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Carb Rejetting with filter change

Discussion in 'Technical and Troubleshooting Torque' started by ricethief, Jan 8, 2011.

  1. Ive got a cbr400 as a trackbike and am looking at getting rid of the airbox and fitting individual filters, either sock ones or pods. The main reason being to give better access to the carbs etc and the air filter is only available from the uk. Will I need to rejet the carbs with this mod or will it be ok without a rejet? I assume it may get lean with the mods, if so what are the indicators that the bike is running lean?

  2. You're correct in assuming you'll probably need to adjust the carburettors - they will be leaned out by the higher rate of airflow and will require new (larger) mainjets and possibly needles as well. If they have an air jet then this may need to be decreased to allow for the higher flow of air through the carburettor.

    I would start with mainjets that are about 3 sizes larger than stock and seeing how it goes. The mixture will also need to be reset (simple screw adjusters, try a quarter to a half turn out from stock) before you ride. If you experience loss of power at low to midrange, I would replace the airjet as described above.

    Indications that its running lean will be loss of power, crackling and/or popping from the exhaust on decelleration and sparkplugs that are white instead of your usual gray or slightly brown colour.

    Cheers - boingk
  3. Having looked at quite a few 400's around various tracks in Vic I can tell you that without exception all ran airboxes....most also have "ram-air" ie holes in the fairing funneled directly to the airbox...

    must be a reason why

    BTW... for parts try RB Imports in sydney. http://www.rbimports.com.au/default.asp

    have a look here as well.,..http://www.400racing.com/
  4. Generally the airbox setup is there to give you the best performance at all engine speeds. I know that the Aprilia RS125 had a brilliant setup, and using a pod - even with rejetting - compromised everything.

    I'd keep the stock airbox and see if you can't get some sort of open-cell foam layer for a filter. Hell I ran oiled, doubled-over pantyhose in my RS125... and that was on the road! Engine showed normal wear at the 10,000km rebuild.

    Best thing you could do is use ram air or even just another set of reverse-taper inlets. Small opening up front, larger at the airbox. This lowers pressure, increases density and provides more power.

    Cheers - boingk
  5. id get a BMC or a K&N reusable filter. regardless it will need a rejeting and readjustment. pod filters work ok on low compression tractor engines like harley and old stroker V8s, i have never seen them used with beneficial results on high revving high compression inline 4s. im sure dynojet will do a kit. but the best results will only be obtained from a dyno reading but. people have harper on about expense but its about $80 per run with jetting reccomendations. dynojets retails for about $220 but can be had alot cheaper. when i did my last carbied road bike i needed to replace the needle with an adjustable one, new mains, same pilots, drill the slide and was meant to replace the diaphragm springs, the stock ones worked better however so i reverted.
    you can get away with doing a plug reading test on a thumper, ive never seen someone recommend it for a i4 but.
  6. As long as all the carbs are at the same settings, I don't see any reason why the number of cylinders would affect things more than a gnat's tadger. I do acknowledge that the middle pots on an aircooled 3 or 4 will run a bit hotter than the outsides, but the factories all seemed to put the same size jets in all the carbs in the 70s/80s so it can't have been that significant.

    Not that plug chops are much use with unleaded petrol. They'll still show gross errors (Ah, The Bone White Plugs of Doom :D) but the days of being able to distinguish individual jet sizes are gone.
  7. Well said PatB. I find that if you use a low ratio of something like valve seal protectant or even just regular motor oil in the fuel (think 1 to 2ml per litre) you'll get a reasonable reading.

    Cheers - boingk
  8. Hmm. Interesting. When I get back to fiddling with the DR I might try this.
  9. well there ya go. someone has reccomended it.

    the firestorm had a larger jet on the no2 (front) cylinder. 175/177 from memory???
  10. Interesting, didn't know liquid cooled bikes had discrepancies between jets... unless its part of the tuning. I know CBR250's have middle cylinders tuned for better low range performance (still 10,000rpm though hahaha), but other than that yeah... interesting.

  11. Boingk - Thanks for the tips, it will give me a starting point to work from.

    Mike - While my version of the 400 has ram air ducts at the front, they dont connect to the airbox like the later cbr400.

    idlm - i ended up ordering unifilter socks.

    Also a couple of other questions, if I got my hands on a wideband, what sort of afr should be ideal for a i4?

  12. Ideally, you're chasing about 12.5:1 air to fuel ratio for maximum power when on the throttle and around 14:1 when cruising for economy.

    - boingk
  13. If it's purely a track bike, I doubt if this is a major concern :twisted:.
  14. Good point Pat.

    Either way, you'll know when its too rich as you'll bog the engine when rolling on throttle too quickly. You'll know its lean when its popping and cracking under decelleration with a closed throttle. You'll know its right when you get on and its running better than it ever has!

    A quick guide is to get the mains right first (top 30% of rpm at full throttle) then set your needle height (mid throttle performance) and float height (low rpm roll on). You want it crisp and powerful up high, with no hesitation or weakness through the midrange... and you should be able to give it full throttle from say 2~3k rpm and have it pull cleanly through until redline. At each stage, make sure the mixture screws and idle are set properly.

    Cheers - boingk
  15. Thanks boingk, i got my filters today so will have a look this weekend.