Bonneville America Forums Forums Technical Forums Performance Mods Heated carb question

I was looking at the forcast for the rest of the week and it looks like I might be able to get a ride in on Friday. That got me to thinking that I don't know how the heated carbs work. I think they heat the gas in the bowl. But at what point does this happen. When the switch is turn on or only starts to heat when motor is running. Is there some type of thermostat that controls all this and at what temp does the heaters kick on.

It heats at a set temp ( I think around 30 ) when the switch is on , if I recall , you leave the switch on for about 30 seconds before you fire it up .

Which switch? The key, or the main power button on the handlebars?

I believe you turn the key on to pre-heat the carb but I do have a question - if you advance the key to the "P" position will it also pre-heat the carbs without the headlight on?

I hate to make my cold battery power the headlamp for 30 seconds before I run the starter on a cold day as it sometimes takes a while if it's really cold out.

Phaedrus and Matt both are good questions. Thanks for the answer Wade.

It's simple, when you turn on the ignition the carb heaters come on if the temperature is below the set value (don't remember exactly what temp). It doesn't use much juice and you don't have to wait even 30 seconds or so. You save more power make it easier to start than if it didn't come on. If it's warm enough to not be needed they don't come on.

Fortunately I rarely need to think about it. It rarely gets cold enough here and those are days I don't ride.

Thanks. Good info.

The heater warms up the venturi to stop carb icing. The power is tapped from the feed to the igniter, so you have to have both the key and the run/stop switch on.

The computer was down for a couple of days. Thanks for clearing up the matter.

we've had this discussion before....but the "heaters" will not help you on start-up, and having the ignition on without starting the bike is to be avoided. The heaters prevent icing from occurring in cold damp weather. "some "folks swear its not so..but the manual makes no mention of using them on startup, it only cautions against having the ignition on without starting. now lets hear it..

The carb heaters are controlled by a temperature switch. They come on below approximately 10c (50f) ambient temperature.
Beyond that, draw your own conclusions....

These are petrol bikes right... So you're confusing a carb heater with a glow plug? (Diesels) Carb heaters are installed to prevent carb icing, and aid the combustion of the fuel in the winter (below 10 degs C). They do come on with the ignition switch if the temp is below set value but don't sit there with the ignition on waiting for the fuel to heat up. Fire up the bike straight away and get the fuel thru the carbs as it warms.

The thermostat is under the right side cover, beside the starter solenoid (or relay). I used to believe the carb heater aided cold starts but after some experimentation, don't believe that is true. Save your battery.

I heard from a very wise person that they only work when the engine is running. guess you could hook up a meter & find out.

The P position on the key switch turns on the parking lights.

Both the key and run/stop switch have to be on to turn on the carb heat. The heater warms the carb throat to improve fuel evaporation and prevent icing. (ask a pilot how important this is) As best I can remember, the heaters shut off when the igniter module reaches around 40F.
The wiring goes from the main fuse to the key switch, then to the run/stop switch and from there to the igniter and carb controller. The carb controller opens the floatbowl vents and, when it is cold enough, turns on the carb heaters whether the engine is running or not.

Thanks for the explanation. Very helpful.