One of the most common questions in the beginner’s forums is how to tune your carb. This varies as "my engine won't idle", my engine sputters when I give it throttle", or "my engine will not reach full RPM". Or variations

I have posted the how to of adjusting your engine a number of times, but I've yet to see a good description of what the adjustment do and how they are related.

Lets start with a control line engine. It has a carb of sorts, just no throttle. The spray bar, where the fuel is introduced into the air stream into the engine is little more than a nub pointing into the throat and the needle adjust how much fule is sprayed out of this nub. The engine runs at full throttle all the time.

Then came the world of RC and the desire to not land at full throttle or to land dead stick. A function Carb was needed. There were a huge number of carb types. Today, the glow engine carbs boil down to two basic types, An Air Bleed carb, or a Mixture control. Most are of the Mixture control type, so this is what I'm going to explain. (I've never used a Air Bleed, so I'm not sure how they work now)

First, the carb has a throttle. Early cars used a vane for a throttle "plate", but our carbs mostly use a barrel with a large hole the size of the venturi of the carb. You can see this opening change as you move the carb. It is basically chocking off the air entering into the engine.

There is a bit more going on though under the hood. There is a spray bar, but this one is different from the Control line engines. It will have a long slot or diamond shaped slot that the fuel sprays from. There is also a Mixture Control valve also incorrectly referred to as the Idle Jet. This valve protrudes into the spray bar and partially blocks the slot. How much the slot is blocked depends on the throttle. At wide open, there is little blocking, but at idle it is mostly blocked. This is accomplished by a spiral grove on the OD of the barrel and as the barrel is turned, it is also moved from side to side, following the barrel spiral. This is what changes the amount of fuel being introduced into the engine in relation to the throttle. As the throttle closes, the barrel rotates to cut air and slides to cut fuel. This is a fixed change and can’t be adjusted. What can be adjusted is the starting point for this mix, IE we can fine tune how much fuel is allowed at idle, and the spiral increases in a set amount additional fuel as the throttle opens.

It is a basically simple setup. However, the initial setup is what is critical. If the mixture control is setup initially way off, it will effect the amount of fuel allowed in at wide open throttle. You adjust the High Speed needle and it really isn't running right. When you throttle down, the mix goes haywire and the engine stumbles or dies. You play with the mixture control and the mid gets better, then the high end gets worse. Soon, you have it putting over at 2000 rpm but you cant get it to run much faster. These are extremes but I have personally got a carb screwed up so bad that I had all of the examples in one day. The photos below show the relation of the throttle barrel, spray bar, and idle mix. The Idle mix moves as a part of the throttle barrel, and is fine adjusted by turning the screw. In the photos, I do not show the throttle barrel turning, only sliding to change the mix. In you engine, you will not be able to see the mix valve at work as the slot is not always visible

So where to start. Read the Instruction Manual. It will tell you what the starting, or factory default settings are. That is where you want to start your adjustments from.

Using a tachometer for every step,
1. Set the throttle for wide open and then adjust the high speed needle for maximum RPM then adjust is rich about 500 RPM.

2. close the throttle down for the slowest RPM you can get and have the engine keep running. Do not change the throttle again for this step. Adjust the mix valve, lean screw it in) very slowly for maximum RPM. You should move the mix valve a maximum of 1/4 turn at a time and the closer you get to where you want it, cut the adjustment down to 1/16 turn, allowing a few seconds for the engine to change to the new setting.

3. Go back to the Wide open and adjust the high speed again.

4. Go back to idle and adjust again. Your idle now should be quite lower RPM that the where you started step 2

5. Go back and do the high speed and idle one more time.

Now for work on the transition. This is fine tuned by the mix control.

6. Run the engine WOT for 15 seconds or so to clear any loading out, the go to full idle. Let it run there for about 15 seconds, then punch the throttle full open as fast as you can. The engine should jump to maximum RPM without stumbling. If the engine dies as soon as you punch it, the mix is set to lean, Open it up 1/16th turn at a time until the engine jumps when hit the throttle. If the engine stumbles, and slowly gains RPM or doesn't gain RPM but just stumbles. the mix is to rich, so again in 1/16th turns lean it out for proper response.

