How Does an Engine Choke Work?

Have you ever set the choke on a mower or chainsaw and wondered exactly why you need to do that every time you start it? I have...
And it's been this long before I actually took one apart to find out what's inside.

[@87amoore]

Understanding that the choke is “choking” off air and not fuel make so much sense now. thank you for this explaination!!!

[@mtcondie]

I appreciate you taking the time to explain the concept of a how a choke operates! Great video! Very thorough explanation. 👍

[@RJSearson12345]

This helps a lot! I always have trouble remembering which way the choke is supposed to start out when I'm trying to start a cold motor, so I'm sure it'll help that I now know a little bit about what it's actually doing.

[@SuperRedshark1]

That helped me to understand the choke. Your explanation was very clear. Thanks.

[@UndulatingOlive]

Diesel mechanic of 13 years, never had to learn what a choke is Simple straightforward explanation, cheers

[@felixneumann9708]

Hey David, first of all pleas excuse my poor english i am a mechatronics apprentice from germany. I watched youre video about the hydrostatic transmission and subcribet to youre channel. I wanted to thank you for the video and let you know that i am amazed at how well you can explain. You manage to bring the passion for technology across and explain better then any teacher in my life so far. Please keep up the good work. Greetings from Stuttgart, Germany

[@PetersonBrosAutomation]

I am so glad to hear that! I love teaching mechatronics in a variety of subjects - automation and digital controls being my favorite. I am glad to hear of your interests and keep up the great work! Greetings back to you from Southwest Washington State.

[@WheresNaldo]

Best video I have found that explains choke function. Thank you!

[@raindropsrising7662]

Appreciate your time putting this video together and explaining this simple feature so well. Fixing up a generator now and this helps so much.

[@craigbikes8831]

Your manner of explanation is extremely clear and easy to understand. Thank you!

[@carlnishi6994]

Really helpful video! Why did it take me 58 years to learn this? Thanks much for a great video : )

[@martybell7930]

Just to reiterate what others have said... Exceptionally well explained.

[@PetersonBrosAutomation]

Thanks, you guys are all real nice.

[@RFD510]

I wondered the exact thing you mentioned in the description, but I wonder no more because of this video. Thank you very much.

[@DaveHonkles]

I came with a question and left with a clear understanding. Thank you.

[@corylowe5557]

Wonderful explanation! Thanks for making this.

[@EricHansen1971]

Fantastic info. I didn't know which was the choke position, where I pull and hold or when it snaps back when I let go. The pull closes the choke - release opens it for normal operation. To many this is obvious but not to me so this was a big help.

[@huminami]

Thanks for the great explanation. It's finally clearer to me. But I was still wondering why Stihl brushcutters usually have a rubber pump for petrol, while chainsaws don't? I always thought the engine was basically the same.

[@HeavensWaiting]

Thank yue so much! I love to know how and why things work the way thy do! And iyts actually really hard to find someone who explains things like this well! Thank yue! ♡♡♡

[@Just_some_guy_who_loves_God]

That was soooo much better then all else that I watch. I feel like I actually understand now.

[@RobertVolkman]

Beautifully explained

[@williamnodvik2913]

Good content. I’d like to see a tear down of pneumatic actuators like those controlled by a PLD. Seems like something you would have around and be able to explain well.

[@PetersonBrosAutomation]

That is a good suggestion - but to clarify do you mean the actuator/cylinder itself or the solenoid valve used to control the cylinder? Both are very interesting, but I have always found the solenoids to be a bit more mysterious. You also have to consider a few precautions when connecting coil devices to a digital programmable logic system. I have seen several PLCs ruined because of a lack of understanding about inductive coil loads. Maybe I could just do a video about each and then my question is pointless :)

[@sfo1mas]

Super helpful...thank you!

[@eamonnocarroll5866]

Excellent video - Thanks so much

[@richfishing92]

This was awesome, thank you! I’ve stranded myself one too many times while out boating. Went far out and had to get towed back. I now know I was flooding my engine. I would smell & see fuel oozing out of my engine onto the top of the water.

[@ajmilagros]

So correct me if I’m wrong, this is why old cars had choke levers , which you would close on engine start up, to reduce air to the carburettor to increase the fuel to the engine. The choke lever would stay out until the engine was warm and you could open the choke again. This is before fuel injectors were a common thing.

[@cultofeyes7260]

Yep I have to do it on my bike every morning because my bike is about two times older than me 😂

[@PetersonBrosAutomation]

Yes, exactly! Once you get a computer in to measure the engine temp and mix the fuel/air however it wants, it pretty much just runs the show... Until one of those magic little sensor stops working...

