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CARB help

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captaindonutbikes:
So gents all help and points welcomed. settle back and i shall tell you the info:

engine: cb550k3 standard with a very mild porting job on the head, more just smoothing than porting, no polishing tho!

carbs: Keihin CR 29mm
Pod Filters
mains: 240
main jets: 120
mixture: 1 1/2 turns out
Needles: right at top position
timing: checked and rechecked (!)
floats: Factory set 14mm
ignition: boyer bransden, with mini coils

So have tried various settings but taken her back to 'out the box' carb settings.

To start her she must be on full choke and i can only get her to 1/3 choke before she dies. i cannot get idle no matter what i do.

when she is on full choke she revs right up with no trouble. when the choke is set at 1/3 when you pull the throttle the engine loses rpm/power. i know she is running lean and i have tried 118 and 122 main jets but i cannot get any kind of idle.

carbs have been bench synced and i have tried different mixtures etc but all the same or worse probs and no idle.

have tried advancing the timing on the stator plate but to now avail.

Need a bit of direction here peeps. how much bigger should i go with the main jets? am i clearly missing something?  ???

(beer tokens for the best answers!)

metalcatfish:
I pressume youv'e tried richening up the idle mix? I dont know if you go in or out on those carbs. I'm running pd46 carbs with a 125 main jet, but i'm still a bit on the lean side, if that helps for comparison. You may have to change the idle jet, presuming everything is good order. Sorry dont know much about them carbs.

Yoshi823:
Have a chat with these people... http://phildentonengineering.com/category.asp?catID=32

Changing the main jet won't have any effect on starting & tickover. It's the mixture screws & float height that affect a closed to 1/8th throttle, then pilot, then the needle. The cutaway on the bottom leading edge of the slide will also have an affect on 1/4 throttle.

http://faq.f650.com/FAQs/CarbMiscFAQ.htm

K2-K6:
Are the "out of box settings" suggested as a start point for that application?

Main jet controls total max demand when wide open slightly trimmed by needle i believe. So has to supply enough to feed the motor at max load, may need someones experience to guide you here.

Needle controls mid range by restricting the amount that flows through main jet at less than full open. Shouldn't require too much shift from central to work (if main is correct) but may require different taper for different application/engine spec.

Idle circuit trims demand when main / needle are effectively closed. So really needs to be considered seperately to the rest to get it starting and running half well.

ffjmoore:
this is what a guy on our form was told.

--- Quote from: traveler on July 16, 2010, 11:57:28 PM ---"Your carbs are on the way. They are set-up for a CB550, but you will still have to make sure that they are jetted correctly for your needs. Try them the way they are and them try the larger 125 main jet to see if it runs better or worse. If it is worse try the 115 jets and see how it runs.  When the bike is warm set the idle around 900 RPM and turn the pilot airscrews in and out 1/4 turn to see where the idle is best. A setting between1/2 turn out and 1 1/2 turns is normal.  When you install them you will see that they are larger than the stock carbs and a tight fit in the rubber carb mounts. Be sure to loosen the carb clampsas far as possible or remove them to make the installation easier. As crazy as it sounds we put a little Windex inside the carb mounts to make them slipin easier. It will evaporate and the carbs will seal better. It helps if you sit the bike in the sun until the carb mounts are warm. Also a person oneach side to push them into place helps. If your carb mounts are too old andhard to stretch just let us know. We stock new carb mounts for the 550s. Please let us know how it goes and we will be happy to help you if you haveproblems. Thanks again for the order and have a great weekend.".


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http://forums.sohc4.net/index.php?topic=51902.75

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