At this setting you can easily change the high speed a click or two for changes in weather. For a change in prop size, I would go through the full adjustment again.

Once the engine is tuned, the only reason you need play with the carb again is when something changes. IE different fuel, different prop, big shift in the weather. Daily adjustments are not needed beyond one or two clicks on the high speed.

This all assumes a fuel system in good order. Air leaks change things. They can be from cracked fuel lines, worn or damaged O rings on the needle and/or mix valves, and on the carb to engine joint. Fuel issues like pinched lines, improperly installed tank lines, etc will also enter into the mix.

Ken and others, I welcome feedback on this. Let me know.

Don

Remember that the barrel is turning and cutting back the air and it goes from WOT to Idle. That turning isn't shown as you couldn't see the valve to spray bar action.

Seems to me a bit has been left out such as how to double check your work. An engine can be tuned perfectly yet I don't see where this was mentioned. Also the relation of so called factory settings to your location in the world.

I see it asked repeatedly "what is the factory setting?" Doesn't matter what it was. It must be set to where you live. This is also a simple task which must be done in a somewhat quiet area. Opening the High end about 3 turns and closeing the low. Attaching a fuel line to the carb nipple long enough to blow into. Simultaneously blowing into the tube and opening thelow end untilyou hear a hissing noise. Theres your factory setting and adjust frome their.

Tuning these things perfectly is not a gift and very simple. There are check to make each time you make an adjustment. First thing to do is get it tuned up as best as you think you have it. The key is "nailing that low end." It can take a couple tanks but I never mess with the low end again in any given season after i fine the sweet spot. The high end is a no brainer. The low can be the root of all evil

After you "think" you have it tuned. remove the fuel line from the carb or pinch it off and wait for the engine to quit. If the rpms increase as the engine quits, low end is too rich. if the rpms go down and quit-too lean. I usually go for a slight increase. Whats the big hurry? Take a couple tanks and get it right.

Also altitude and weather effect the engines performance substantially. Approximate 3% loss in power and performance for every 1000ft altitude and even worse in the summer with the high DA. With this you should consider getting the more powerful engine in any given class. Meaning if your at 5000ft and IMHO, I stay away from the "LA types etc. Some may disagree and this is fine if they have first hand knowledge

I'm not sure what you mean by Double checking. If you are refering to pinching lines or holding nose up to check your settings, my belief is that this is all covered using a Tach to set the engine. After you have experience, you can trade the tach for your hearing and judgment to some extent. That comes from experience.

I wasn't aware that O.S. for instance had diffeent starting setting for the U.S and For Austrailia for example? What are you getting at here??

Don

Removed double post
Don

No the pinch test is high end for those squeezing every last bit of lean rpms. Tilting th plane up is also for the highend side of it.

What I'm getting at here is all those settings your being told to go by in the manuals are based on sea level. Also in an unrelated area are all those who by temp gauges and aim that at the engines to tell them-nothing. There are no specs.

I have had several engines overheat in my day and it the indications of this is when its too late and you better be in the right area to bring it in for landings

The factory settings are a broad range starting point. One that usually allows you to get the engine running so you can do a proper tuning later.

As for altitude tuning, my method is as good as you are going to get. You are tuning for maximum performance with a bit of leaway for the prop unloading in flight. What else can you do to tune for alititude??

As for overheating, there are a number of issues. Tuning, which I covered, wrong prop, and poor air curcliation come to mind. I am addressing the tunning issue.

Look, this post is in the beginners forum. I wouldn't claim it is the absolute answer to anyones tuning problems. It will give you an engine that you can depend on. It may not be that absolutely maximum performance you can get from the engine, but it will get you up in the air with a minumum of fumbling around on the ground. It will keep the engine running for the full flight and will give you an engine that will respond when needed. All of this is based on an assumption that the engine that can run correctly with a proper tuning.