[@cultofeyes7260]

@@PetersonBrosAutomation damn still responding? Pretty impressive you keep on top of it

[@PetersonBrosAutomation]

@@cultofeyes7260 Give a nerd some free time and yeah, he'll respond to comments. Sometimes.

[@cultofeyes7260]

@@PetersonBrosAutomation well that deserves a sub mate

[@righteous1229]

ENLIGHTENING VIDEO!!!

[@fairplayer916]

Dude thank you. I actually understand the principles of the choke now

[@motofeig6473]

Amazing work. Thank you

[@thefox787]

i searched why does an engine needs to choke for Microsoft flight sim 2020 and this came up XD But i understood the concept

[@bonsailogic3205]

good explanation!

[@Garage_Beers]

Wow. Really great video. Very clear. Super helpful. Thanks so much

[@liamfitzsimons9246]

Great video

[@josephkraus]

Awesome video thank you, maybe do a video explain exactly how a compass works, you explain things very well

[@LarryGoggins000]

Awesome video! Thank you for the knowledge

[@mpoulin]

Good stuff man!

[@fairplayer916]

So the reason why a engine with say a dirty carb may only run with choke on is because it's not spitting out as much fuel as intended, so you can compensate, or balance it by choking the engine. Am I correct?

[@armandodelao1273]

Now I know what it does thank you

[@badonebadone4778]

If it still fires but doesnt last due to energy being lost to heating the engine could you not just start it a few times with normal mix?

[@Hockeyfan9884]

Awesome, and very informative, Thank you.

[@s70rk]

Great explanation, thank you kindly.

[@MariaGonzalez-ei1pv]

My car is carburetor 2 barrel cutlass supreme v6 .this what I i do in my mornings i pump the gas padle like 3 times and then turn the car on ,now the car is idling ,then I give it a few minutes ,1 minute I step on the gas n idle goes down now that's perfect..this morning I step on the gas start the car n it did not idle .my rpm was at 1000,,which it should be above 1000 at idle ,it's weird,help.great video well explain

[@francoisr.wevers2522]

Excellent!

[@yannickbigras2890]

Thank you for the explanation very clear.

[@christiantiofack2939]

Thank you sir! Clear explanation

[@Sam-jm7tx]

Thank you, good content simply well explained.

[@Speckled_Trout]

Very helpful thanks!

[@MidnightMogwai]

You’re a legend. Thank you!

[@richardthomas4112]

Very well explained, thank you!

[@streetscience8646]

This video is perfect

[@mitchjohnson4714]

Thanks so much. This is very awesome. Before I ask my questions, let me say that I did not grow up with small engines like so many people did. I basically don't have the experience of doing this. I've used them once or twice, but it was by fiddling with these levers that I started the engine and made it work. So my first question is what is the vocabulary? Do you say "choke it"? Do you say "choke the engine"? Do you say "choke the air supply"? Maybe "close the choke?" or "open the choke?" That sounds good because I think people say "open the throttle." So if you were to ask me, I'd say that "open the throttle and close the choke" sounds like a good way to communicate what you're doing. And that brings me to my next question. So to start the engine on a cold day, would you open the throttle and close the choke? That way you'd have mixture rich in gasoline? If so, I guess the unspoken part is that a mixture rich in gasoline will produce more heat, but will do so at the expense of lots of fuel and incomplete burning (because of lack of oxygen). Yeah, it's still not clear to me why a mixture rich in fuel starts better. Is it because it releases more energy in total or just because with how cold it is, it's more likely to have a chain reaction with lots of fuel molecules more densely packed (in contrast to a normal fuel-air mixture)?

[@PetersonBrosAutomation]

Honestly, that's about the extent of my experience as well - fiddling with levers until it worked. Growing up in the post-fuel injection world, I've never really needed to worry about it much. TI can only answer your first question with my limited experience, and the phrase I have heard most often is to 'set the choke'. As far as technical terms, I'd rather be able to explain a concept than speak the right words. You said 'close the choke', and since it is a flap that closes when set, your phrase would be most appropriate. However, since I haven't really heard that phrase in practice, I would tend to avoid that proper terminology in favor of the more generally understood terms. But that's just me. I'd rather help someone understand rather than be technically correct. Maybe people use that term, and I just don't know. There's a lot of things I don't know, so I just roll with what makes sense. Next question, starting on a cold morning is a tricky balance. Opening the throttle allows more of the mixture into the engine. The choke controls that mixture %. If you open the throttle too quickly with a rich mixture, you run the risk of flooding the engine with more fuel than it can burn, since the oxygen supply was already cut low. The best case is to turn it on with the choke fully closed, and as the engine heat up, open the choke valve while opening the throttle at the same time until the engine is running smoothly. But the timing is tricky. Even a slightly colder morning will change the timing. People with lots of experience can just do it all by sound and feel and get it fired up easily. The rest of struggle. A lot. Then we go buy a battery powered tool :)

