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Steve, Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 6994
Experience:  30+ yrs. experience, MMI, HD technician, HD early, engine rebuild/performance, bike builder, kit bike program manager, CCI 2001-2004
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My 2003, 1200 EFI equipped Fatboy is developing a strange

Customer Question

Hi, my 2003, 1200 EFI equipped Fatboy is developing a strange noise at certain RPM's. It sounds a little like a high pitched vibration or hum that last a few seconds between shifting gears. If the bike is under load in 2nd @ 20 mph, and 30 mph in 3rd and now even 4th. But, it only happens at specific places on the torque curve, between being under load or no backpressure. I am starting to hear a knock sometimes also now and then. I changed fuel already and it didn't help that much.
JA: Do you have the torque specs?
Customer: I only have the manual.
JA: What is the model of your '03 motorcycle?
Customer: FLSTF
JA: Are you fixing your bike yourself? What have you tried so far?
Customer: I have changed fuel. It helped a little at first then returned to the same thing.
JA: Anything else you want the mechanic to know before I connect you?
Customer: No.
Submitted: 9 months ago.
Category: Motorcycle
Expert:  Steve replied 9 months ago.

where is the noise coming from?

tell me everything you can.


Customer: replied 9 months ago.
The noise (humming vibration sound) seems to come from the front of the bike, under the fuel tank. It doesn't happen if I am not accelerating or even if I am decelerating. It seems to happen when shifting gears and between gears, in second gear at about 20 mph, 30 mph in 3rd gear, 40 in 4th and as I said only if I am not accelerating or decelerating. Also, after it is thoroughly warmed up, I can hear a knock at idle when I come to a stop. After reading up a little I am starting to suspect it is in the EFI system somehow. I already changed fuel. The engine runs fine and doesn't seem to be any mechanical problem.
Expert:  Steve replied 9 months ago.

the EFI system does not hum or knock.

it seems to me you are describing a chain noise.

the first thing I would do is adjust the primary chain to see if it has any bearing on the noise.

I would also listen to ensure the noise, especially the knock, isn't coming from the bottom front area of the crankcase, where the balancer shafts and chains run.

let me know if you need more help.


Customer: replied 9 months ago.
Ok on the EFI. I have a belt drive so I sort of ruled out the possibility of a loose chain. But you are right, the noise does have the characteristics of a loose chain of some sort. Is there a chain inside the crankcase? The noise comes from under the fuel tank but I can't pin point exactly where. It does knock at idle after it is warmed up good. I just had it serviced. oil, transmission fluid, filters but I don't think that has anything to do with it. I checked all the fluids already they seem to be ok. I will listen closer to see if I can pin point where the knock noise is coming from as you suggest.
Expert:  Steve replied 9 months ago.

let me know if you need more help.