Why Engine doesn't run smoothly and sometimes stalls when it's cold?

[godudi]

Hi

I am trying to understand why my bike won't run smoothly until the engine warms up? I mean once the engine is started and running what would make it stall? why makes it stumble, surge etc. when cold?

I have a monster 1100 evo in the present, and used to have monster 696 in the past.
When you ride a short distance, you never get the chance to enjoy riding 

Can anyone explain in a simple way?

David

[Dellikose]

This is just my assumption (more knowledgeable members can confirm/deny)...

The engine PCM has to constantly monitor the air/fuel mixture to match engine temperature and gradually bring it to the optimal running temperature. When you reach the right temperature, you can now run the best air/fuel mixture without worrying about the engine not being able to keep up (stall). Temperature is important to get the right oil viscosity for the engine to safely run and be lubricated.

Seems to make sense to me...I guess

[godudi]

This is just my assumption (more knowledgeable members can confirm/deny)...

The engine PCM has to constantly monitor the air/fuel mixture to match engine temperature and gradually bring it to the optimal running temperature. When you reach the right temperature, you can now run the best air/fuel mixture without worrying about the engine not being able to keep up (stall). Temperature is important to get the right oil viscosity for the engine to safely run and be lubricated.

Seems to make sense to me...I guess

Thanks for your explanation and sorry for my dumb questions, but I still don't understand why air/fuel mixture has to match the engine temprature. And finally, why its operation is not smooth when cold?!

[thought]

 i'm guessing that your evo is pretty much brand spanking new and it'll smooth out once you get some more miles on it.

my 796 used to be pretty rough and would sometimes stall when cold when i first got it, but after 2k miles it never happens anymore.

a lot of m1100evo owners are saying the same thing about the stalling, but almost every single one has less than 2k miles on the bike... some less than 1k.

give it some time and it'll get better.  and one option that will def give you better low end smoothness would be to get termis... or at least open up the airbox and reflash the ecu.

[Dellikose]

Thanks for your explanation and sorry for my dumb questions, but I still don't understand why air/fuel mixture has to match the engine temprature. And finally, why its operation is not smooth when cold?!

If your monster is new, then you are probably in the break in period as thought has said.

For the temperature, it as more to do with oil circulation. Oil becomes less viscous the warmer it is. The less viscous it is, the easier it flows within the engine. I'm pretty sure that with newer synthetics it's not that big of a deal, it should be fine after a couple minutes.

How short is a short trip for you?

[bond0087]

To preface this response, I don't know a lot about the new monster engines specifically, and while I am an engineer, thermodynamics is pretty far removed from my specialty. I do have a pretty good understanding of general automotive topics, though.

I'd say that the reason has little to do with your oil viscosity. While everything that was said about the oil is true, I think the effects are much smaller than the air/fuel issues.  Basically, for proper combustion, you need the right amount of fuel for how much air is coming in, and you need that fuel to be in a fine mist (good atomization).  When the engine is cold, the fuel atomization is not as good as when the engine is hot, in other words, the spray is not as even and fine at cold temperatures as it is at high temperatures. When the injector spray hits hot engine components before entering the combustion chamber, the fuel quickly evaporates into a fine mist, and mixes evenly with the intake gas, making for good combustion.  When the engine components aren't hot, however, the fuel can essentially pool up on surfaces, then be sucked in by the intake gas going past, which doesn't lead to as even and fine a distribution of fuel particles, meaning worse combustion.  You could compensate for this by just putting in more fuel than you need, but that'd be bad for emissions, so it's probably being very conservative on the amount of excess fuel it uses as below operating temperatures.  For simplicity's sake, I'm ignoring how the fuel controller tries to compensate for this situation, but I think the short answer to your question is "Poor fuel atomization below engine operating temperature".  Of course, I could be completely off base (and please let me know if I am; I'd like to learn something new), but that's my best guess.

[jason 900sie]

i believe the last response you got. my monster has 20k on it and still stalls if i just fire it up and go without letting it warm up and once it is warm runs like a dream. just sayn.

[godudi]

To preface this response, I don't know a lot about the new monster engines specifically, and while I am an engineer, thermodynamics is pretty far removed from my specialty. I do have a pretty good understanding of general automotive topics, though.

I'd say that the reason has little to do with your oil viscosity. While everything that was said about the oil is true, I think the effects are much smaller than the air/fuel issues.  Basically, for proper combustion, you need the right amount of fuel for how much air is coming in, and you need that fuel to be in a fine mist (good atomization).  When the engine is cold, the fuel atomization is not as good as when the engine is hot, in other words, the spray is not as even and fine at cold temperatures as it is at high temperatures. When the injector spray hits hot engine components before entering the combustion chamber, the fuel quickly evaporates into a fine mist, and mixes evenly with the intake gas, making for good combustion.  When the engine components aren't hot, however, the fuel can essentially pool up on surfaces, then be sucked in by the intake gas going past, which doesn't lead to as even and fine a distribution of fuel particles, meaning worse combustion.  You could compensate for this by just putting in more fuel than you need, but that'd be bad for emissions, so it's probably being very conservative on the amount of excess fuel it uses as below operating temperatures.  For simplicity's sake, I'm ignoring how the fuel controller tries to compensate for this situation, but I think the short answer to your question is "Poor fuel atomization below engine operating temperature".  Of course, I could be completely off base (and please let me know if I am; I'd like to learn something new), but that's my best guess.

Thank You. Your answer is very reasonable.

[suzyj]

Agree with bond.  I'll add that the EFI on our monsters is not terribly sophisticated - while the air-fuel ratio is pretty good at normal operating temperature, when you deviate significantly from that, it drifts significantly from stoichiometry.

[ChrisH]

Don't the o2 sensors need to heat up to be 100% on the money too? My 696 with almost 7k miles does the same thing, but it was drastically improved when I reflashed the ecu and removed the o2 sensors from the ecu program. That leads me to believe that the sensors play a part in it.