Author Topic: CB750 Idle issues  (Read 34885 times)

0 Members and 1 Guest are viewing this topic.

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
CB750 Idle issues
« on: July 19, 2012, 04:05:15 pm »
Details:
1975 Honda CB750F motor
657B carbs, Cleaned, rejetted w/ Velocity Stacks
    #120 Main Jet
    #40 Idle Jet
    Idle Mix screw - 1 1/4 turns out.
4 into 2 Headers with shorty tips and glass packed, 8" baffles.
Stock coils and points
Float Height on all bowls (see pic)


The Problem:
First, it doesn't start with the choke on.  I must remove the choke to start it.  When cold and no choke, it fires the instant I touch the start button and revs nice under throttle.  However, the idle has to be set high...because it wont idle below 2000 rpm.  If I set it lower than that, it will idle about 30 seconds...while it just starts to drop revs and finally die.  Other than a bad idle, it runs great, pulls hard, and smells like the mixture is correct.

Possible "solution":
I found this "fix" for bikes with idle issues due to velocity stacks and/or open exhaust...interesting read.
http://www.hondachopper.com/garage/carb_info/idle_screw_mod/idle_screw_mod.html

« Last Edit: July 19, 2012, 05:02:13 pm by brycegp »
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0

Offline xnoahx

  • Enthusiast
  • **
  • Posts: 115
  • 78 750K
Re: CB750 Idle issues
« Reply #1 on: July 19, 2012, 04:34:58 pm »
Isnt the idle on these supposed to be around 1000 rpm? 

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
Re: CB750 Idle issues
« Reply #2 on: July 19, 2012, 05:02:53 pm »
Isnt the idle on these supposed to be around 1000 rpm? 

SEE EDIT...haha...it won't idle below 2000 rpm.  typo...sorry.
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0

Offline phil71

  • Master
  • *****
  • Posts: 1,813
Re: CB750 Idle issues
« Reply #3 on: July 19, 2012, 05:30:30 pm »
check the points?

Offline Elan

  • Expert
  • ****
  • Posts: 916
    • Vital Motor Parts
Re: CB750 Idle issues
« Reply #4 on: July 19, 2012, 05:44:13 pm »
how did you land at #120 for jetting? did you do the plug test?
Oil Pump Kits are Available on eBay!See my eBay store!
http://stores.ebay.com/Vital-Motor-Parts?_trksid=p2047675.l2563

k6 build   http://forums.sohc4.net/index.php?topic=79833.0

76 750k

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
Re: CB750 Idle issues
« Reply #5 on: July 19, 2012, 06:03:26 pm »
check the points?

points look good.

edit: points removed. pamco electronic ignition installed.
« Last Edit: July 30, 2012, 10:34:04 pm by brycegp »
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
Re: CB750 Idle issues
« Reply #6 on: July 19, 2012, 06:06:18 pm »
how did you land at #120 for jetting? did you do the plug test?

I did a lot of research here on what size jets to go to when opening up the air/exhaust flow.  I found #120 to be the go to size for bikes similar to my setup so I started there.  And it seems to run nice with them...so there I stayed.
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0

Offline w1sa

  • Hot Shot
  • ***
  • Posts: 444
Re: CB750 Idle issues
« Reply #7 on: July 19, 2012, 06:23:09 pm »
What you say indicates rich mixture in the low speed circuit.........The fuel level in your pic looks slightly high (at the joint line?)........I'd lower the fuel  levels in the bowls a few mm below the joint line.

Are you sure the air passages are clear?

Try running at idle with the tubes fitted to see whether the levels changes, but it's a good bet the plugs are fouling at low speed.... lowering the fuel levels might correct it to an acceptable level.

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
Re: CB750 Idle issues
« Reply #8 on: July 19, 2012, 08:07:44 pm »
What you say indicates rich mixture in the low speed circuit.........The fuel level in your pic looks slightly high (at the joint line?)........I'd lower the fuel  levels in the bowls a few mm below the joint line.

I guess not exact in the pic...but the levels are about 2mm below the joint line...
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0

Offline w1sa

  • Hot Shot
  • ***
  • Posts: 444
Re: CB750 Idle issues
« Reply #9 on: July 19, 2012, 08:57:49 pm »
If you're satisfied the ignition is firing per spec and delivering good spark at idle.........continue to lean the low speed mixture by turning the airscrews out @ say 1/8 increments (checking results) to a max of two turns (total). If that does not change the idle mixture (and even if it does to a lesser extent) you've probably got blockage to air passage(s).

I think you could go to 4mm with the fuel level (most aim at 3-4mm)............the float bowl fuel level will obviously influence the effect of the airscrew setting (the higher the fuel level...the higher the airscrew turns out required)

Also....to confirm fouling......the plugs do come out sooty once it ceases to run....don't they  ??
« Last Edit: July 19, 2012, 09:07:05 pm by w1sa »

Offline phil71

  • Master
  • *****
  • Posts: 1,813
Re: CB750 Idle issues
« Reply #10 on: July 20, 2012, 08:21:41 am »
What does it mean "points look good"?

