will look something like these:

N (1550): 0, 20, 22.5, 25, 100
ID1 (1650): 100, 30, 27.5, 30, 100
ID2 (1750): 100, 32.5, 30, 32.5, 100

D. OPERATION

[rkeith2]

Paul -

I am using 87 octane and 4 1/2 oz Blue marble per gallon. 1 3/8 turn out on both needles.

I lower my throttle and increase my pitch to leave the ground at around mid stick and transition from 4 stroke to smooth 2 stroke at that point.

Head temp about 2 - 3 flanges back from the plug are 220 F or so.

Hope this helps.

Ron

[PaulH]

Keith,

By my calculations, you are mixing at about 28:1. That sounds a bit high, but if it's working for you, I won't knock it.

I'd like to add a section on radio programming to including things like the correct throttle curve to use in normal and stunt modes, the relation of throttle to pitch curve, etc. An operational section that explains the correct way to spool up would be good too.

Any takers? Anyone? Bueller? Anyone?

[rkeith2]

Yep -

Saves the engine on the loooooonnnnng flights

[1275mini]

Guys i am going to use my boat mix in my vario gasser but i think the mix is a bit high at 16:1 am comments

[rbort]

Barry:

WAAAY too much oil for the heli, it will burp like hell. Keep it around 32:1 for best performance. Even at 6oz per gallon its too much, you are at 8oz per gallon...

-=>Raja.

[1275mini]

Thanks Raja just found this little program it is aimed at the r/c car but there is a neat converter for fuel

http://www.jskelly.co.uk/download.htm

R/C pit tools

[PaulH]

I always get a chuckle out of how many people want a computer program to do fuel/oil mixtures. Just divide by 128! If you want to run a 32:1 mix, then 128/32 = 4 oz of oil. A 40:1 mix is 3.2 oz. A 50:1 mix is 2.6 oz.

[broncoholic7]

Hey Paul!
Great idea! I myself would love to see some set ups for curves.
I have been looking and asking but still havenot found any basic ones yet. I hope someone does step up!
Looking forward to the 107th FF!

[PaulH]

The thing to remember with Walbro carbs is that the butterfly is not linear like the intake on a nitro carb. In other words, a 50% throttle setting does not open the carb half way. A gas engine is at half power when the butterfly is around 30% or 35%.

I know I have seen a couple of discussions about throttle curves either here or at RR. I'll have to do some digging this evening if no one else wants to jump in. If I recall correctly, a generic/starter five-point curve for Normal mode was 0-25-35-60-100. This is assuming zero degrees of pitch at mid stick (3/4 stick hover). Stunt modes are U-shaped, such as 100-65-40-65-100, but the exact values depend on the desired head speed.

For setting up stunt modes, I cheat: I use a RevLock. :mrgreen:

[rbort]

Curves:

No, 60 is WAY too much throttle in my opinion. My curves are more like:

N: 0, 20, 22.5, 25, 100
ID1: 100, 30, 27.5, 30, 100
ID2: 100, 32.5, 30, 32.5, 100

I use 1550, 1650, 1750 head speeds. The numbers above are from memory and I may have tweaked them a bit but for sure they are less than 40 in a hover in any mode. At 60 your head will be screaming unless you like that!

Paul, are you going to the Tewksbury FF on the 16th?

-=>Raja.

[scofow76]

Thanks for the info guys! My first gasser is on order. I look forward to some operational tips and spoolup.

Thanks,
Scott

[PaulH]

Okay, note to self: dont' post on HeliFreak when I'm in a boring meeting. Neither task gets my full attention.

The numbers I typed would be more appropriate to a 1/2 stick hover than a 3/4 stick hover. Raja is absolutely correct that a throttle value of 60 would probably blow the head apart at a low pitch.

[rbort]

Now that's pretty funny! You're in a meeting, with your laptop. Clicking here and there and typing away. People think you are taking notes. You are reading/writing posts in helifreak. Little do they know! Got that privacy screen guard so your neighbor can't see what is on your screen?

You gave me a good laugh! So are you coming to Tewksbury FF or not? You can bring your laptop if you like!

-=>Raja.

[PaulH]

No privacy screen... I just sit in a location where no one can see my computer. It works well! I usually get more work done during meetings than when I'm in my office. :mrgreen:

I doubt that I'll be going to Tewksbury on the 16th. The 107th's annual fall fun fly is the next day. I plan be at the field on Saturday helping the rest of the crew getting everything prepared for Sunday.

Are you coming to our first-ever heli fun fly on Sept 30th? I'm the only person with a gasser heli there. I wouldn't mind the company. Broncoholic (Mike) will be up from CT so that I can help him get his gasser tuned. Any help would be appreciated.

[Teej]

Originally Posted by PaulH

I always get a chuckle out of how many people want a computer program to do fuel/oil mixtures. Just divide by 128! If you want to run a 32:1 mix, then 128/32 = 4 oz of oil. A 40:1 mix is 3.2 oz. A 50:1 mix is 2.6 oz.

