Here is what happens when bike has been warmed after 15/20 miles of riding but not before traveling before 15/20 miles:
Cruising at 55mph..hit normal highway bump then speedo needle drops to 0 then bike has mild hesitation, slight jerking to and fro (bike acts and sounds as if spark plug firing is too advanced or retarded, a feeling similar to lugging the engine).. after 2/3 seconds, speedo and bike go back to normal operation..repeats same problem when next bump is hit.
No backfiring happens.
Might this be caused by a bad speedo or a faulty ignition timer module or both or something else?
Negative and positive ground wires were checked and tightened.
Speedo sensor replaced.
Gas cap replaced.
Overflow tube checked OK.
Vacuum lines are not cracked.
Voltage regulator tested OK.
Fresh gas---93 octane.
New spark plugs and cables.
Original coil.

All comments and tips are appreciated.

 

First of all, what year of bike?

Assuming that it's a 2004+ Sportster:

Something's coming loose when it gets hot and interrupting most of the current.

The speedo runs off of an impulse sender reading off of the transmission output shaft. It's not connected to the ignition circuit and could fail completely without affecting anything else. Similarly, the ignition circuit could fail and it wouldn't take out the speedometer circuit.

The only thing that they share is grounds.

Also, there's no such thing as a positive ground wire on these bikes. They're a negative ground electrical system. You need to check the ground not only at the battery, but also where it connects to the crankcase. Additionally, that's just the main ground. In addition to the point where the negative cable attaches to the crankcase, there is a second ground point on the crankcase on the left side.

Disconnect both grounds and clean them (at the crankcase end, not just at the battery terminal). Just tightening them often won't really help because that doesn't remove corrosion. Then go through the fuse panel and check that all the fuses and relays are seating tightly (clean these as well, can't hurt). Especially check the big maxi fuse.

Report back with the results of that.

 

Remove the two 90* connectors on the coil......Give the slide on connector a good squeeze, slide connector back onto the stud on the coil. Problem should be fixed.

 

Ooops....it is a 2003 1200 standard...about 11000 miles.
Ooops....I placed the word "ground" in the wrong place.

Also the symptoms only happen when the bike has reached its normal operating temperature. Never happens after firstly starting and riding a few miles. Only happens when hot after 15 to 20 miles.

I will try all of your tips and suggestions.

Many thanks.

 

Thank you very much to everyone.
I've heeded all of your tips.
Started with the cheapest and simplest and working towards the more expensive replacements/repairs.
Well, luck was with me on the first attempt.
Took out the battery to clean the terminals, connectors and connections.
Lo and behold the positive battery cable was severed from its connector at its connection with the starter stud.
Thank goodness the bike just didn't quit entirely while on the road.
It gave me early fore warnings via the speedometer.
Who would think that a static wire connection would suddenly sever itself?
But it happens as many have experienced....now I too have that distinction.
Thanks again and hopefully someday I may be able to reciprocate with helpful information and tips too.

 

Now that some info. Mine has been doing the same thing.

 

Im having the same exact issue with my 05 883c but when I hit a bump the speedo dies and when it comes back the check engine light is on for a 1/2 second. And most of the time when this happens the bike will back fire. This will only happen when I hit a bump, but if I'm cruising at 40-50mph and tqwist the throttle open the bike will backfire/sputter and the check engine light will come on. I'm gonna go thru and check the fuse box, battery terminals, and grounds.