Offline flybox1

  • My wife thinks I'm a
  • Really Old Timer ...
  • *******
  • Posts: 14,301
Re: CB750 Idle issues
« Reply #11 on: July 20, 2012, 09:09:15 am »
What you say indicates rich mixture in the low speed circuit.........The fuel level in your pic looks slightly high (at the joint line?)........I'd lower the fuel  levels in the bowls a few mm below the joint line.

I guess not exact in the pic...but the levels are about 2mm below the joint line...
Hondaman indicated the fuel level should be 4mm below the bowl mating surface. 
your idle jet might be sitting in too much fuel. get this set correctly first.
enjoyed your build, btw. 8)
« Last Edit: July 21, 2012, 08:11:04 am by flybox1 »
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline lucky

  • Really Old Timer ...
  • *******
  • Posts: 6,717
Re: CB750 Idle issues
« Reply #12 on: July 20, 2012, 07:15:49 pm »
Set the floats like it tells you in the book!!! End of story.
That first photo of fuel all the way to the top of the float bowl is way too high.

Do it like the book says by measuring the float height.


I know of no other way to tell you.
« Last Edit: July 21, 2012, 03:51:03 pm by lucky »

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
Re: CB750 Idle issues
« Reply #13 on: July 20, 2012, 10:59:16 pm »
why so hostile, Lucky?  geez...sometimes you are a trip.  you may be a mechanic by trade but you act like its all just common knowledge.

i don't need you to respond if you can't be friendly, END OF STORY.
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0

Offline lucky

  • Really Old Timer ...
  • *******
  • Posts: 6,717
Re: CB750 Idle issues
« Reply #14 on: July 21, 2012, 03:45:49 pm »
why so hostile, Lucky?  geez...sometimes you are a trip.  you may be a mechanic by trade but you act like its all just common knowledge.

i don't need you to respond if you can't be friendly, END OF STORY.

I was not angry at all. I am sorry that I came off as hostile. Please accept my apologies.

It amazes me how many people on this forum taught by others to measure the float bowls with this clear tube method.
While it may be valuable to see the fuel level and compare them between carbs, it is
not as important as  getting  the float level correct.
The Clymers' manual does show the clear tube method on Page 60 at the top of the page for marking the fuel levels AFTER you set the floats.
But #7. says, "Check and adjust the float level as described under Float Level Adjustment Chapter six.

CHAPTER SIX:
Bottom of page. Illustration 13.  " The height of the float above the carburetor body should be  1.023 thousandths(26mm).

The illustration #13 is very important.

I just feel sorry for owners when they have carb problems and their motorcycle runs poorly and they only listen to this "clear tube nonsense".

Now people will come after this post and dispute all that I say. You just have to choose which method makes more sense to you.

 Please just read any workshop manual about how to set the float level with a float gauge or measuring device.

If Honda Man said 4mm below the top of the float bowl, still adjust the float levels properly yourself and then see if the clear tube shows the fuel 4mm below the top of the float bowl.





« Last Edit: July 21, 2012, 03:54:21 pm by lucky »

Offline phil71

  • Master
  • *****
  • Posts: 1,813
Re: CB750 Idle issues
« Reply #15 on: July 21, 2012, 10:03:07 pm »
Oh god man, it's not that critical. If the floats fill 3/4 of the bowl and there's no leaks or overflow  , you're golden. Clean carbs and jets are way more important , and you're making it into more than it is. I'll bet the farm that high float levels are NOT the problem here (but yeah dude, that's kinda high). Even the Honda manuals are pretty blasé about it. R.E.L.A.X.

Offline lucky

  • Really Old Timer ...
  • *******
  • Posts: 6,717
Re: CB750 Idle issues
« Reply #16 on: July 22, 2012, 12:19:59 pm »
I agree Phil 71.

When i have removed any carb float bowls that were adjusted the float bowl was about 3/4 full.

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
Re: CB750 Idle issues
« Reply #17 on: July 23, 2012, 06:44:39 pm »
Thanks for the post Lucky.  I do respect your opinion so I have set all the floats to 26mm.  After starting it, I was getting spitting out of cylinder 2 carb velocity stack....with blasts of pressurized fuel...out of the carb.  SO...I'm starting from scratch. 

I've been holding off...but now I'll going to install my 3.3ohm coils, Pamco electronic ignition, new plugs wires, caps and plugs. 

I'm pulling the carbs.  I've got a second set of carbs that I have scrubbed with a fine toothbrush using 2 cans of carb cleaner...and fitted with new internals, i.e. rebuild kit.  Tips on which notch to clip the main needle on would be very welcome.

