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Author Topic: 78' CB750K Cylinder #4 rich?  (Read 1011 times)

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Offline zeus87

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78' CB750K Cylinder #4 rich?
« on: June 23, 2008, 10:57:50 AM »
When i start my bike after it has been sitting for a while, it puffs black smoke out of the #4 cylinder exhaust pipe.  Also when engaging the clutch to coast after being on the throttle, it is the only pipe that backfires.  Any ideas?  All tune-up was done (carbs only bench synced though)., k&n factory style, mac drag pipes, mains were drilled out with a 3/64 drill bit
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Offline scondon

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Re: 78' CB750K Cylinder #4 rich?
« Reply #1 on: June 23, 2008, 11:04:56 AM »
Vaccum synch carbs, check. Switch #4 main jet with #1, check. New plug with proper gap, check. Fresh wire on #4 lead, check. Compression test if these don't make any difference.
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Offline BobbyR

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Re: 78' CB750K Cylinder #4 rich?
« Reply #2 on: June 23, 2008, 11:16:54 AM »
One other thing on 77-78 carbs is that the mixture screw is actualy a gasoline mixture screw. Try turning in all the way gently and back it out  1 to 1 1/2 turns and see if that makes a difference. If not we go deeper.
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Offline zeus87

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Re: 78' CB750K Cylinder #4 rich?
« Reply #3 on: June 23, 2008, 09:13:10 PM »
hmm im going to go deeper.  I just replaced the plugs, did a 2 mile strip at 3500, and did a plug chop...all cylinders look the same....deffinetly running lean at that range.  Heres whats going on.  Its groggy but still responsive from idle-3500 and above it screams, but with the choke it is smooth as glass from idle-3500, but will sputter and cut out by 4000..  do you thing i should try shimming the needle? if so, i put a washer under the factory clip, raising the needle correct?  or should i just drill the main again to 1/16 (i believe that is the next size up).  It runs really good all around (especially from 3500 and up), but the lean condition scares me a little.  Grey tip, with a spotless white insulator.

P.S. if i dont let it run for a good 5 minutes, it will stall out when accelerating from a stop without the choke engaged.   Thanks everyone!
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