won’t rev past 5000 backfires when I close the thottle

Posted: Sat Jul 20, 2024 8:38 am

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

Posted: Sat Jul 20, 2024 10:42 am

"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.

Posted: Sat Jul 20, 2024 11:21 am

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

John

Posted: Sun Jul 21, 2024 9:17 am

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

Posted: Sun Jul 21, 2024 9:22 am

easier to visualize

Posted: Sun Jul 21, 2024 10:33 am

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

On a roll Joined: May 03, 2024 Posts: 104 Location: Canada

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

Posted: Thu Aug 08, 2024 3:12 pm

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

Posted: Thu Aug 08, 2024 3:16 pm

Anybody know where I can get a left side cover?

Posted: Thu Aug 08, 2024 3:30 pm

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!

Posted: Fri Aug 09, 2024 5:32 am

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!

Posted: Fri Aug 09, 2024 11:09 am

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...

Posted: Fri Aug 09, 2024 1:24 pm

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.

On a roll Joined: May 03, 2024 Posts: 104 Location: Canada

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

Posted: Sat Aug 10, 2024 9:05 am

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…..

Posted: Sat Aug 10, 2024 9:10 am

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

Posted: Sun Aug 11, 2024 10:49 am

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.

On a roll Joined: May 03, 2024 Posts: 104 Location: Canada

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

Posted: Mon Aug 12, 2024 12:30 am

FLAN wrote:

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