The outside of a motorbike engine seen up close near the exhaust

Home Repair And Restoration

What Is A Carb Heater?

By Ren Withnell

A carb heater, or more correctly a carburettor heater is something, anything that will heat up a carburettor. Logical huh.

But why, what is it for? Carburettors get cold. They get cold for 2 reasons. I'm offering a basic explanation here. If you wish to find a more technical explanation then a web search will take you into a world of science, physics and chemistry. Here I'll keep it simple.

As air is sucked through the carb into the engine the air changes pressure, it drops.  When pressure drops things get cold. The deodorant in a can is pressurised, when you spray it on your armpits it goes from being squashed to free in the air and makes your whiffy bits feel cold. Imagine that happening inside your carburettor every moment you ride and you can imagine how chilly things can get.

When liquids evaporate they make things cold too. Your sweaty body is evaporating water to keep you cool, it's nice on a hot day to spray yourself with some extra water. Petrol as it mixes with air does the same.

So the air is being cooled by pressure changes, add to that some petrol evaporating and the temperature falls. On a hot summer's day this is not typically a problem, things don't get cold enough even with the effects described above. However during winter in those cold, dark and damp conditions it's all too easy for ice to form.

On a cold, crisp winter evening it's not unusual to find frost on cars. That frost is the moisture in the air freezing onto the surface of the car. Inside a carburettor it's even colder so more frost can form. The engine is constantly drawing air through the carb so there's a lot of passing air to draw moisture from.

This frost, or ice, starts to block things up. Tiny holes and passageways where fuel passes through can become blocked. Things that move inside the carburettor get jammed by the ice. Even the airway itself can gather enough ice to restrict air flow. In short at best the engine will run incorrectly, typically rich. At worst the bike will stall or stop and you'll have to wait for the heat of the engine to melt the ice before you can carry on.

So how is this overcome? By heating the carb with a carb heater! Most carb heaters I'm aware of are nothing more than a brass fitting with a wire attached to it. Inside the fitting is a tiny electrical element much like a kettle's and this gets hot as electricity from the charging system is passed through it. This heat spreads around the metal carb body and should be warm enough to prevent ice forming. Some systems use hot water in the cooling system to much the same effect. 

small brass threaded fitting, a carburettor heater
A typical electrical carb heater.

Of course most modern bikes are injected. These don't require the pressure drop to "suck" petrol into the engine, the fuel is pumped at high pressure into the air. As there's less pressure drop there's less temperature drop too. It is physically possible for an injector system to ice up but it rarely happens under our temperate conditions.

I hope that helps!

Reader's Comments

tahrey said :-
Fun thing with the single injectors used on bikes like the CBF, though (which between that and the way the tappets work look like the cheapest possible conversion from the CG's carb-and-rocket setup)... they essentially sit in the same place the carb did, and thus aren't quite so far along the manifold thus close to the heat of the engine as a full from-scratch injection fuel system design.

My first car, an early 90s Polo had something very similar. Engine looked almost exactly like the carb model, but it had the injector sitting where the carb would be.

Two or three particularly harsh winter mornings in Wales saw not only the whole body encased in ice, but the injector suffering from a version of Carb Icing too. I think it actually had a heater, but it took pouring a kettle over the air filter "bin lid" housing and shoving socks in the intake to mitigate the pressure drop in order to get the bugger to start. Was fine once running.

Distances are a lot less on a bike of course, but then again so is the heat.
29/10/2018 02:51:13 UTC
Ren - The Ed said :-
As far as I've seen most modern bikes with FI have a setup much the same as the CBF125 - the injector and throttle body placed where the carburettors once were. Essentially the injector simply replaced the carb. I also know the CBF125 is available in some countries with carburettor, it would be a simple swap.