If you have suggestions on how to go about tuning an engine differently than what I posted, please contribuite them. Otherwise, please PM me with your response.

Don

Campgems, thank you for the detailed understanding of what's going on inside our carburetors. For me, having a picture in my head of what I'm actually adjusting will make things make so much more sense; it already does.

I'm not sure what the disagreement is about, but I think this is great information for us new guys. It is enough to get the engine running properly and get comfortable tuning the engine, and with experience we will later be able to fine-tune for different conditions or requirements.

Thanks so much for your time in doing this, we appreciate it!

Kind of like going to the flea market eh? Everyone is bringing a bunch of crap but someone is leaving with something good.Guess we have two topics here, How tha carb work vs how to tune it. Two separate areas.

OK, I'll explain the Air Bleed carb. These are found on mostly lower end engines, and for some reason, on many 4 stroke engines. They are simpler to make, work pretty well, but they have their disadvantages as well. The biggest disadvantage is reduced fuel draw at idle. Most newbies will be finding them on engines like the OS LA series.

The Air Bleed carb has the same throttle barrel as the 2 needle or mixture control carburetor. It has a hole bored thru it to serve as the venturi. In this carb, the fuel inlet jet is commonly just a tube dead ending in the centre of the throttle barrel. The throttle barrel typically does not move side to side when opening and closing, it simply rotates. This simplifies machining. When running at full throttle, it's operating just like the C/L venturi described by Campgems. The engine is pulling air through the carburetor, and the air is accelerated to high speed by the smallish opening of the throttle. This creates a low pressure area in the venturi (or throttle barrel), and this draws fuel into the engine. The main mixture needle, or high speed needle, just provides an adjustable restriction so we can control the ratio of fuel being delivered relative to the airflow.

This setup works well with the throttle set anywhere above about 1/3 throttle, but below that the mixture begins to get hard to keep within the necessary range. Manufacturers of air bleed carbs design their carbs so that if there were no air bleed (or it was closed), near idle the mixture would be grossly rich. The air bleed allows us to admit a little extra air to bring that mixture back to ideal. When we open the air bleed screw, we are admitting a little more air into the carb below 1/3rd throttle, but it doesn't affect higher throttle settings at all.

As the throttle is closed from wide open, for the first 2/3rds of its rotation, it's open only to the big air intake on top. As it keeps closing, it begins to uncover the air bleed hole inside the carb. As it keeps closing, more and more of the air the engine is breathing is coming from that air bleed hole. We simply adjust that hole's opening with the air bleed screw until the engine will continue idling for a long time without starving for fuel (too much bleed air, lean) or loading up (too little bleed air, rich).

With any glow carb type, be it 2 needle or air bleed, after you tune (ideally a couple hundred RPM's rich of peak at WOT, and an all-day idle with good transition), it's a good idea to either pinch test or nose-up test the plain. Both accomplish the same result, trying to prove that the engine is rich enough that it stays happy even when pulling fuel uphill when in a nose-up attitude. In this respect, the nose-up test is a bit better, as the pinch test only proves that we're richer than peak, but with a little experience, the pinch test is easier to perform, especially with larger planes. Ever nose-up test a 120 sized 3d aerobat? Nope, me either.

**uploaded better image....**

J

I understand the "nose up" test and why it is performed. I have performed this test in the past up till very recently and may even use it again. HOWEVER, ever since I've learned to PROPERLY tune an engine, the nose up test always passes without adjustment.

Tuning seems to be a mystery until a person finally gets it right and understands how things work. After that it's, "Man that's soooo, easy! why did I have so much problem before?" .....and I just recently got it in me head

Thanks for the excellent work on the post and pictures.

Jubrry, a double thanks on your air blead write up.

Maybe we can get RCKen to do a proper editing job on both of ours and include it in his Gettingairborne site.

Don