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.