I've not heard of any bikes suffering "injection icing". Logically it is perfectly possible though so I am open to being further enlightened.
29/10/2018 06:48:35 UTC
said :-

30/10/2018 10:02:24 UTC
tahrey said :-
Having seen the image of the injector on the other page mentioning it now, I wonder if the design has been modified in the light of lessons learned over the last 20ish years. That pepperpot type arrangement with a broad, slightly recessed head and many holes, mounted off to one side, looks like it would be less likely to get iced up - and certainly not entirely - vs what I saw on the car, which was something more like the nozzle of a fire sprinkler, or the shape used for "injector fault" warning lights, and was sat right out in the very middle of the airflow where it would get hit with the greatest cooling effect.

The only other explanation I can think of it for the car was that the HT leads were acting up, as they were hugely iffy for some reason, and even when going for high quality aftermarket alternatives, I still probably replaced them 3 or 4 times in as many years. And I had problems of sudden idiosyncratic power shortfall some years later in another (also SPi) car whilst driving along in wet, fairly cold conditions on the motorway, a good 40+ miles into the journey, which were to all intents and purposes the same as carb icing but in a situation where you wouldn't have expected it so much but would have quite readily blamed the ignition system instead (it also ate a couple sets of leads, but less frequently).

Thing is, duff HT leads usually led to missing on just one, or sometimes two cylinders, not all of them, and then mainly low revs/full throttle... idling was unaffected and when running the problem could be cleared by downshifting and lifting off a little. Plus it only actually fired up after the ignition had been on a while - allowing the carb / throttle body heater time to work, if it was actually even functional on that car - and I had been attempting remedies that performed dual functions of somewhat emulating a choke (reducing intake air mass, on a car which to my knowledge lacked a MAF, using only a lambda sensor to ensure correct mixture once warmed up, so forcibly enriching the mixture) and heating up the environment around the throttle body ... yet, if anything, should have *negatively* affected the current carrying ability of the leads by wetting them with splashes and run-off. Ice isn't conductive as far as I know, and can't sink in to rubber that's gone a bit porous, but hot water can. On the other hand, maybe just having the bonnet open slowly dried out any moisture they might have been holding?

Third explanation, maybe the battery voltage was low due to the temperature, and having the ignition on, juicing the TB heater and keeping the CDI unit charged caused enough internal heating to raise it to the point where the starter could turn AND the spark plugs fire (I've definitely seen the bike suffer the conundrum where there's only enough charge to turn the engine over OR make a spark, but not both - thankfully it's a lot easier to bump-start). But again, I've had it start up just fine in even colder, but bone dry conditions, and generally the reality seems to be that once a battery's reached that discharged state, it's not going to get any better until recharged either off the mains or by a running engine, and leaving any kind of load connected actually just makes it worse.

So, hum.

This is only for models where the FI is a single injector and sensor-equipped throttle body acting pretty much a direct, computer controllable swap for a carb and more conventional butterfly throttle, by the way. Most multipoint systems (which I would expect includes most multicylinder bikes except for the very earliest conversions; SPi is *extremely* old-hat for cars, after all, even the cheapest and smallest engined models ditched it 15+ years ago) sit rather closer to the actual inlets, which means more or less touching the head on a bike, making icing up less of a risk and something that would have to affect all of the individual injectors to an extreme degree to prevent it starting and running at some level, and the general up-to-date arrangement is one of direct injection which pretty much completely precludes such a problem as the compression stroke itself should produce enough heat to melt any ice layer just long enough for the injector to fire, warming up the cylinder.

