Subtle idle variation - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 18 (permalink) Old 09-01-2013, 10:21 PM Thread Starter
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Subtle idle variation

I tend to be a bit of a perfectionist as a mechanic and sometimes I need to be told when good enough is good enough.

Since rebuilding my carbs my bike has been running much better. No more stalling, to more flooding out. I'm going on longer rides and building confidence in the bike.

The one thing its doing that still bothers me is that the idle will vary a bit. When the bike is fully warm it will vary by about 50 RPM. It will idle around 1100 for 15-20 seconds, then drop to 1050 for a few seconds than back. Is this just normal? I replaced the plugs and the intake boots were in great shape when I pulled the carbs. I don't think its an intake leak but the problem is too spurious and subtle to really determine if a soapy bubble test affects it.

So should I keep looking into it or is it good enough?

-Robert
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post #2 of 18 (permalink) Old 09-02-2013, 08:51 AM
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You may be getting a little bit of fuel past the float needle and seat. But I would not worry about it. I would use a little sea foam and ride.

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post #3 of 18 (permalink) Old 09-02-2013, 11:20 AM
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50 rpm's? Most could not hear the difference. Are you sure it is not just the tach? It would be somewhat common for an analog gauge.
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post #4 of 18 (permalink) Old 09-02-2013, 11:25 AM
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As quirky as these carbs are, that's going to be hard to pin down. If it runs well and isn't giving you fits, it may best to be happy with it until the problem gets bad enough to find. 50 rpm variation isn't all that bad. edit: The #1 thing I think you could try is tweaking the air screws just slightly and see if you can even it out.

Could be going lean or going rich at times. I'd say it's uneven fuel delivery in one direction or the other. Could have missed one or two dirty holes in the emulsion tube, the fuel picks up, then drops off.

Maybe a small vacuum leak, maybe dribbles of fuel coming through the petcock vacuum.

I don't think the float needle and seat would be the culprit as far as leaking, when running, I think the needle is open to some degree all the time. Now float height could be making it too rich momentarily. Then there's the coasting enrichers that also have a needle and seat to deliver fuel during high vacuum (really think that's the source of decel popping).

Loose valve guides can cause erratic idle, but your going to see a lot of oil usage and plug fouling when they're that bad.

I usually try to discourage people from chasing those idle hiccups and split second stumbles when the vehicle runs good enough otherwise. Vehicles we drive aren't supposed to sit around idling for long periods anyway.

I know I'm going to have to get into my carbs. The Seafoam cure lasted one ride and it's back to having a rev limiter. It's also had a bit of idle hang at times, even though the boots are tight, no vac leaks, etc. When I first put it on the road, it had decel pop, after several hundred miles it went away. The coasting enrichers started working, I'd say. Going to try and put off carb cleaning until the snow flies or she gets sold in the divorce. I don't need to be hitting 9000 rpm anyway.

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post #5 of 18 (permalink) Old 09-02-2013, 12:18 PM
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You saying does this when the engine is warm....do you really mean warm or could you say - hot ??

When the bikes running at idle and sitting still with a hot motor much of that heat is warming up the air going into the airboxes. This will actuallly cause the RPMs to drop some. The bikes RPMs down, it puts out a bit less heat, air a bit denser now, RPMs go back up a bit. It can repeat this cycle for awhile.

That would be my guess as to what's happening if everything is functioning normally...

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post #6 of 18 (permalink) Old 09-02-2013, 12:23 PM Thread Starter
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Quote:
Originally Posted by wmsonta View Post
50 rpm's? Most could not hear the difference. Are you sure it is not just the tach? It would be somewhat common for an analog gauge.
I can feel it too. But I'm a bit hypersensitive to how the bike runs and especially idles since before my last carb work it would die after idling. That problem seems solved now though. The float pin plastic was deteriorated so after idling for 20-30 seconds, when warm the idle would jump up and down a lot (up to about 2000 RPM) before totally dieing. The only way to start it then was to turn the fuel valve off. So that issue is now resolved but I'm sensitive to the idle as a result.

-Robert
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post #7 of 18 (permalink) Old 09-02-2013, 01:43 PM
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When the bikes running at idle and sitting still with a hot motor much of that heat is warming up the air going into the airboxes. This will actuallly cause the RPMs to drop some.
When my bike was getting too hot, the rpm at idle always rose and stayed there. At least 1500-1800. That was the first tip off that got me checking the gauges. When that happened I got out of the stoplights and traffic and cooled her off, then I took the fast way home.

I'm still thinking RG1 might balance this out with the air screws, but it's going to need a light touch if it works.

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post #8 of 18 (permalink) Old 09-02-2013, 03:45 PM
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Maybe a small vacuum leak, maybe dribbles of fuel coming through the petcock vacuum.
That is where I found my 100-200 rpm idle variation to be. I utilize 3/16" clear vacuum Line (not fish tank stuff) I had purchased from a Hardware Store. It was visible, the leaking Petcock allowed tiny amount of fuel to get sucked into vacuum port.

Rebuilt Petcock, problem solved and replaced/renewed 3/16" clear vacuum hose.
btw: I also utilize 1/4" clear vacuum tubing for fuel line.

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post #9 of 18 (permalink) Old 09-02-2013, 05:45 PM
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I have had bikes where the idle was very erratic. It constantly went up and down. There are a bunch of possibilities. Jets could be clogged, float level could be off, it could be a vacuum leak, an intermittent fuel supply to the carbs, poor carb venting, carbs not synchronized, pilot screws not adjusted properly, a leak around or in the diaphragm, or even to much of a difference in compression between cylinders. It's rarely the ignition, but it's possible.

One of the neat things about carbs is that if everything is right, they can be tuned to achieve a dead stable idle. I have never seen a fuel injected engine (car or bike) with a nice smooth idle, and the throttle response is never as good either. When you rev a carbed engine and release the throttle, it drops back to idle instantly. When you rev a fuel injected engine, it usually takes 2-4 seconds before it comes all the way back down to idle.

I think a stock carbed engine in good mechanical condition should have a nice steady idle. I do have one engine (in a truck) that won't idle smoothly, but it has a long duration cam.

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post #10 of 18 (permalink) Old 09-03-2013, 12:54 PM Thread Starter
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Quote:
Originally Posted by VN750Rider/Jerry View Post
One of the neat things about carbs is that if everything is right, they can be tuned to achieve a dead stable idle. I have never seen a fuel injected engine (car or bike) with a nice smooth idle, and the throttle response is never as good either. When you rev a carbed engine and release the throttle, it drops back to idle instantly. When you rev a fuel injected engine, it usually takes 2-4 seconds before it comes all the way back down to idle. .
The reason the injected engine takes a couple of seconds is for emissions. reving (or any acceleration) requires an increased mixture and the slower drop to idle helps run everything through the cat before the idle comes down. Ultimately its suppose to prevent cars from building up emissions while driving and then blowing them all out at a stoplight. When I have one of my injected cars on the computer and I set the actuator to run a specific RPM it goes right to it.

-Robert
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