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engine missfiring / running rich

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by damo03, Mar 5, 2008.

  1. I recently swapped engines on my 1981 cb250 to a cb400n engine and swapped overthe carbys as well as I assumed the jetting between the 250 and 400 carbys would be different. The 400 seems to be running very rich as it has a backfire when coming off throttle and misses when not under load (misses when cruising at low speed and not accellerating).

    I didnt change over the cdi unit, would this be different?

    Im wondering if this would be a jetting issue, blocked jets, or the carby would need a overhaul. Also if someone could confirm wether the jets are the only different bits between the carbys. I might put the 400 jets into the 250 carbys and see how that goes.

    Any input and ideas would be appreciated.

  2. First, eliminate the possibility of a loose exhaust.

    Your Honda dealer should be able to tell you what the jetting differences are. The parts microfiche will say what the jet sizes are. Can't see the CDi causing the misfiring you describe. Are the air filters the same? Maybe less air's getting in if the 250 had any restriction upwind of the carbies. Also what do the plugs look like after a plug chop? (ie, run engine up hard and then turn immediately off).

    You plan to put 400 jets in 250 carbs sounds like a good idea.
  3. so just to get it right, you've got the 250 carbies on the 400? Why?

    A popping on closed throttle actually indicates a lean idle mixture. there is an explanation here: http://www.mikuni.com/tg_backfires_in_exhaust.html

    If the carbies are the same, swapping the jets over may get you the right setup, if they were right in the first place.
  4. Thanks for the replies so far.

    Just to clear things up a bit more the 400 engine and the 400 carbies were swapped over, and the air intake / filter is the same for both models. The misfire is only cruising at low to mid revs at little load. Accelerating hard there is no miss.

    The off throttle backfire is present when backing off at any speed / rev.

    A friend I spoke to rekins that the fuel air mixture ratio might be stuffed so I may have to adjust that.

    Thanks for that link Ibast. It's quite possible that there is a leak in the exhausts so i'll clear up the misfiring first then worry about exhaust leaks / backfiring.
  5. Swapping over the main jets won't affect the problem at the throttle position you have mentioned. Check also for muck in the float bowls in case it is actually running lean.


    Trevor G
  6. Sounds more like lean than rich. Hard to tell over the net, mate.
  7. Do you get the cheaper 250 rego still, with the 400 fitted???
  8. Popping usually means a lean mixture, check the exhausts for leaks etc. particularly at the head. It doesn't happen when accelerating due to more fuel going into the engine.

    Check the plugs too, and make sure they are the right ones for the 400N, just in case and clean them while they're out.

    Also I take it you gave the 400 carbies a good clean out before fitting? After 25 years there may be "some" wear and /or dirt in them.
  9. Mate.

    There are a few very small leaks in the exhaust which Im going to rectify, so thanks for that diagnosis of the backfire /popping.

    Plugs were near new from the 250 engine that are the correct type. The carby didnt get dissasembled when i swapped it over due to a time shortage but I plan to do that in a couple of weeks as I now have two carby rebuild kits with needles and jets so I'll do a complete overhaul and see how it goes...Im hoping whatever the problem is (dirt in carby, wrong air mixture, worn needles etc) will be resolved.

    Cheers for the replies.