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Ltbordo
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Joined: Jun 19, 2024
Posts: 21
Location: Atlanta

PostPosted: Sat Jul 20, 2024 8:38 am Reply with quote Back to top

More good marching orders,

Valve clearance is my next chore and I will add checking exhaust donut gaskets to my list!

The coils look old, but have no signs externally of any failure! That was a weird one for me after a lot of years playing with diagnostics

Even after valve clearance and exhaust gasket checks, the carbs are gonna come off again for a more thorough cleaning…. Thanks, Sean G.!


Moving into valve clearances procedures can you guys tell me off the top your head… What 25mm typical OEM valve shim thickness size for these motors was when new? 2.8mm (280) or so?

John

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genesound
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Joined: Jul 24, 2007
Posts: 44863
Location: Studio City, CA

PostPosted: Sat Jul 20, 2024 10:42 am Reply with quote Back to top

"What 25mm typical OEM valve shim thickness size for these motors was when new?"

Well of course they didn't come with one size in them. They were installed with the proper thicknesses to achieve proper clearances.

I think 750 heads used to come with mostly 280 to 300 shim range when new. The larger heads (900, 1000, & 1100) heads were more like 270-285.

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Kindly read this linked thread with the rules before posting
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ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
81 CB900C, 83 CB1000C. 
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Ltbordo
Getting up steam
Getting up steam



Joined: Jun 19, 2024
Posts: 21
Location: Atlanta

PostPosted: Sat Jul 20, 2024 11:21 am Reply with quote Back to top

Thx genesound…….is there ANYTHING you don’t know about these machines?

John

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Ltbordo
Getting up steam
Getting up steam



Joined: Jun 19, 2024
Posts: 21
Location: Atlanta

PostPosted: Sun Jul 21, 2024 9:17 am Reply with quote Back to top

E E E E

005 005 005 004 004 004 (003)004

1cyl 2cyl 3cyl 4cyl

005 005 005 005 005 005 005 005

I I I I

realistically are these readings anything to worry about?
parentheses is lowest valve lash clearance

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Ltbordo
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Joined: Jun 19, 2024
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PostPosted: Sun Jul 21, 2024 9:22 am Reply with quote Back to top

easier to visualize

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genesound
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Joined: Jul 24, 2007
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PostPosted: Sun Jul 21, 2024 10:33 am Reply with quote Back to top

Those should be ok. Ideally, the inside #4 exhaust is a bit snug and could be a half size thinner shim.

Here's some info about half sizes:
https://www.cb750c.com/modules.php?name=Forums&file=viewtopic&p=8892#8892

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NMT - New Member Thread, a Must Read, click here!!! - Information For New and Veteran Members.
Contains SITE RULES, how to enter required bike information in your signature, thread and picture posting...
Also some important DOHC wrenching tips, and some other information, shop manuals.
Kindly read this linked thread with the rules before posting
pictures or asking lots of questions that are already answered in it.

ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
81 CB900C, 83 CB1000C. 
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FLAN
On a roll
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Joined: May 03, 2024
Posts: 104
Location: Canada

PostPosted: Sun Jul 21, 2024 6:42 pm Reply with quote Back to top

Wow you are real close. As mentioned get that exhaust up a bit and you are good to go.

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1982 CB750 Nighthawk 
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Ltbordo
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Joined: Jun 19, 2024
Posts: 21
Location: Atlanta

PostPosted: Thu Aug 08, 2024 3:12 pm Reply with quote Back to top

Old spark plugs looked good but new ngk non resistor Heat range 8 plugs also made a difference- no more back firing!!

Carbs are off for an extreme Sean G kind of cleaning and then we’re all done!

Interesting back from the grave reno!!

For any who are interested here was the laundry list when it was brought in:

No key
No battery- obviously…
Seized engine
Flat front tire
Seized starter motor
Intermittent start switch
Clogged brake lines
Frozen master cylinder
Clogged carbs with leaks
Intermittent ignition with bad caps
Leaky tank petcock
Dirty tank
Clogged #2 exhaust
Seized gas tank latch
No front brake stop switch
Missing side cover
Broken rear brake lens

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Ltbordo
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Joined: Jun 19, 2024
Posts: 21
Location: Atlanta

PostPosted: Thu Aug 08, 2024 3:16 pm Reply with quote Back to top

Anybody know where I can get a left side cover?

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genesound
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Joined: Jul 24, 2007
Posts: 44863
Location: Studio City, CA

PostPosted: Thu Aug 08, 2024 3:30 pm Reply with quote Back to top

Ltbordo wrote:
Old spark plugs looked good but new ngk non resistor Heat range 8 plugs also made a difference- no more back firing!!

Carbs are off for an extreme Sean G kind of cleaning and then we’re all done!

Interesting back from the grave reno!!

For any who are interested here was the laundry list when it was brought in:

No key
No battery- obviously…
Seized engine
Flat front tire
Seized starter motor
Intermittent start switch
Clogged brake lines
Frozen master cylinder
Clogged carbs with leaks
Intermittent ignition with bad caps
Leaky tank petcock
Dirty tank
Clogged #2 exhaust
Seized gas tank latch
No front brake stop switch
Missing side cover
Broken rear brake lens

Wow, you've come a long way!

_________________
NMT - New Member Thread, a Must Read, click here!!! - Information For New and Veteran Members.
Contains SITE RULES, how to enter required bike information in your signature, thread and picture posting...
Also some important DOHC wrenching tips, and some other information, shop manuals.
Kindly read this linked thread with the rules before posting
pictures or asking lots of questions that are already answered in it.

ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
81 CB900C, 83 CB1000C. 
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Ltbordo
Getting up steam
Getting up steam



Joined: Jun 19, 2024
Posts: 21
Location: Atlanta

PostPosted: Fri Aug 09, 2024 5:32 am Reply with quote Back to top

One last thing on those old bad coils and my “starter fluid” check holding at WOT

with old bad coils: the revs would NOT go past 5000rpm with backfiring and backfiring would get WORSE with starter fluid

With new coils: the revs SHOOT up to 9500 rpm with small backfiring (with the new plugs even the backfiring went away)

Old coils simply ran out of “juice”at 5000 rpm!

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SeanG
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Joined: Oct 14, 2007
Posts: 6513
Location: Seattle, WA

PostPosted: Fri Aug 09, 2024 11:09 am Reply with quote Back to top

You really need to complete your sig and activate it for your posts!

That being said, it shouldn't be a surprise that 40+yo coils, subjected to constant high temps being right above the cylinder head would start to fail...

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Sean
Seattle
´77 XLCH (Blackie the barhopper)
´80 CB900C (the faster RED color)
´01 FLSTFBi (The Silver Flame)
´05 FLHTCU/I (Blu )

MacGregor Carb Cleaning Manual:
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=13611

New (to you) bike checklist:
https://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=5822

Carb Sync Process Explained:
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=5245

Carb Idler Arm Explained:
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=10610 
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genesound
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Joined: Jul 24, 2007
Posts: 44863
Location: Studio City, CA

PostPosted: Fri Aug 09, 2024 1:24 pm Reply with quote Back to top

Those old coils develop a few issues that degrade their performance. They weren't all that good to begin with.
The HV contacts corrode and might even rot off altogether. The cases also get cracks, and then arc and lose power.


"...complete your sig and activate it for your posts..."

The sig was only off for that one post. I fixed it. All good.

_________________
NMT - New Member Thread, a Must Read, click here!!! - Information For New and Veteran Members.
Contains SITE RULES, how to enter required bike information in your signature, thread and picture posting...
Also some important DOHC wrenching tips, and some other information, shop manuals.
Kindly read this linked thread with the rules before posting
pictures or asking lots of questions that are already answered in it.

ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
81 CB900C, 83 CB1000C. 
View user's profile Send private message Send e-mail Visit poster's website
FLAN
On a roll
On a roll



Joined: May 03, 2024
Posts: 104
Location: Canada

PostPosted: Fri Aug 09, 2024 8:47 pm Reply with quote Back to top

Good job. Did you check Ebay for a side cover?

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1981 CB750 Custom
1982 CB750 Nighthawk 
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Ltbordo
Getting up steam
Getting up steam



Joined: Jun 19, 2024
Posts: 21
Location: Atlanta

PostPosted: Sat Aug 10, 2024 9:05 am Reply with quote Back to top

I will check eBay today.

Sean , my sig is showing my year and bike when I look back at them. What else should I add to it?

Not too good at forum stuff. Took me a couple of tries to upload that valve clearance pic…..

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Ltbordo
Getting up steam
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Joined: Jun 19, 2024
Posts: 21
Location: Atlanta

PostPosted: Sat Aug 10, 2024 9:10 am Reply with quote Back to top

Btw

I’ve chucked Qtips it my dremel and polished the living daylights out of the float valve seats!

Still leak!!

Where may I buy just 4 rubber tipped float needle valves? They look great 10x iPhone mag. But they are the last variable.

John

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genesound
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Joined: Jul 24, 2007
Posts: 44863
Location: Studio City, CA

PostPosted: Sun Aug 11, 2024 10:49 am Reply with quote Back to top

Don't buy cheap needle valves. They can have all manner of issues like plating coming off, tips coming off, wrong length, wrong width, etc.

_________________
NMT - New Member Thread, a Must Read, click here!!! - Information For New and Veteran Members.
Contains SITE RULES, how to enter required bike information in your signature, thread and picture posting...
Also some important DOHC wrenching tips, and some other information, shop manuals.
Kindly read this linked thread with the rules before posting
pictures or asking lots of questions that are already answered in it.

ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
81 CB900C, 83 CB1000C. 
View user's profile Send private message Send e-mail Visit poster's website
FLAN
On a roll
On a roll



Joined: May 03, 2024
Posts: 104
Location: Canada

PostPosted: Sun Aug 11, 2024 9:28 pm Reply with quote Back to top

I've had decent success with Randakk's Cycle Shakk for carb parts.

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1981 CB750 Custom
1982 CB750 Nighthawk 
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genesound
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Joined: Jul 24, 2007
Posts: 44863
Location: Studio City, CA

PostPosted: Mon Aug 12, 2024 12:30 am Reply with quote Back to top

FLAN wrote:
I've had decent success with Randakk's Cycle Shakk for carb parts.
thumbs up

_________________
NMT - New Member Thread, a Must Read, click here!!! - Information For New and Veteran Members.
Contains SITE RULES, how to enter required bike information in your signature, thread and picture posting...
Also some important DOHC wrenching tips, and some other information, shop manuals.
Kindly read this linked thread with the rules before posting
pictures or asking lots of questions that are already answered in it.

ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
81 CB900C, 83 CB1000C. 
View user's profile Send private message Send e-mail Visit poster's website
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