Sportster 1200 sputtering at low throttle
Thread Starter
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6th Gear
Joined: Jul 2021
Posts: 10
Likes: 3
From: Ontario
Sportster 1200 sputtering at low throttle
The problem: The bike functions completely normal when cold and warmed up with choke off. Once the bike gets more temperature and heat soaking it's way throughout the bike it will sputter or stall when giving low throttle. It will idle fine, it will ride fine if you can give it enough fuel to get past low throttle and get the rpm's up. https://www.youtube.com/shorts/bqin1ljYlyw
I've been digging into this problem a fair bit and I've concluded that it's unlikely a fuel delivery issue. The CV carb looks clean, the fuel filter looks clean and there isn't any apparent disruption in fuel. I replaced the ignition coil, plug wires and plugs hoping it was a bad coil; it was not. I'm trying to think of how heat could be causing a failure. I was thinking maybe however unlikely that it's bad rings causing too much friction for the engine to rotate at low rpm but I'm not getting excess smoke and it idles fine at low rpm. Not sure where to go next.
My next thought is:
1) Something may be malfunctioning in the carburetor where fitment changes when hot, maybe atomizing is being affected.
2) There's another loss of ignition elsewhere.
3) Loss of compression or vacuum somewhere that worsens as the seals heat up. I did try to induce an rpm surge spraying carb clean around the exterior of the carb near the gaskets, no surging.
I've been digging into this problem a fair bit and I've concluded that it's unlikely a fuel delivery issue. The CV carb looks clean, the fuel filter looks clean and there isn't any apparent disruption in fuel. I replaced the ignition coil, plug wires and plugs hoping it was a bad coil; it was not. I'm trying to think of how heat could be causing a failure. I was thinking maybe however unlikely that it's bad rings causing too much friction for the engine to rotate at low rpm but I'm not getting excess smoke and it idles fine at low rpm. Not sure where to go next.
My next thought is:
1) Something may be malfunctioning in the carburetor where fitment changes when hot, maybe atomizing is being affected.
2) There's another loss of ignition elsewhere.
3) Loss of compression or vacuum somewhere that worsens as the seals heat up. I did try to induce an rpm surge spraying carb clean around the exterior of the carb near the gaskets, no surging.
Last edited by ZKG; May 24, 2023 at 09:23 PM.
Stellar HDF Member
Joined: Jun 2018
Posts: 2,296
Likes: 494
From: WI
Wow. If that is not coming out of the head from the head gasket, and it is actually coming out of the spark plug hole, which seems kind of unlikely, but if it is coming out the spark plug hole, I don’t even know how that engine is running.
Hey Max,Dan, have you ever seen that much oil coming out of a spark plug hole cause I haven’t. Seriously, I think you have a head gasket problem, cracked head, I pray you sure you get it figured out. I’m watching wanting to learn.
Hey Max,Dan, have you ever seen that much oil coming out of a spark plug hole cause I haven’t. Seriously, I think you have a head gasket problem, cracked head, I pray you sure you get it figured out. I’m watching wanting to learn.
Last edited by Harley1004; May 25, 2023 at 11:21 AM.
Thread Starter
|
6th Gear
Joined: Jul 2021
Posts: 10
Likes: 3
From: Ontario
Wow. If that is not coming out of the head from the head gasket, and it is actually coming out of the spark plug hole, which seems kind of unlikely, but if it is coming out the spark plug hole, I don’t even know how that engine is running.
Hey Max,Dan, have you ever seen that much oil coming out of a spark plug hole cause I haven’t. Seriously, I think you have a head gasket problem, cracked head, I pray you sure you get it figured out. I’m watching wanting to learn.
Hey Max,Dan, have you ever seen that much oil coming out of a spark plug hole cause I haven’t. Seriously, I think you have a head gasket problem, cracked head, I pray you sure you get it figured out. I’m watching wanting to learn.
Last edited by ZKG; May 25, 2023 at 11:40 PM.
Tourer
Joined: May 2011
Posts: 469
Likes: 158
From: Alberta
You have a stumble, hesitation or bog, whatever you want to call it. It happens at a transfer point in the fuel delivery system. For example: going from idle to 1/8 throttle which is primarily controlled by the mixture screw. The pilot jet provides fuel for this circuit.
