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Carb mystery

S_JH

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Livonia, MI
I've rode an 82 CB750C for a few years.

This year, I've been unable to get it started. After testing for spark (it had spark) and swapping out the air filter (should have done that before trying to start it), it still wouldn't start.

I left it alone for a while, as I thought I needed a complete carb rebuild, and I didn't have any free weekends.

This past week, I tried to drain gas from it. The carb bowls were dry; nothing came out after I opened the drain plug.

I removed the tank, and it had plenty of gas. The hose from gas tank to the top of the carbs was full of gas. But there was nothing in the carb bowls. (Which would explain the engine not starting!)

What could be the cause of that?

Do I still need to take the carbs out and clean them out? Or should I look for something else?
 
I was figuring it would be "clean the carbs". Though I'll look into the T where the line from the fuel tank is connected at the top of the carbs.
 
I may be too late here but before you rip into the carbs think about this. Fuel in the tank, is it fresh or has been there for months? Stale fuel does not ignite easily.
With the fuel tap "on" does fuel run easily out with the pipe disconnected or does it just dribble out. Dribbles? gummed up tap.
Does your bike have that awful automatic cut-off tap? If so disconnect it. This gadget only opens when the engine is cranked, it is opened by the vacuum in the inlet tube from #2 carb The vac tube may be cracked, disconnected or the diaphragm is perished, cracked, no vacuum, the damn thing won't open. Rig a line direct from the fuel tap to the feed line to the carbs, fuel tap on, any fuel in the carb bowls? Yes? Just by-pass the auto tap. Block off the vac tube on #2 carb.

To test the auto tap disconnect the vac line on #2 carb with the manual tap "on" gently suck on the vac tube this should open the tap and fuel should flow to the carbs. No fuel, there's the problem.
Check the vent tube, if this blocks, the tap won't open.
If fuel is reaching all the carbs but is not getting into the bowls then most likely the needle valves are stuck due to varnish, rust or whatever. Drop the bows and jiggle the floats to see if the needles are free. If not, then remove and strip the carbs.
As you can see the last thing you do is dive into the carbs, NOT the 1st.

Hope this helps
 
Last edited:
Thanks for the helpful steps.

I've been busy enough that I haven't touched the motorcycle much...other than to take the tank off, and check whether there was a plug in the line between the fuel tank and the carbs.

I'll have to look closely at that vacuum line, and find the vent tube.
 
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