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Refresh OEM Carbs

473 Views 1 Reply 1 Participant Last post by  david.jennings.orpa
Some of you 'Old Hands' out there who have successfully brought a project back to life, I need your help. I have a 1971 CB 175. Top end rebuilt, valve clearances are in spec, new plugs, points & coils and plug caps. Ignition timing set per spec. Ignition gap on points set per spec. New battery installed, starter motor runs good. Seems like it is ready to start and run. And yes, the spark plugs are sparking.

Set the OEM carbs to initial one full turn out on the pilot air screw and the Throttle stop screw. Fuel on, switch on, full choke set, hit the starter. It starts after the bowls fill-up. It is running lumpy with the choke on at about 3,000 RPM. Turn the choke off. And...WHHEEEEEE!!!!!!

The engine revs from 3K to 8K RPM before I get my hand of the choke. I have not touched the hand Throttle at all! The amount of fuel getting into the engine is from the carb without me advancing the hand Throttle. I am so surprised, I fumble for the kill switch, but this model does not have one on the bars. The sound is tremendous, constant and very nice. Not missing a beat. I finally get my hand onto the key to shut it off. Silence.

As exciting as that was, it is not good. I have a new #92 main jet and a new #35 pilot jet installed in the carbs, along with new floats, new float valve and float valve seats. The bowls are filling up and are not leaking. The float height is adjusted to spec.

So I adjust the air screw to the closed position, the Throttle screw all the way out such that it is not causing the slider to move up at all. Set the choke, hit the starter, and it does exactly the same thing again. I can run it for 1 - 2 minutes on the choke at about 3K RPM, but moment I turn the choke off the engine begins to increase speed quickly to the redline!

I changed the main jets to the smallest ones I have on hand (#90) and repeat the process again, with the same results.

I think the needle valves on the sliders may be set too high, allowing too much fuel past the needle at 'idle'. I think m next step is to lower the needle a notch or two and see if that helps. Otherwise, I am open to suggestions.
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And its fixed! The Throttle cable splits into two under the tank. The left hand cable segment was not seated in the cable splitter, placing extra strain on the left carb slide (holding it nearly WFO!) Running great now!
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