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1995 CBR 600 F3 - Choke stuck?

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Old 07-10-2018, 10:32 PM
RedBull's Avatar
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Default 1995 CBR 600 F3 - Choke stuck?

My boss recently brought over a pretty beat up '95 600 F3 so my dad and I could try to fix it up into a road ready or stunt only bike. I had it running today but after a few start ups it absolutely refused to start up again.. just cranking until the battery would die. We finally took the tank off and got down to the valves.. I'm not sure of the technical term.. but the choke "plates" are stuck closed. The choke cable actually does not pull the little slider that should open/close the choke plates. You can however move the slider by hand but that does not make the plates even budge. I've never messed with anything like this before but I can usually take apart and put anything back together. What is he slider that the choke cable pulls on supposed to be connected to? Has anyone dealt with this before? I can open the choke plates one by one manually (very easily) with my fingers or a screwdriver so they're not physically stuck or gummed up in any way but they will not stay open.

Any insight would be great. I can get pictures sometime if it would help. I regret not taking any now..
 
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Old 07-11-2018, 01:15 AM
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There is no valve or choke plate...engaging the choke activates a specific circuit on the carb that sends extra fuel to the engine (as opposed to sending less air by blocking its flow with a choke plate - either way you end up with a rich mixture, which is what a cold motor needs). The throttle cable is connected to the throttle plates, which are at the exit of the carb just adjacent to the head (pic):



At the input side of the carb you'll see the vacuum piston slide. You can, with the engine off, lift it up and when you let go, a spring will close it again. This piston is vacuum controlled, in that it is nearly closed at idle and slides upward with increasing RPM. Lift up a slide and look inside and then open the throttle...now you can see the throttle plates move. Attached to the diaphragm that lifts this slide is the main metering needle, which progressively enriches the fuel mixture as the slide moves up. The slide itself helps keep intake air velocity high by limiting airflow at lower RPM's, improving throttle response.

There's tons of info online, but basically if you have spark and you can't get the bike started, the carbs will have to come off and get disassembled and cleaned. PITA the first time, but if you're mechanically inclined you'll be fine.

(Edit: Although carbs are the most likely issue, know that a clogged petcock or fuel screen inside the tank could also be an issue if you can't get fuel to the carbs. I can't remember if the '95 has a vacuum petcock or not, but know that they have a history of being fussy...I replaced the vacuum petcock on my '91 with a non-vacuum unit from a '98).

I strongly recommend a good silicone grease for any and all neoprene rubber gaskets that you encounter during the disassembly & re-assembly. And note that the vacuum slide diaphragm's are NLA, so treat them like gold and coat them with a thin layer of silicone grease to help preserve and protect 'em (I use the Dow Corning stuff, which is totally safe for anything and has 100's of uses besides bikes and automotive - Link below).

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Last edited by EchoWars; 07-11-2018 at 02:42 AM.
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