So my view from watching your video your problem is just off idle. You often have to play a bit with the idle screw and the mixture screw as too much opening of the throttle plate ( idle screw ) opens the transfer ports on the bottom of the carb. You want to have as few of these open as possible. So the first thing I would do is try adjusting the mixture screw.
I have no idea what jetting you have in the bike, but it isn't going to be easy to eliminate your problem with those short pipes.
So my view from watching your video your problem is just off idle. You often have to play a bit with the idle screw and the mixture screw as too much opening of the throttle plate ( idle screw ) opens the transfer ports on the bottom of the carb. You want to have as few of these open as possible. So the first thing I would do is try adjusting the mixture screw.
I have no idea what jetting you have in the bike, but it isn't going to be easy to eliminate your problem with those short pipes.
Thread Starter
|
6th Gear
Joined: Jul 2021
Posts: 10
Likes: 3
From: Ontario
You have a stumble, hesitation or bog, whatever you want to call it. It happens at a transfer point in the fuel delivery system. For example: going from idle to 1/8 throttle which is primarily controlled by the mixture screw. The pilot jet provides fuel for this circuit.
So my view from watching your video your problem is just off idle. You often have to play a bit with the idle screw and the mixture screw as too much opening of the throttle plate ( idle screw ) opens the transfer ports on the bottom of the carb. You want to have as few of these open as possible. So the first thing I would do is try adjusting the mixture screw.
I have no idea what jetting you have in the bike, but it isn't going to be easy to eliminate your problem with those short pipes.
So my view from watching your video your problem is just off idle. You often have to play a bit with the idle screw and the mixture screw as too much opening of the throttle plate ( idle screw ) opens the transfer ports on the bottom of the carb. You want to have as few of these open as possible. So the first thing I would do is try adjusting the mixture screw.
I have no idea what jetting you have in the bike, but it isn't going to be easy to eliminate your problem with those short pipes.
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Tourer
Joined: May 2011
Posts: 469
Likes: 158
From: Alberta
A warm engine requires less fuel than a cold engine. So it could be at operating temps it is running too rich or loading up on fuel. Not sure what pilot jet you have in the carb. If you haven't changed anything then it could be something as simple as a piece of dirt in the wrong place. Also vacuum leaks can cause erratic behaviour, but they are easily diagnosed at idle.
The area where you have the stumble is just off idle so you have to work on the areas that affect that portion of the carbs circuits. Namely the pilot jet and mixture screw. Nothing else carb related is actually involved at that point.
The area where you have the stumble is just off idle so you have to work on the areas that affect that portion of the carbs circuits. Namely the pilot jet and mixture screw. Nothing else carb related is actually involved at that point.
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Joe Kucinski
Grand HDF Member
Joined: Apr 2007
Posts: 3,727
Likes: 3,073
From: Brighton, MI
You did not mention intake manifold seal replacement. Leaky (old) manifold seals can do exactly what you describe... As the cylinders heat up they "grow" in length. When they grow in length, the seals lose their seal. They get hard with use (heat cycles) and time. Consider replacing. Make sure manifold is properly positioned when installing. Some air filter mounts will "pull the carb and manifold out of alignment and you will never know it unless you really look closely for it.
Thread Starter
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6th Gear
Joined: Jul 2021
Posts: 10
Likes: 3
From: Ontario
A warm engine requires less fuel than a cold engine. So it could be at operating temps it is running too rich or loading up on fuel. Not sure what pilot jet you have in the carb. If you haven't changed anything then it could be something as simple as a piece of dirt in the wrong place. Also vacuum leaks can cause erratic behaviour, but they are easily diagnosed at idle.
The area where you have the stumble is just off idle so you have to work on the areas that affect that portion of the carbs circuits. Namely the pilot jet and mixture screw. Nothing else carb related is actually involved at that point.
The area where you have the stumble is just off idle so you have to work on the areas that affect that portion of the carbs circuits. Namely the pilot jet and mixture screw. Nothing else carb related is actually involved at that point.