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Discussion Starter #1
I have a 1979 CM400M with two Keihin 24b carbs.
New manifold orings. All jets cleaned out.
Compression 150 lbs. both.
No leaks that I can find.
Won't rev as no vacuum to raise slides.
will idle but after while will rev then quit.
Mufflers not clogged.
Had been sitting for long time not running.
Have run out of options that I can think of.
Anyone got fresh ideas??
 

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Just FYI, manifold vacuum doesn't raise the slides, the air flowing past the bottom of the slide creates the vacuum that lifts them. When you open the throttles, the rpm increase would cause the slides to raise, assuming the rpm does increase. First, check to see that the slides do easily rise; removing a carb and turning it upside-down should cause the slide to fall to the open position. If that happens, you very likely have a blocked main jet.
 

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Discussion Starter #4
I have replaced the manifold orings & inspected the compliance fitting manifolds for damage. The carbs are securly fastened & I can find no leaks.

I have disassembled both carbs, soaked all jets in a cleaner, blown air through every orifice I could find. No blockages that I can find.

The slides seem free although I didn't try the upside down trick.

If the piston goes up & down, & the valves are opening, & there is compression, there should be vacuum when the piston decends. But the guage reads 0 - 2 and the engine won't rev. :confused:
 

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Small hole or tear in the slide diaphragm?
I manually operate the slide a few times to make sure it's pushing air, and that nothing is binding. Even a small hole in the diaphragm can make the slide not lift.

I've got a Mikuni carb put together on my bench right now ready to install once I get to work. The slide and diaphragm seem to be working fine, so all that's left is the running test.
 

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Discussion Starter #7
There are no diaphrams.

hondatwins.net
/forums
/63-fuel-supply-carburation
/14104-keihin-carb-sizes-other-useless-information.html

See post #9
 

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Discussion Starter #9
I reset the valve clearances, checked the CID and the AC Generator. I have come to the conclusion that the vacuum plate in the upper chamber is worn, or the walls of the chamber are worn causing a bad seal, but I won't know if that is the problem until I can find some replacement carbs.
 

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Replacement carbs are usually not cheap but I think it would be time to get to looking, it is nice and warm(it is here anyways).
 
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