In addition to the above, it could be a fuel flow issue:
1. Check all the vacuum lines to your petcock. Edit, 1989 did not have a vacuum operated petcock
2. Check your fuel line to ensure it is in good shape.
3. Remove your gas tank once the fuel is low and on reserve and check your filter screen for any debris and such. If it has never been replaced it might be a good time to replace it.
4. Try running the bike on reserve with a partial fill fuel tank, if it does not have the issue the problem is the fuel screen or crap in the tank covering the fuel flow.
i fully cleaned the fuel tank when I bought it since it hasn’t ran in 15-20 years. Cleared all the rust and coated. replaced petcock and screen. New fuel lines so I know it’s getting great flow. I even removed the fuel cap while the high idle is happening to verify it’s not pressure locking the flow…
This is reminding me of a situation I had with a recently purchased Sporty. Turned out the pilot (mixture) screw was turned all the way in (i.e. “0 turns out”), and the idle screw was turned all the way up to compensate. It ran, but weird things would happen in this configuration, like random and temporary high idle. (What was really weird was I had to drill out the cap to get to the pilot screw, meaning the pilot screw was somehow set like this from the factory!)
Have you done the idle-drop procedure? How many turns out is your pilot screw?
This is reminding me of a situation I had with a recently purchased Sporty. Turned out the pilot (mixture) screw was turned all the way in (i.e. “0 turns out”), and the idle screw was turned all the way up to compensate. It ran, but weird things would happen in this configuration, like random and temporary high idle. (What was really weird was I had to drill out the cap to get to the pilot screw, meaning the pilot screw was somehow set like this from the factory!)
Have you done the idle-drop procedure? How many turns out is your pilot screw?
my mixture screw is turned about 1 7/8 out. Using a 48 slow jet with drag pipes and high flow filter. I installed a quick adjust mixture screw when I rebuilt the carburetor.
one weird thing I did notice is at 0 turns out. The bike will idle low but not die. I would think the bike should not continue to run at 0 turns out but idk.
one weird thing I did notice is at 0 turns out. The bike will idle low but not die. I would think the bike should not continue to run at 0 turns out but idk.
Could be float level set too high, or a leaky float needle letting fuel build up too high and dribble into the airstream when it is not supposed to. If you rebuilt the carb, some of the after market float needles with the black viton tip are rubbish and leak from the get go.
Or sometimes if the tiny ball valves in the accelerator pump get hung up with gunk, they can let excess fuel get sucked up through the accelerator pump nozzle when the pump is not pumping, which dribbles excess fuel right into the air stream and would cause fast idle. Or a torn diaphragm in the accel pump might do that too.
Or it could be the choke actuating mechanism with the high-idle cam hanging up, if your carb has that.
Subscribing to the belief that most carbutator problems are electrical, if your 89 Sportster still has the original mechanical auto advance unit in behind the ignition sensor under the "points" cover, it could well be the auto advance unit hanging up giving you too much advance at idle, making it idle fast, and then eventually the advance weights pull in and the idle drops to normal. Usual cause of this is weak springs, which can be replaced. Or worn pivot pins and holes in the weights, in which case replace the whole unit.
There are some real junk aftermarket advance units available that are more trouble than a secondhand lawnmower. Best one to get is the Vulcanworks unit or maybe Riviera. Even better is a decent electronic ignition that does not use steam-engine technology such as spinning weights and springs. I kid you not, that is how steam engine speed governers worked in the 1800s.
You can check the operation of your existing auto advance unit with a strobe timing light. You should see the timing mark drop smoothly back from full advance to full retard as you close the throttle and drop the revs from about 3,000 to idle. If it idles high and the timing mark is still showing full advance or close to, you got your huckleberry.
48 pilot is perhaps too big. Most of us go no larger than a 45. When did you install the 48 jet? 1 7/8 turns out is not generally the norm either. 2.5 - 3.5 is usually where to set it with a 45 jet. Drag pipes aren't the best for performance, usually a flat spot happens. Of course, none of this may be the issue causing the idle changes.
Could be float level set too high, or a leaky float needle letting fuel build up too high and dribble into the airstream when it is not supposed to. If you rebuilt the carb, some of the after market float needles with the black viton tip are rubbish and leak from the get go.
Or it could be the choke actuating mechanism with the high-idle cam hanging up, if your carb has that.
Yes, IIRC there is a three sided float needle and a four sided one. Make sure you replace with same type.
my mixture screw is turned about 1 7/8 out. Using a 48 slow jet with drag pipes and high flow filter. I installed a quick adjust mixture screw when I rebuilt the carburetor.
one weird thing I did notice is at 0 turns out. The bike will idle low but not die. I would think the bike should not continue to run at 0 turns out but idk.
If the drag pipes have no baffles or lollypops, this could be part of the issue (no back pressure) along with carb adjustment or idle/low idle jet/emulsion tube obstruction.
If the bike doesn't die with the air/fuel screw turned all the way in the pilot is too big. 48 is pretty big and that would be my first guess as to the high idle issues.
11-26-2023, 08:18 PM
