Topic: Cold starting

[thelowmax]

This may seem like a obvious issue but…

Why is my bike so hard to start when it's cold? When I garage it or in the warmer weather it starts up at the first kick every time. But late in the fall if I've left it out for a while, it needs a lot of choke, idle finesse and repeated attempts. what is it exactly that is causing the problem?

Cold oil? Contraction? Gremlins? Anybody?

[bjbuchanan]

Full choke and unless you have fast idle cam carbs like the PD ones that have them, which you probably don't then choke and work the throttle.

Don't play with the idle know like some people do, just gear up before start-up. Some bikes are "cold blooded" but you haven't listed anything. IIRC you have a 750 but either way save the year make model in your sig line

[DustyRags]

It's a physics thing with the combustion. Essentially, combustion (burning) requires both fuel (gasoline) and oxygen (air). That means that it can only burn where the twain shall meet. Pour a cup of gas in a can and set it on fire, and you've got a nice, slow flame. Pour it out on the ground, and you've got much more surface area and it burns faster.

Apply that same theory to a carburetor: the more it atomizes (sprays, they don't actually break things down to atoms) the fuel, the more surface area there is to burn, and the fuel burns faster. That's why we have carbs in the first place.

If you REALLY want something to burn, vaporize it. Basically, let it evaporate. Now you haven't got droplets, but individual fuel molecules floating in the air. Those don't just burn, they (virtually) explode (technically, an explosion happens simultaneously while "exploding" vapors just burn very, very quickly, but that's nitpicking). Explosion=sudden expansion, and that's what gives the engine power.

So, if you want the bike to run well, you gotta hit that perfect mix of gasoline vapor and air. Too much gas, and you've got a rich burn- you're not burning all of it, and puking out clouds of smoke. Not enough gas, and you're burning it hot and fast, but not expanding as much as you could.

So when you've got a cold bike, with cold air and cold fuel, the carbs mix the air and fuel droplets, but they're not evaporating entirely. Rather than getting a nice mixture, you're getting a lean mix, along with droplets of fuel that can't explode, but have to burn slower. The choke or enricher changes that mix so there's more fuel. It still doesn't evaporate completely, but since there's more of it, you now have a better mix of air/fuel vapor, with droplets of fuel floating around. Better mix=better burn=runs better/at all.

As the engine warms up, the fuel evaporates faster, so rather than exploding the vapor and then burning the droplets, it just explodes the vapor because the droplets have all evaporated. Now you're rich- you close the choke/turn off the enricher, and you're at proper running mix.

Make sense?

[HondaMan]

One of the 'things' about Kettering ignitions that is not well-understood by the average user is: the tungsten points conduct poorly when they are cold. This was always an issue with the old cars, and with our bikes it remains. This is one area where my ignition really shines: it adjusts for the cold, boosting the power to the coils over the points power until the points warm up enough to do their job. I don't "tout" it much, because most folks don't understand it, but it's true. The most common 'thanks' I get back on these boxes is the comment on how much it improved the cold-start issues, particularly on the bikes with the PD carbs (aka "lean burn" mix carbs).   

[thelowmax]

It's a physics thing with the combustion. Essentially, combustion (burning) requires both fuel (gasoline) and oxygen (air). That means that it can only burn where the twain shall meet. Pour a cup of gas in a can and set it on fire, and you've got a nice, slow flame. Pour it out on the ground, and you've got much more surface area and it burns faster.

Apply that same theory to a carburetor: the more it atomizes (sprays, they don't actually break things down to atoms) the fuel, the more surface area there is to burn, and the fuel burns faster. That's why we have carbs in the first place.

If you REALLY want something to burn, vaporize it. Basically, let it evaporate. Now you haven't got droplets, but individual fuel molecules floating in the air. Those don't just burn, they (virtually) explode (technically, an explosion happens simultaneously while "exploding" vapors just burn very, very quickly, but that's nitpicking). Explosion=sudden expansion, and that's what gives the engine power.

So, if you want the bike to run well, you gotta hit that perfect mix of gasoline vapor and air. Too much gas, and you've got a rich burn- you're not burning all of it, and puking out clouds of smoke. Not enough gas, and you're burning it hot and fast, but not expanding as much as you could.

So when you've got a cold bike, with cold air and cold fuel, the carbs mix the air and fuel droplets, but they're not evaporating entirely. Rather than getting a nice mixture, you're getting a lean mix, along with droplets of fuel that can't explode, but have to burn slower. The choke or enricher changes that mix so there's more fuel. It still doesn't evaporate completely, but since there's more of it, you now have a better mix of air/fuel vapor, with droplets of fuel floating around. Better mix=better burn=runs better/at all.

As the engine warms up, the fuel evaporates faster, so rather than exploding the vapor and then burning the droplets, it just explodes the vapor because the droplets have all evaporated. Now you're rich- you close the choke/turn off the enricher, and you're at proper running mix.

Make sense?

Totally makes sense. Thanks.