Question about Harley Davidson FXSTB Night Train Motorcycles

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I have a 04 night train with carb and it does back fire and then there is a mist of oil that spill out of my air filter.

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The backfiring though the carb is probably due to a lean mixture. The carbs are jetted extremely lean to meet EPA emissions levels. Also, the crankcase pressure is vented into the air filter to be recycled though the engine per EPA requirements. The same thing as the PCV valve on your automobile engine. When the engine coughs, it's will blow out a mist of oil because of this.

Now, you can have the carb rejetted to richen the mixture. I'd go up one size on the slow jet. Also check to make sure the accelerator pump is working properly. With the engine OFF, the air filter off, crank the throttle to the wide open position. You should see a stream of fuel squirt into the carb from the brass accelerator pump nozzle.

Since the crankcase pressure is vented to the air filter, oil will accumilate in the air filter. You must periodically remove the covers and clean the filter out. There are kits available on the aftermarket parts venue that will allow you to reroute your crankcase pressure away from the air filter.

Good Luck

Posted on Aug 03, 2010

  • dauger Aug 05, 2010

    tank you for the info. i will check if my carb is well calibrate.

  • wd4ity
    wd4ity Aug 05, 2010

    If you can find someone with an Exhaust Gas Analyzer and a Dynometer, this will help. You can check you fuel air mixture throughout the engine's RPM range and change the jets accordingly.

    If you're just going to try to get it better yourself, this is what I do with the carbs. Remove the carb from the engine. First drill and remove the "anti-tamper" plug that covers the idle mixture adjustment screw. Do this carefully so as not to damage the screw.

    If your low speed jet is a #45 go up to a #48 or a #50. Generally, I only move one step at a time. These jets make more difference than you think.

    Then I remove the slide and the diaphragm that operates it. With the slide out, lift the needle out of the slide. Install a 3mm flat washer on the long end of the slide. This will RAISE the slide the thickness of the washer which is about 0.023". Use a small dab of grease to hold the washer onto the needle while you reinsert the needle into the slide. This slight raising of the needle will enrichen the mid-range mixture just a bit. Reassemble the slide and diaphragm being very careful not to punch a hole in the diaphragm.

    I leave the main jet alone unless I have problems with the bike at high rpm or wide open throttle acceleration.

    Put the carb back on the engine using a new spigot seal (rubber ring around the intake manifold opening for the carb) so you don't have any vacuum leaks. Check carb for proper operation. Start the engine and allow it warm up a bit and adjust the idle mixture for best idle.

    See if this helps

    Good Luck




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