Hello, I have a 2001 Sportster 1200 (stock) Carburetor has been rebuilt. Still will not run unless choke is out all the way. Also pops and backfires out the carb. Compression on back cyl. is 120 pounds and 110 pounds on front cyl. I bought it like this. Thinking just clean the carb. Back spark plug black (I would think from the choke being on) and front one looks normal.
I welcome any ideas. Also low speed jet is a number 45 Thank You

 

How many miles? It's running rich and the timing could be off or an intake leak.

 

It has 29,700 miles, just the back plug is rich and no intake leak

 

Never worked on a sporty carb, but that sounds like main jet issues to me. Will it idle ok?

 

I took the carb off and took it to a HD dealer and had them go through it, did not make any difference.

 

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Sounds like your mounting AC bracket isn't seated straight or the gasket between the carb and the manifold is loose .
Had the same issue exactly last week. Replaced the rubber gasket and took good care on mounting the AC bracket and it returned to normal operation.

 

The choke is actually an enrichener, it adds fuel for better cold start up.
If your sportster only runs with enrichener fully out it's telling you it needs more fuel. Maybe bump that pilot jet up a size and see if it helps.
If not it's got an intake leak and additional air is getting in the cylinders, thus requiring more fuel.

 

If it ran before this issue, jetting should not be your problem. Intake leak is the likely issue.

 

You've probably got an intake leak somewhere. Go over it good. You'll never tune it properly otherwise.

Also 120 compression is low end of FSM specs (too low in my opinion), and 110 is below spec. Are you checking compression with the throttle wide open?

 

The choke does not add fuel, it restricts airflow, thus enriching the A/F ratio. Since these carbs work off vacuum, the suction pressure increases on the fuel to draw more fuel in thus aiding in cold starts.