Slow to start
Thread Starter
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Stage III
Joined: Sep 2025
Posts: 3
Likes: 2
From: California
Slow to start
I have a 2020 Heritage 107 with a stage 4 upgrade kit installed. Bike was starting and running great until last week. Now its very slow to start and sometimes I hear a soft backfire with a white puff of smoke coming out the airlifted and exhaust. The Harley mechanic said its the battery. I replaced it with a brand new one and still the same problem. She runs great once she starts up. Amy ideas? Spark plugs? Injectors? Stator?
Grand HDF Member
Joined: Aug 2008
Posts: 3,602
Likes: 9,927
From: Florida's gorgeous East coast
Im afraid all I can offer are some generic thoughts, no experience with your specific bike but maybe some will help. In no particular order:
1.) Long ago I learned not to fully trust a brand new battery. Have it load tested, or swap it with one that is starting someone elses bike just fine. I got a new battery from a dealer that had a bad cell . Chased parts all over the starting system, ignored the new battery because, its brand new. Thats rare, but it happens.
2.) Once you KNOW the batterys good, use a meter to check voltage to see if you can pinpoint where the loss is happening.
3.) Cable/ wire condition can contribute. Unseen corrosion inside insulation, even internal breaks in the copper strands. Something as simple as which way the front wheel is turned can hurt starting efficiency. It changes the stress on the little wire on my FXRS that runs from the start button on the right grip in a bundle, down the neck. Found a break that way.
4. ) How does it start if you jump the posts on the solenoid with a screwdriver/ knife blade, etc.. That takes everything out of the system and sends power directly to the starter. Might help eliminate some possibilities.
Hope this helps.
1.) Long ago I learned not to fully trust a brand new battery. Have it load tested, or swap it with one that is starting someone elses bike just fine. I got a new battery from a dealer that had a bad cell . Chased parts all over the starting system, ignored the new battery because, its brand new. Thats rare, but it happens.
2.) Once you KNOW the batterys good, use a meter to check voltage to see if you can pinpoint where the loss is happening.
3.) Cable/ wire condition can contribute. Unseen corrosion inside insulation, even internal breaks in the copper strands. Something as simple as which way the front wheel is turned can hurt starting efficiency. It changes the stress on the little wire on my FXRS that runs from the start button on the right grip in a bundle, down the neck. Found a break that way.
4. ) How does it start if you jump the posts on the solenoid with a screwdriver/ knife blade, etc.. That takes everything out of the system and sends power directly to the starter. Might help eliminate some possibilities.
Hope this helps.
Last edited by guido4198; Sep 5, 2025 at 04:57 AM.
Seasoned HDF Member
Joined: Aug 2008
Posts: 5,076
Likes: 1,008
+1 on the above
i would do a starter draw test to actually see what amps the system is needing. if it is high, might need to rebuild the starter if no other mechanical issue arise. if the draw down drops voltage too low, it might not start up snappy.
after inrush subsides, voltage will rise so the long crank
i would do a starter draw test to actually see what amps the system is needing. if it is high, might need to rebuild the starter if no other mechanical issue arise. if the draw down drops voltage too low, it might not start up snappy.
after inrush subsides, voltage will rise so the long crank
Seasoned HDF Member
Joined: Mar 2010
Posts: 38,681
Likes: 6,470
From: Honah Lee
I have a 2020 Heritage 107 with a stage 4 upgrade kit installed. Bike was starting and running great until last week. Now its very slow to start and sometimes I hear a soft backfire with a white puff of smoke coming out the airlifted and exhaust. The Harley mechanic said its the battery. I replaced it with a brand new one and still the same problem. She runs great once she starts up. Amy ideas? Spark plugs? Injectors? Stator?
When you push the start button, is the motor cranking over slower than it use too when you attempted to start it? Or are you saying it cranks over longer before gas motor is running evenly with both cylinders?
Do you have a vote ohm meter and a low amperage maintenance charger set up?
That softbackfire out the exhaust and airfilter is usually a cylinder with too much gasoline in it firing up too slow and when intake valve is open. That's not a battery issue. There is probably a issue with a part of those stages. Probably the tuner but that me just guessing.
Could be a slightly fouled plug with a weak spark.
Last edited by Jackie Paper; Sep 5, 2025 at 06:59 AM.
Road Master
Joined: Jun 2019
Posts: 952
Likes: 528
From: Richmond TX
If you find that your battery and all connections are good and the problem persists, you might have an issue with one or both ACR's (Automatic Compression Releases). Jackie Paper has asked for clarification on what 'slow to start' means. If it turns over quickly but doesn't fire right away, it's not the ACR's. If it turns over very slowly, then this could be a place to look. Some bikes won't even turn over if the ACR's are both stuck closed, but you don't mention your cam or compression ratio, so it's possible they're your problem.
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Stage III
Joined: Sep 2025
Posts: 3
Likes: 2
From: California
Slow meaning in have to rev the throttle to make sure its getting gas. Then a soft backfire and small white cloud of smoke then it starts slowly. Once it starts and warmed up the bike is fine
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Seasoned HDF Member
Joined: Mar 2010
Posts: 38,681
Likes: 6,470
From: Honah Lee
What do you mean with the words slow to start? Do you mean?
When you push the start button, is the motor cranking over slower than it use too when you attempted to start it? Or are you saying it cranks over longer before gas motor is running evenly with both cylinders?
Do you have a vote ohm meter and a low amperage maintenance charger set up?
That softbackfire out the exhaust and airfilter is usually a cylinder with too much gasoline in it firing up too slow and when intake valve is open. That's not a battery issue. There is probably a issue with a part of those stages. Probably the tuner but that me just guessing.
Could be a slightly fouled plug with a weak spark.
When you push the start button, is the motor cranking over slower than it use too when you attempted to start it? Or are you saying it cranks over longer before gas motor is running evenly with both cylinders?
Do you have a vote ohm meter and a low amperage maintenance charger set up?
That softbackfire out the exhaust and airfilter is usually a cylinder with too much gasoline in it firing up too slow and when intake valve is open. That's not a battery issue. There is probably a issue with a part of those stages. Probably the tuner but that me just guessing.
Could be a slightly fouled plug with a weak spark.
??Rev the throttle while holding the start button in and motor is cranking from the starter only gives a modern fuel injected motor brief shots of extra air. The air shouldn't be needed.
The procedure for starting a flooded FI motor is to hold the throttle wide open and crank. That sends a signal to the ECM to not give it fuel.
Not sure what signal revving is sending to ECM while cranking.
Glad once it's warmed up, it's fine now.
Last edited by Jackie Paper; Sep 6, 2025 at 08:05 AM.
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