long question about carb needles/rich mixture

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Hi all,

This is rather long, but only because I need to fully describe my bike. I have a 75 CB550F. I just cleaned the carbs, and I need to get it to start. Right now, it usually won't start, and if it does, it's only on 1 or 2 cylinders. Here are the specifics:

-Carbs are clean, and I bench-synched them with the drill bit method. -It has an aftermarket muffler. I don't know what kind, all it has stamped on it is the mostly illegible word "winning" on the mounting bracket. I put one of those cheap, universal baffles in it. -It's got Dyna S ignition (from previous owner), new Dyna coils, new wires, new, correctly gapped plugs. -It's got spark. -The wiring is all correct. -New battery. -Static timing done. -Valves adjusted. -Stock airbox, but not attached because working on carbs. -I don't know what size main jet or needles. -When it runs, it's very, very rich, as evidenced by the extremely sooty condition of the plugs upon inspection. If it won't start, the plugs are always wet with gas. -When I got it to start on the 2 cyclinders (1&2), it would only start, from cold, with half choke. -The needles were in the second from bottom position. (where they were when I got the bike) -In order to lean it out a bit, I raised the needles to the second from top position. The result was it would only backfire through the carbs, and not start. -I then moved the clip back down one more position, and I get no reaction whatsoever.

My questions are, would the extremely rich condition prevent it from starting? (I imagine so)

Does raising the clip position (thus lowering the needles) lean out the mixture, or do I need to get smaller jets?

Does the backfiring through the carbs indicate that I made it too lean?

I'm really getting frustrated here. ANY help would be greatly appreciated.

Sorry for the drawn-out question, but I figured it'd help if I completely outlined the situation.



-- Zac Twining (ztwining@gslis.utexas.edu), April 22, 2002


Hello Zac,

I just got my '75 CB550F running recently and was having some problems with too rich conditions. First the needles do not control starting or idling. There are idle jets for just this. There are little brass screws on the side of the carbs, for a starting point make sure they are clean and set about 1-1/2 turns out from full in. Second check your float height. I found mine to be too high causing the rich condition. I am still working on mine so I can't help with the needles, or main jets, haven't gotten that far yet.

Good luck,


-- Scott Kulas (kulas@mindspring.com), April 25, 2002.

Re: Carb problems. Ive had just this kind of problem before.

If your sure the 3&4 have spark (if not check the respective coil connection), I would overgap a set of new spark plugs (like 3mm or so). You clearly have too much fuel flowing through your carbs. Adjusting the idle mixture to lean is next( A previous respondant mentioned this). I expect that the main culprit is the float bowl in the carbs. A full battery, Dyna coils, over gapped plugs and no airbox should get you started. Then you can start adjusting your carbs on a per cylinder basis. -And go back to a more or less sane spark plug gap.

Good luck marc

-- Marc Robinson (jrobinson6@cox.rr.com), April 27, 2002.

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