[@mitchjohnson4714]

@@PetersonBrosAutomation Oh yeah, that all sounds familiar. Now that you describe that process, I remember that the way I did it was turn one lever and then start it and then throttle it until it got how and then turn that first lever the other way. so that lever was the choke. I think I will say "open the choke" if nobody gets too irritated because I think I've heard people say "open the throttle" and I like how it describes what's happening so you can keep it in kind. So I'm tinkering with my lawnmowers and it looks like they both have a choke and throttle, but I don't think I'm normally supposed to be fiddling with those. Are they automatically controlled somehow?

[@tenswr6469]

The term used to close the choke is usually "choke it" or "set the choke."

[@gingarella1]

Amazing video Really helpful and clearly explained. Thank you

[@AndrewEisenberg]

Very clear explanation. Thank you!

[@themachinerydoctor9117]

Very helpful well explained video thanks

[@arnoldchimuwhezwa3016]

Thanks so much

[@noahway13]

What changes in an engine when it gets warmed, and how does that affect the gas ratio from carb? What exactly needs to be warmed so that it can process leaner gas?

[@everts1991]

Good explanation! Thanks!

[@MiguelGarcia-rv3nj]

Great info Thank you

[@Mr11ESSE111]

Is it good if chain start to rev/spining while still on choke ? Is it good to tune carburetor on stihl 038 because of that on LA on lower to avoid that ?

[@PetersonBrosAutomation]

You would probably want to avoid heavy loads (like spinning the chain and throttling up) while on choke. You have reduced the air supply, so it might die out if you ask for too much fuel. With this small engine, you don't need to wait long. With a bigger car engine, you may need to start up a few minutes early and let it warm up idle before taking off the choke and hitting the accelerator. With new fuel-injected cars, they can monitor the temperature and change the fuel-air mix right away, so it's much easier to get going quicker on cold mornings. The choke doesn't increase the amount of gas - it simply reduces the air supply in the mix. That is not good for loading down the engine.

[@janhuijghebaert7293]

Clear explanation! Thanks!

[@FAMEAcademyNY]

thank you!

[@krystlebarba980]

Thank you ! really says it all.

[@illyriaschmidt7768]

Summer season, van won't idle an stalls when put in gear. Should I use the choke?

[@PetersonBrosAutomation]

If it does this all the time, even when the engine is warmed up, I don't think choke is the problem. I have heard of this being an issue with vacuum leaks, like a vacuum hose that comes disconnected and it runs really rough and stalls at an idle, even when just sitting at a stop light.

[@SL1CK1307]

Thanx

[@ClarkPokemanMaster]

Suppose the engine is flooded. How to resolve it? I watched a video saying turn the throttle to max, start the engine many times, burn the fuel out, engine will start in the end? What is your opinion? Throttle to low, start the engine many times? Any idea?

[@PetersonBrosAutomation]

If you aren't in a rush, then you can unscrew the sparkplug to let the gas evaporate from the cylinder. If you don't have a lot of time, you can shut off the fuel supply (usually engines have a valve somewhere in-line with the tank) and try to start a bunch of times. The fuel should eventually burn off, but then you'll need to turn the valve back open and re-choke because it will still be cold.

[@ClarkPokemanMaster]

Thanks for your video. I have a question. When I set a choke, I can start my engine. However when I start to push it back, it will die. I have to push it slowly. How long should I put the choke on(pull out the choke knob)? I have a 4 stoke outboard boat motor. Thanks.

[@PetersonBrosAutomation]

Good question, but there isn't one certain amount of time for any engine, it can change. If it's a cold morning, or cold wind blowing, it will take longer to heat up. That engine is probably bigger than this chainsaw engine, but if it's anything like a lawnmower or something, then probably less than a minute I would think? I actually haven't ever used an outboard motor, so hopefully someone who has a motor like that can comment with a bit better advice!

[@Justin-ty7vm]

And can anyone say what happens when we pull choke in ideal slow speed condition

[@crouton3455]

I still don't really understand why using the choke helps with starting the engine. If you just use less air then that wouldn't run any better than the normal mixture right? Or is the trick that you use less air but also increase the overall throttle?

