Bike would sustain a steady 74mph indicated. Starting with the stock 110 jets, I made a 5th gear wide-open run up the 2 mile long 7% grade just outside of town. I went a step further and tried some different main jets. I have not yet gone through a full tank of fuel, but economy seems to be vastly improved. It will pull smoothly (for a thumper) almost right off idle with little to no jerking or shaking. The solution is simple: Replace the needles and emulsion tubes! That's what I've done, and drivability issues are completely gone. According to FactoryPro, in some cases issues due to wear become noticable in as little as 10k miles. Over time, the wear becomes significant enough that the fuel mixture becomes very rich at part throttle conditions. One of the issues with the BST carbs, and all carbs with a moving needle, is that the needle and the corresponding jet (in this case, emulsion tube) wear. Its the most reliable and consistent.There was nothing wrong with the spark plug or ignition on mine. You can not rely on stamping sizes, as recent posts by another member clearly showed how different and inconsistent aftermarket jet orifices are. A complete cleaning of the carb internals would be a good step to insure what's in there, where needle clips are set (a huge impact on fuel delivery) and jet sizes. Fix what's actually broken by diagnosing each system in proper sequence: ignition first, then valves, then carbs. Manipulating fuel height lower to compensate for sooty plugs is a bad idea. Fuel performance and other "surging" is then diagnosed properly by the other systems involved (ignition, valves, induction/exhaust). However, a mixture of jets, needles, and screw changes coupled with poor performance from the ignition certainly will.įuel level in your bowls should be set to the proper height, irrespective of your plug colors. Non-stock needles will effect the fuel performance, but I can't imagine as much as you describe. Looking for thoughts and opinions even if you feel I am not on the right track.Ībsent of jet gauges, you can not be certain someone prior to you has not drilled open the orifice diameter of the jets that are installed. I have checked the coils and they ohm out good, Dyna S Ignition System and the air screws need to be set at 2 1/2 turns out or it will not idle at all. I’m not real jazzed about the second option. Buy an aftermarket set of emulsion tubes or used if they can be found or buy 022A needles. Installed the factory Mikuni needle and it was corrected. The other reason is I chased the same issue with another motorcycle when I blindly installed the Factory Pro needle with a stock emulsion tube and fuel economy was horrible afterward. I pulled a couple emulsion tubes out but there was no part number stamped on them. Two reasons: The 100 main jet that was in the carbs when I bought it, I think the PO bought the wrong kit and replaced the emulsion tube with the wrong one. What I am highly suspicious of is the needle jet/emulsion tube, that it could be for a 550K 022A model. The carbs also have the stock needles with the Keihin part number 273004. The carbs originally came installed with 100 mains (from the PO) and have since put in the proper 98 mains. Full throttle the bike pulls hard and has no surging. I bought the bike this spring and it has run poorly and comsumes a lot of fuel ~26-27 mpg with a lot of surging from 1/4 throttle to 3/4 throttle and black sooty spark plugs. 1975 CB550F with 069A carbs, stock exhaust, stock airbox, properly set up carbs etc.
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