Carb help needed please. 400F

8 replies to this topic
  • RyJames

Posted March 26, 2011 - 06:17 PM


I got my freshly cleaned carb back into my 98 400F (Bone stock) today and after kicking the crap out of it, it finally fired up, but it's running choppy (backfiring and popping out the exhaust). This bike ran great before being put away for storage in the winter.

I did an oil change, new plug, tank flush, but I did move the fuel screw when I had the bowl off. Could this be a simple matter of re-adjusting the fuel screw by 1/4 turn at a time to get it running right again? My guess is that I'm getting too much fuel. Even after warmup, the bike requires choke to fire back up and won't idle on it's own. Would greatly appreciate any help you can offer. I'd love to be able to ride a bit tomorrow.

  • grayracer513

Posted March 26, 2011 - 07:08 PM


If it was getting too much fuel, why would using the choke make things better?

What you probably have is a varnished pilot jet. The metering orifice in the pilot is only about .015", and a film of dried fuel residue will reduce the effective size of the jet by as much as 50%.

As far as the pilot screw is concerned, you can't just stick it back in and expect it to work. Base screw settings for your engine range between 1.5 - 2.5 turns out from gently seated. You will also want to be certain that there is a washer and an O-ring under the spring on that screw.

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  • RyJames

Posted March 27, 2011 - 04:53 AM


Thanks for your reply, gray.

The jets aren't clogged, as I had the carb apart and all jets sat in carb cleaner and I double checked that they were clean. I didn't just stick the mixture screw back in. I set is 1.5 turns out, as stated by the manual. So if it's not getting fuel via the pilot jet, why would it be backfiring as if it's running a rich condition? Would turning the mixture screw out another 1/2 turn be enough to make a difference in it's condition? Then maybe I'll know if it's just a matter of tuning the mixture screw to make it right. This FCR carb is a pain. Sure would be nice to have a mixture screw on the outside of the carb like everything else I've owned.

  • grayracer513

Posted March 27, 2011 - 08:08 AM


So if it's not getting fuel via the pilot jet, why would it be backfiring as if it's running a rich condition? Would turning the mixture screw out another 1/2 turn be enough to make a difference in it's condition?

Why do you think a rich condition would cause backfiring? Most times, it's the other way around. I'll ask you again: If it was too rich, why does the choke improve the way it runs?

And you need to understand that most carb cleaners you can buy today won't dissolve dried varnish the way the old dip can stuff did, so you may need to use a piece of fine wire or the shank of a #80 drill to clear the pilot. Again, you can have a jet that you can see light through, blow carb cleaner through, and still have its size effectively reduced by a film of varnish. Varnish may also be built up in the pilot circuit passage itself, either leading back to the mouth of the carb, or up into the carb throat.

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  • William1

Posted March 27, 2011 - 08:13 AM


Put in a NEW pilot jet and retest.

  • RyJames

Posted March 27, 2011 - 08:15 AM


Taking everything you've said into consideration, I'm going to go ahead and purchase a new pilot/mixture screw, float valve, and bowl gasket. I'll give the carb body another thorough cleaning and start with the mixture screw set at 1.75 turns out and see how she runs.

As far as my assumption about too much fuel causing the backfiring, I guess I was wrong. Obviously if it's running with the choke but not without, then the problem is clearly a lack of fuel delivery. I appreciate your help!

  • SledParty

Posted March 31, 2011 - 10:13 PM


I'm pretty much having the same problem, did you fix yours and how??

  • SledParty

Posted April 04, 2011 - 11:57 PM


try disconnecting the TPS sensor on the carb. I've been reading and this seems to cause a lot of problems for some folks.

  • mikewrf18

Posted April 05, 2011 - 12:08 AM


Put in a NEW pilot jet and retest.

What William1 said, "New Pilot Jet." Also, on a side note, when cleaning passages they need to be blown out with compressed air.

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