Paul:

No offense, but you're contributing to the confusion. While this post is correct, your sticky at the top says 32:1 is 4 oz oil _per mixed gallon_.

Not only is that incorrect (4 oz per mixed gallon would be 31:1, not 32:1), but the math gets difficult much faster.

[PaulH]

Fixed.

Originally Posted by Teej

No offense, but you're contributing to the confusion. While this post is correct, your sticky at the top says 32:1 is 4 oz oil _per mixed gallon_.

Not only is that incorrect (4 oz per mixed gallon would be 31:1, not 32:1), but the math gets difficult much faster.

[DebianDog]

My 2 cents: The only reason vendors and engine manufacturers "recommend Blue Marble" is because most people cannot tune their motors. You can lean out a motor on a full synthetic FAST!

[1275mini]

Guys found today looking round the web throught it may help or not


Close both low and high speed needle valves. Do not close tightly, just until you feel them seat.
2– Open the low speed needle 1 1/2 turns and the high speed needle the same.
3– Adjust the butterfly or main idle screw so when the screw comes in contact with the throttle assembly it opens it about 1/8 “. This will cause the engine to idle high, but necessary for proper adjustment.
4– Supply water to the engine if it is water cooled. It is best to allow the boat to draw from a remote source such as a bucket if your boat has a water pump. A simple piece of tubing can be plugged into the water inlet.
5– With your boat secure on the stand, carefully start the engine. Be extremely careful of the spinning propeller. Always stay to the side of the boat and away from the spinning prop. Never have loose clothing or tools present in the prop area.
6– Starting with the low speed needle turn it in “clockwise” and listen as the rpm’s go up. There will be a point that a “woowing” sound will be present, just before the engine stalls out. Now back the needle back to the 1 1/2 turn out starting point and do it again until you can distinctly hear that sound. The next time when you hear that “wooing” sound, turn the screw out about 1/4 turn from that point. The engine will now start to operate at high idle, but more smoothly.
What we just did is cause the engine to go to a lean idle state, (too little fuel) at the low end and quit. The “wooing” sound I am referring to is to give you a present indication of what a lean state in a two cycle engine will sound like. The sound is caused by too little fuel being drawn into the combustion chamber resulting in a situation where the engine can not sustain operation. The fuel is being burnt off at a rate greater than the combustion timing is calling for, and the engine starts to pulsate. By realizing this sound, you will now be able to identify when your engine is too lean at low end.
8– The high speed needle is a bit trickier… The high speed needle MUST be set under load. When an engine is put in a loaded situation they tend to lean out, therefore it is more of a guess than a science if you were to try to adjust on land. The best way to properly adjust the high speed needle is to launch the boat, get it up to temperature, a couple of laps at high speed will do, and then return to shore making small 1/8 turn increments (clockwise) starting from 1 1/2 turns out, followed by another lap or two. Do this until your boat has reached its top speed, or until you lean out… Lean out? No problem just open it a hair to where it was just before the last adjustment. All kidding aside, a lean indication on top end is indicated by a couple of things. First it is best to throttle up quickly, if the engine _ _ it’s and gets but the boat slows down and falls off pace after a lap or two you are border line lean. Just back it off a smidg. If it bog’s or dies when you open throttle from slow to full you are way lean, again back the needle out and give it another go. Running the engine too lean at the top end can and will cause engine damage or severely shorten an engines life span if allowed to run in this state
A rich top end is opposite, the boat will have a raspy sound at top end and not go as fast as it may be designed to go. This is not a bad thing, you may fowl a plug or two each season, but generally no real damage will come from this. Some boaters actually prefer to operate their models on the rich side.
The best way to check and make sure your engine is operating correctly at top end is to “read the plug”, this is pretty easy to do. Remove the spark plug and clean the electrode with a wire brush, get it as shinny as possible. Make sure there is no wire pieces stuck in the plug when you reinstall it. Run your boat for a half a dozen laps or so at top speed. Return to shore and remove the plug when it cools down. Look at the electrode, is should be a medium brown in color. This means that all is well with both carburetor settings and choice of pre-mix oil. If the electrode is white or grayish in color, you are running too lean. Richen the high speed needle and perform the plug check once again. Do this until the plug reads the safe color.

[rbort]

I'm still up in the air on the 107th. I'm going to the AMA airshow in CT next weekend. Today was an AWESOME day at Tewksbury, too bad you didn't make it. I ended up flying and talking to a bunch of great guys all day long. I think just about everyone from Massheli group showed up.

I'm also not keen about flying over salt water marsh, and having a canoe nearby for retrieval. Not that I plan on retrieving anything, but the thought of it is just not very comforting. I may come and do a Howard thing, sit under the tent and watch other people fly, chat, help out with other people's machines and such :-)

-=>Raja.