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Author Topic: 76 CB750 Cylinders 1 & 3 Running hotter  (Read 684 times)

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Offline Big D

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76 CB750 Cylinders 1 & 3 Running hotter
« on: August 08, 2016, 06:25:39 AM »
Please help, I have a 1976 CB750, and cylinders 1 & 3 are running about 60 degrees hotter (checking temps at exhaust just past head with gun)
Bike does run rough as if not firing at these two cylinders but smooths out with higher rpm
I checked spark and have no issues there, getting fuel, carbs have been cleaned and pilots and passages are clear.
very confused as obviously coils are 1&4 and 2&3, so why the issue with cylinder 2 & 4?
any ideas
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Offline riffman12

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #1 on: August 08, 2016, 11:20:18 AM »
pull the plugs. do cylinders 1 and 3 look leaner than 2 and 4?
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Offline calj737

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #2 on: August 08, 2016, 11:40:03 AM »
Also please confirm that your coils/plug wires are set up to serve 1&4 from the left coil, and 2&3 from the right.
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Offline Big D

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #3 on: August 08, 2016, 04:13:14 PM »
Checked all 4 plugs and they all look good, a little black but clean and all look the same, I should mention I am running 4 pod filters, and 4 into 1 Cycle Xchange exhaust, new points and timing set statically, points set between .012 and .016
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Offline Big D

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #4 on: August 08, 2016, 04:15:05 PM »
I did confirm coils are correct as I set on motorcycle as far as right and left
I really appreciate the help guys
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Offline Big D

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #5 on: August 08, 2016, 04:16:32 PM »
the carb to manifold boots are old but are not cracked or dry rotted, so I don't think I am pulling in air there, unless these things can leak even when looking good, they are stiff
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Offline ekpent

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #6 on: August 08, 2016, 07:12:31 PM »
  My infrared gun I play with does not seem to be super consistent when checking head pipe temps, especially with ones that are chrome.
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Offline riffman12

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #7 on: August 08, 2016, 11:10:47 PM »
interesting that the plugs are all the same color when 2 cylinders are way hotter.

Any way you have an exhaust leak on 2 of the cylinders? Did you put new exhaust gaskets in all 4? Did you by any chance not remove the old gaskets and thus "double up" on some?

Have you syncd the carbs?
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Offline calj737

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #8 on: August 09, 2016, 06:30:11 AM »
Please pull all four plugs and post a clear picture of them in profile.
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Big D

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #9 on: August 16, 2016, 12:51:39 PM »
sorry for the late reply, I did install new exhaust and new exhaust gaskets, I removed all 4 old rings and cleaned face.
I don't think I have any exhaust leaks
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Offline Big D

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #10 on: August 16, 2016, 12:53:58 PM »
I will pull all 4 plugs and post a pic, I found that #4 is what I believed to be fuel fouled.
My assumption at this point is weak condenser, I think my spark is weak at #4 and #1.
I am planning on replacing both condensers next
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Offline BomberMann650

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Re: 76 CB750 Cylinders 1 & 3 Running hotter
« Reply #11 on: August 16, 2016, 01:45:49 PM »
+1 sync
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