Certainly I've not suffered any such problems due to cold or wet weather ever since moving into the multipoint age myself about ten years ago, with a 2000-vintage Megane, and the current Micra with its direct injection simply laughs at wintery conditions... though the idle does sit rather high for a few minutes and the clutch/gearbox absolutely hate the cold. I've had some other problems with the hardware, including failing pencil coils (essentially the old HT lead problem, but more tightly localised)/synchros/engine mounts in the Meg, endless sensor/EGR/turbo/gearbox/immobiliser woes on a couple of diesel Clios (never trust the French or Italians with electrics), and exhausts and brake pads made out of brie on the Nissan, but the fuel systems and most of the ignition parts have behaved themselves admirably. Including the Megane shrugging off some of the coldest conditions I've yet lived through, despite taking more than 15 minutes to get up to sufficient temperature to run the heater and demist the windows (after they'd already been scraped... on the inside) on a fast idle after starting up.
08/11/2018 12:46:47 UTC
Ren - The Ed said :-
Diagnosis is an incredibly frustrating thing to do. Motorcycle and car engines are complex with many parts that all have to work in unison to run well. As you've read around the blog you'll have seen me struggling with various problems and their numerous potential causes. I'll be publishing another one soon that has got me wondering.

Take poor starting as you've mentioned. Is it the battery, HT leads, a LT corroded wire, a sensor, injector, compression due to valves, rings or gaskets and so on.

There is the triangle as it was taught to me. Spark, Fuel and Compression. Without these 3 items engine no go brum brum. But each of these 3 simple items are complicated. I'm tired now just thinking about it. I'm going to lie down.
09/11/2018 08:06:29 UTC
Upt'North said :-
Just a general know it all comment.
Don't under estimate how much importance you should give to earth leads. Bad earth's can show up some really stupid faults.
Undo them, clean them, fasten them, grease them. Nice winter job listening to Michael Ball on Radio 2. Except for listening to Michael Ball on Radio 2.
09/11/2018 09:21:07 UTC
Ren - The Ed said :-
Yeah, I reckon the old earth is overlooked all too often, by myself as well as others. Duly noted.

I like radio 2 but I'll pass on the Michael Ball show thanks.
09/11/2018 09:53:31 UTC
james said :-
I have a broken carburetor heater how can I remove and replace it
21/10/2019 09:49:07 UTC
Ren - The Ed¹ said :-
The carb heater should unscrew from the carbutettor and the new one would screw back in!

I'm afraid without knowing what bike you have etc etc it's hard to be more specific. Also are you sure it's the heater? Not the wiring or even the thermostat that turns the power to the heater one and off?
21/10/2019 02:06:27 UTC
Bob said :-
I learned about these when I had a BMW F650 which suffered with carb icing. I was suprised that BMW sent it out like that, it gets very cold in Germany.
As described, the carbs on the F650 are on long inlet stubs and so don't get heated by the engine. After many failed attempts with fuel additive and IPA I fixed the bike by adding a 50W 10R resistor to each carb float bowl, controlled by a switch mounted on the bars.
I found that the bike ran so much smoother that I ended up using the heaters on dry and not-so-cold days.
I have never had any icing issues with EFI
22/10/2019 01:24:35 UTC
Ian Soady said :-
I had a carb heater on the Honda NX650 Dominator I used for year-round commuting some years ago. It never did much as I remember - disconnecting it seemed to make no difference. I did find Silkolene FST fuel treatment seemed to help with icing however.
22/10/2019 02:13:28 UTC
James said :-
I have a 05 Yamaha Road Star 1700 the heater is broke off inside the carburetor the small tip of it is still within the wiring harness how do I get that piece out of the carburetor
23/10/2019 09:26:51 UTC
Ren - The Ed¹ said :-
Urgh, that sucks James.

I don't know how mechanically adept you are. If it were myself I'd be looking for a stud extractor. The problem is brass is quite soft and so is the alloy of a carb. I'd be concerned the stud extractor could push open the soft brass then the soft alloy ruining the carb.

You could drill it out but you'd have to be very accurate so as not to damage the threads in the carb.

I might try soldering something onto the brass then using that to turn out the remaining bit.

If you're not too sure I'd be seeking the assistance of an engineering shop. They can clamp the carb accurately and drill out the ruined carb heater.
24/10/2019 09:14:54 UTC
Upt'North said :-
I'm with Ed, if you're happy taking the carb off, then take the carb to a trusted bike or engineering shop. That's no place to start attacking with your Black and Decker hand held.
Good luck.
24/10/2019 09:37:51 UTC

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