[@PetersonBrosAutomation]

When you start the engine, the colder it is, the more energy is required. However, you also need more energy to run at full throttle. You can't easily get both. To get more heat, you need to burn more fuel, which means a rich mixture. There are a few possible combinations that will make more energy: 1. More gas + less throttle = lots of heat, but lots of exhaust 2. More gas + more air = less heat, but more efficient to provide more energy. In other words, more of the energy is motion, not heat. However, there are combinations that don't work so well: 3. More gas + more throttle = not enough oxygen to burn enough to keep it running. 4. More gas + less air, pretty much the same as #1, but only works if you keep throttle low. So when you first start the engine when it's cold, you need #1 (less air, same as #4) to build up enough heat to start the engine or it will suck too much of the heat energy into the block and try to die. But if you ran that lean mixture all the time, and pulled the trigger for full throttle, it would not have enough oxygen to burn and would die out. If you tried to run the lean mixture with extra gas all the time, you'd build up a lot of heat, but blow out a ton of exhaust with a really inefficient mix. EPA would be mad. I hope this makes sense and answers your question.

[@crouton3455]

@@PetersonBrosAutomation Thanks for replying, it does answer my question at least a little bit. So if i understand correctly you kinda want 2 different things for different situations? A mixture that generates more heat to warm up the block on startup, and then a mixture that generates more movement to actually run the engine once it has started?

[@PetersonBrosAutomation]

Exactly. You see the same with cars which throw out a lot more exhaust for a few minutes on cold mornings (choke), but then lean out the mixture for more efficient power once they are up to temp.

[@TheMomo2nd]

I’m sorry I almost fell asleep watching this, its felt like school, droning on and on. Im sorry if it’s harsh. 😞 You did get the facts across, thanks!

[@PetersonBrosAutomation]

School, but with a chainsaw. Gotta be better than regular school right? I gotcha, I'm well known for talking too long. Gift, curse...?

[@TC-dw6wg]

@@PetersonBrosAutomation great gift?

[@zakmiller9363]

So do you have to use the choke when it’s cold outside? Or do you have to use the choke every time you being to start the engine?

[@PetersonBrosAutomation]

It's more pronounced when it's cold, but really, you would need to use it anytime the block is a lower temperature than at full operation. If the block is colder, it will suck thermal energy out of the combustion, leaving less for moving the cylinder, since you only get a certain amount of BTUs (energy) out of one injection of gas. So even a hot day will need a little bit of choke, but only for a short time until up to full temp. A cold day simply makes it take a lot longer because you are a) starting from a lower temp, and b) it is cooling faster even while warming up. I was starting up an old wood chipper the other day (hot day for Oct here in SW WA) and needed choke for less than 10 seconds, then half choke, then full throttle no choke and it worked great.

[@zakmiller9363]

Thank you. Much appreciated

[@mba2ceo]

Do U even need a CHOKE if it starts with OPEN ?

[@urmomsh0use]

A+

[@user-qd6py1fw3u]

A++ Thanks

[@tripacer8259]

Some people just can't "get this"! I do not know why.....

[@estegueydijo....]

when I was younger I thought it was to choke the tihs out of it so it will run faster or stronger but ended up shutting off the engine 😂

[@AleksejsKružkovs]

ok so, choke close - more fuel, choke open - less fuel

[@PetersonBrosAutomation]

Not quite - the choke flap only controls air, not fuel. The ratio of fuel to air still changes like you said, but not by changing the fuel amount, just the air. Full choke = less air No choke = more air Think of it this way, if you are choking on a piece of food, what is happening? You are getting no air to your lungs. That's a helpful way to remember choke, it means the same thing for engines and for humans.

[@hydraboss1208]

Summary: engine needs warm, so cut off some air with choke so more gas and gas go boom and make engine warm then release choke so air go suck suck now go fast

[@yeetyete90]

The chokes on weedwackers and chainsaws like the symbols no clue what any of them mean so i just put it on one and try and if it dont work i try another one😂

[@PetersonBrosAutomation]

Same. Then if it still doesn't work, yell at it a few times, go have a beer and come back later.

[@submeitsfreebruh]

I hate shit auto choke thing in scooter. Like its that hard to pull the cable like in old bikes. Takes more than 5 minutes to start the bike with this auto choke shit. I hate it.

[@tomlewis4748]

Should we really trust an 'expert', who can't focus a camera? When he is in wide shot, the background is perfectly clear, yet he's fuzzy. When he shows things in a close-up, the background is again also perfectly clear, yet the thing he is showing us is fuzzy. This is not quantum physics. Anyone purporting to be a technician should be able to figure this out pretty quickly.

[@tsunchoo]

my guy, you gotta focus on something! - good video otherwise

[@JohnSmith-dh3kx]

Sorry but your explanation is incorrect.

[@bobbyjones9017]

blurry video

[@jaywest]

Great explanation. Thank you!