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post #1 of 8 (permalink) Old 11-06-2008, 05:44 AM Thread Starter
 
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Idle?

Ever noticed some engines have a nice smooth idle, while others tend to oscillate? Last time I heard the zx-rr it seemed to jump to 3k rpm then almost die and then do it again. I've heard several old muscle cars behave in the same way. My bike tends to do that when its cold, but smooths out as it warms up.

What causes that?



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post #2 of 8 (permalink) Old 11-06-2008, 08:51 AM Thread Starter
 
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A little bird tells me that its caused by a lean mixture. Well that makes some sense. Race engine would run somewhat lean to get more power. And far be it for an old muscle car to not be in a good state of tune.
Now what causes an engine to run lean when cold?
More viscous gas that the pump has trouble pumping?
Cold injectors stick a bit?



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post #3 of 8 (permalink) Old 11-06-2008, 11:24 AM
 
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Quote:
Originally Posted by Vash View Post
Now what causes an engine to run lean when cold?
this is half of the answer, colder air condenses, and air condenses more than fuel, thus having more air that fuel... but that can only go so far becuase there is fire/heat reminiscent in that engine that will heat up the air before it gets into the chaimber....


my bike is a bit clunky at low RPM
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post #4 of 8 (permalink) Old 11-07-2008, 04:00 AM
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I had a similar problem with my VF500F, but it was carb'd. I haven't offered advice mainly because it was still a problem when I crashed it. Things I got advice to try (and they didn't resolve it) included changing the boots between the carbs and engine, changing the tubing from the engine vacuum to the fuel tank, resynching, cleaning the carbs, lubing choke, and others that escape me right now.

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post #5 of 8 (permalink) Old 11-07-2008, 05:42 AM Thread Starter
 
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I really don't think I have a problem, as it surges less than 500 rpm, and calms down in minutes. I was just wondering what causes that.



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post #6 of 8 (permalink) Old 11-07-2008, 06:18 AM
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I simply haven't had any surging on my '05 at all. And my friend's '06 GSXR doesn't surge either. Both are smooth as silk. Is that normal for that ride?

From my past engineering experience, and given that the fuel/air ratios are now computer controlled, my gut says somebody didn't do a good job with the process control on the chip. I used to do chemical systems' flow and level controls on very large systems and they would operate exactly as you state at startup and part of our job was to change the flow and level controller reaction rates to properly react to system changes.

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post #7 of 8 (permalink) Old 11-07-2008, 06:36 AM Thread Starter
 
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Hmm, I was just thinking that they are using a weak air temperature sensor. It may do fine telling you if the incoming air is up to temp or not, but cannot tell the difference between a cold engine on a summer morning, and a cold engine on a winter morning.



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post #8 of 8 (permalink) Old 11-09-2008, 03:32 PM
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The nail has been hit on the head already. Cold air is more dense and leans out the mixture, the cold start circuit/choke (carb) will counteract this and enable the engine to start/run. The engine still runs poorly though because it still hasn't reached operating temp and combustion is erratic. A computer controlled engine will only further make things worse if it's setup to monitor and adjust fueling during cold start warm up because it'll constantly be chasing the ideal fuel mixture as it goes too rich or too lean.

A good tune-up and carb/computer setup will make the erratic running less noticeable though while the opposite will of course do the opposite.

Some muscle cars do something a bit different in the rough idle you notice. They "lope" while running due to their aggressive cam profiles which allow them to get more fuel and air into the cylinder but sacrifice some vacuum vital to a smooth fueling at idle (ala the rough idle).
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