Why does my idle speed keep jumping around?
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Grand HDF Member
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From: Behind you, making faces
Why does my idle speed keep jumping around?
I'm loving the performance of my 1994 Fatboy after the re-jetting the other night, but now I'm having a problem that I hope is something simple.
I can set the idle to what feels about right after warming the bike up, but then if I goose the throttle, it comes back to an idle that's WAY too high. Conversely, sometimes when sitting still and idling, the idle speed will drop to a point that the engine threatens to die and has a couple of times.
Have any of you experienced this? Could it be the mixture screw setting? The throttle cable(s)?
Any help is appreciated.
I can set the idle to what feels about right after warming the bike up, but then if I goose the throttle, it comes back to an idle that's WAY too high. Conversely, sometimes when sitting still and idling, the idle speed will drop to a point that the engine threatens to die and has a couple of times.
Have any of you experienced this? Could it be the mixture screw setting? The throttle cable(s)?
Any help is appreciated.
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Grand HDF Member
Joined: Jul 2017
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From: Behind you, making faces
Okay, now a stupid question....how many vacuum hoses are supposed to be attached to the carb? I only seem to have one, a little 1/4-inch job that attaches on top.
Grand HDF Member
Joined: Apr 2007
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From: Brighton, MI
For 1994 bikes, the vacuum line from the carb goes to the VOES. The voes can develop a leak from just the age of the part. Or it could be the vacuum hose to the voes, also as mentioned the intake manifold to head or carb to manifold seal. I would replace the hose first and see if that fixes your problem. I think you recently replaced the manifold O-rings?
If the hose is good, and the voes is original, might be worthwhile to test the voes with a meter and a vacuum gauge (like mighty-vac or similar) . Even if they don't leak, the switch inside the voes has contact points that get oxidized over the years and the resistance increases. The meter will tell you what the resistance is. Not saying that is the cause of your problem, but I am saying that it may be worthwhile to test the voes for a leak. You don't have to remove the voes to testm but you do need to get at the wire connections and the vacuum line at the carb, so maybe fuel tank removal. You can also hear the voes click when the vacuum is applied triggering the switch inside.
If you don't have a mighty vac, you can at least check for a leak by applying suction. If it holds it good, if it don't then it leaks. The meter is optional but those are just a few bucks at harbor freight if your on a budget...
It's just another suggestion in what to check, good luck. YD
Slight variation in idle speed from a cold bike to a warm bike is normal, mine idles higher when it is warmed up, so I adjusted my idle to 1000 rpms when at normal operating temp nut it should not rise and fall like your experiencing. YD
If the hose is good, and the voes is original, might be worthwhile to test the voes with a meter and a vacuum gauge (like mighty-vac or similar) . Even if they don't leak, the switch inside the voes has contact points that get oxidized over the years and the resistance increases. The meter will tell you what the resistance is. Not saying that is the cause of your problem, but I am saying that it may be worthwhile to test the voes for a leak. You don't have to remove the voes to testm but you do need to get at the wire connections and the vacuum line at the carb, so maybe fuel tank removal. You can also hear the voes click when the vacuum is applied triggering the switch inside.
If you don't have a mighty vac, you can at least check for a leak by applying suction. If it holds it good, if it don't then it leaks. The meter is optional but those are just a few bucks at harbor freight if your on a budget...
It's just another suggestion in what to check, good luck. YD
Slight variation in idle speed from a cold bike to a warm bike is normal, mine idles higher when it is warmed up, so I adjusted my idle to 1000 rpms when at normal operating temp nut it should not rise and fall like your experiencing. YD
Thread Starter
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Grand HDF Member
Joined: Jul 2017
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From: Behind you, making faces
For 1994 bikes, the vacuum line from the carb goes to the VOES. The voes can develop a leak from just the age of the part. Or it could be the vacuum hose to the voes, also as mentioned the intake manifold to head or carb to manifold seal. I would replace the hose first and see if that fixes your problem. I think you recently replaced the manifold O-rings?
If the hose is good, and the voes is original, might be worthwhile to test the voes with a meter and a vacuum gauge (like mighty-vac or similar) . Even if they don't leak, the switch inside the voes has contact points that get oxidized over the years and the resistance increases. The meter will tell you what the resistance is. Not saying that is the cause of your problem, but I am saying that it may be worthwhile to test the voes for a leak. You don't have to remove the voes to testm but you do need to get at the wire connections and the vacuum line at the carb, so maybe fuel tank removal. You can also hear the voes click when the vacuum is applied triggering the switch inside.
If you don't have a mighty vac, you can at least check for a leak by applying suction. If it holds it good, if it don't then it leaks. The meter is optional but those are just a few bucks at harbor freight if your on a budget...
It's just another suggestion in what to check, good luck. YD
Slight variation in idle speed from a cold bike to a warm bike is normal, mine idles higher when it is warmed up, so I adjusted my idle to 1000 rpms when at normal operating temp nut it should not rise and fall like your experiencing. YD
If the hose is good, and the voes is original, might be worthwhile to test the voes with a meter and a vacuum gauge (like mighty-vac or similar) . Even if they don't leak, the switch inside the voes has contact points that get oxidized over the years and the resistance increases. The meter will tell you what the resistance is. Not saying that is the cause of your problem, but I am saying that it may be worthwhile to test the voes for a leak. You don't have to remove the voes to testm but you do need to get at the wire connections and the vacuum line at the carb, so maybe fuel tank removal. You can also hear the voes click when the vacuum is applied triggering the switch inside.
If you don't have a mighty vac, you can at least check for a leak by applying suction. If it holds it good, if it don't then it leaks. The meter is optional but those are just a few bucks at harbor freight if your on a budget...
It's just another suggestion in what to check, good luck. YD
Slight variation in idle speed from a cold bike to a warm bike is normal, mine idles higher when it is warmed up, so I adjusted my idle to 1000 rpms when at normal operating temp nut it should not rise and fall like your experiencing. YD
So I'll try the vacuum line first. Mine was a bit of a pain to get on. Maybe I should also look at the seating of the carb, because maybe I didn't get it quite right? I'll have a look tomorrow.
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From: NC USA
And, I assume that year has a vacuum operated petcock so there will probably be a "T" in the vacuum lines. Eliminate all those possible leak areas before you go any further.
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Grand HDF Member
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From: Behind you, making faces
When I first installed the carb back onto the bike after re-jetting, the idle was low but it was consistent. But then I removed it again to increase the mixture a bit, and since then, the idle has been all over the place.
So I'll remove it again tonight to make sure it's seated properly.
So I'll remove it again tonight to make sure it's seated properly.
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Thread Starter
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Grand HDF Member
Joined: Jul 2017
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From: Behind you, making faces
I think I may try that. But first, I want to be 100% sure that the carb is properly seated against the manifold and that the boot looks healthy.
