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Carburetor problems - need help with diagnosis


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I’m having a strange problem on my 1993 RC36. I recently had the carbs apart to replace the o-rings on the fuel and air tubes that connect them. I took lots of photos and took my time, disassembling and reassembling them very carefully. When I went to synchronize them once back on the bike, I did so this time with the air box off so I could watch the slides.  This is where the problem arises. 2 of the slides rise and fall normally when I blip the throttle, but the other 2 don’t move. I pulled those slides out and checked the diaphragms again, as I did when I initially disassembled the carbs, but see no tears or holes.  Is there anything else that would cause that symptom other than an air leak in the diaphragms? TIA.

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The two things you need to make the slides move are a vacuum signal from the carb throat to the top of the vacuum diaphragm, and a free air vent to the bottom of the diaphragm. That creates the pressure differential which causes the slide to rise. You can simulate the same thing by blowing air (with your mouth) into the vent tubes, which should cause the slides to rise.

 

I would at least check that the slides can move freely by pushing them up by hand. 

 

You may need to check that you haven't got a blockage of the hole in the bottom of the carb slide (which is where the vacuum signal to the top of the diaphragm comes from). 

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Thank you very much for the advice, but I could use a little clarification. I blew air into the main vent tube, that leads to all 4 carbs. That didn’t cause the slides to rise (probably not enough pressure when split to all), but I could hear the air moving, so I then placed my hand over each velocity stack in turn as I blew, and could feel air rushing past my hand each time.

The slides do move entirely freely. The clarification I need is about the vacuum signal hole. I pulled one of the slides back out, and see 2 holes at the base, other than the one the needle passes through. Are those the ones you are referring to, or is it a different location within the carb?

 Thanks again.

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Almost certainly the slides that don't rise have the diaphragms installed incorrectly.  :blush:   it's easy for them to slip out of their groove while installing the cap and spring.

 

the hole you need to blow into is the large one (about  1/2 " diameter), find it  and test b4 mounting carbs on bike. don't worry about the holes in the top vacuum chamber as they never cause trouble.

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Well....that didn’t work. I noticed that one of the slides was sticking a little, so I swapped the 2 left ones. They then moved smoothly and freely, and had the same rebound as the 2 right slides that have been working properly. I’m very sure i got both diaphragms in the groove this time. But once I got the carbs back on, the problem remains. Any other ideas? Anyone?

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  • 1 month later...

Have you got the hole in the outer edge of the diaphragm properly around the very small brass tube in the top of the carb?

You can see the part I mean in your picture of the slide, at the top left next to the small spring.

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