I'll install all the above.  Then, reset the float height, reset the timing and fine tune the idle mix and HOPEFULLY this f#cker will perrrrrrrr.......wish me luck.
« Last Edit: July 23, 2012, 09:12:11 pm by brycegp »
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0

Offline lucky

  • Really Old Timer ...
  • *******
  • Posts: 6,717
Re: CB750 Idle issues
« Reply #18 on: July 23, 2012, 09:36:22 pm »
You need to tell us which of the three types of carbs you have. What are the serial numbers?

Two different needle clip positions. 3 or 4?

Also with velocity stacks you will want to drop the clip to raise the needle for those stacks. One step.

No wonder a carb was spitting back at you . It is real lean.
But now that you are putting in electrical parts AND taking the carbs off and putting on other carbs ...if the new carbs work better you will not know what was the cause of the improvement. But usually you do not get all you want in one swoop like that.

Why don't you put in the new electrical parts .Check the carbs over that you are working with now, and make NO changes yet and see what happens?   Then if it is the same, find out which carbs you have and then raise the needles by lowering the clip one notch and THEN see what you have.

Make sure all 4 carbs have the same mixture screw opening exactly.
Try different mixture screw openings by changing them 1/4 turn at a time.
Let the engine cool down between tryouts.
« Last Edit: July 23, 2012, 09:40:13 pm by lucky »

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
Re: CB750 Idle issues
« Reply #19 on: July 23, 2012, 10:08:25 pm »
both sets, current and spare, are 657B and 5 clip positions...

I haven't pulled the carbs yet.  The reason I want to is that I relied on someone else to clean and rebuild them.  After all I've done with this bike, other people always seem to fail me.  And now I just want to verify that the carbs are perfectly clean and rebuilt as they should have been.  I've seen evidence already that proves the contrary...blocked air passages to be specific.  So...I don't trust they are performing as they should until I install a set that I can verify.

I did install the Pamco Ignition.  I haven't started it up with it tho.  Update coming...



« Last Edit: August 04, 2012, 02:41:50 pm by brycegp »
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0

Offline lucky

  • Really Old Timer ...
  • *******
  • Posts: 6,717
Re: CB750 Idle issues
« Reply #20 on: July 23, 2012, 10:23:07 pm »
both sets, current and spare, are 657B and 4 clip positions...

I haven't pulled the carbs yet.  The reason I want to is that I relied on someone else to clean and rebuild them.  After all I've done with this bike, other people always seem to fail me.  And now I just want to verify that the carbs are perfectly clean and rebuilt as they should have been.  I've seen evidence already that proves the contrary...blocked air passages to be specific.

Yes.. that is valuable info and you cannot depend on others unless you know them for many years.

Check out this little exerpt from HondaMan.


657 A & B's

HondaMan
Honda's recommendation on the "sooty 657A carburetor calibration" was to first try dropping the needles
 one notch. If that wasn't enough, to go to the 110 jet was the next step.

In actual use, dropping the needles has more effect on daily riding than the smaller jet, since you're riding
 most of the time in the lower 1/4 throttle range where the combined too-rich idle and too-rich needle are
 causing the issue. Thus, dropping 5 on the mainjet will only drop it about 0.5% until the slide clears the
wasp-waist in the throat: then it goes to about 2.5%. Dropping the needles drops it a little over 1% per notch.



The early K1 pipes (HM300) had no baffles inside. The later ones had one baffle near the upsweep corner(s)
 on each pipe, which increased the backpressure enough to make the 115/notch4 calibration too rich, and
 the 'unofficial advice' was issued via the factory reps' word-of-mouth. BTW: the 115 was the original jet in
the 657A carbs.

Some other things to watch for on the 657A: the air screw seats were thinner than the later carbs. So,
 turning the air screws in to seat, then out 1 turn, slowly widens that seat. Today, you may find that you
 are better off with about 7/8 turn instead. This seat got about 0.5mm thickerin the 657B, among other things.


Also, the 657A carbs often had 25mm float levels, and this level was set from the tiny flat pedestal
 that is right alongside the floats: not by using the notch on the sides of the carb body. If you attempt
to meausre a 657A carb that is properly set from those side notches, it will appear to be set at 24mm,
which leads some folks to adjust it. Don't...


The 657A and the original 4-cable K0 carbs both suffered very soft brass tangs on those floats that
were also made from brass. Check for a tiny dimple in the float tang of each one, and polish it off
with either a needle file or such, then reset the float level. This little dimple makes the float valves
 stick and occasionally overflow the carbs for no apparent reason. The 657B until K3 was also
 famous for this, but had plastic floats.


LUCKY now... Just thought I would throw that in there to think about.
A little history never hurts, but I have not see those brass floats with the dimples since I was 17 years old. That was back in 1969?

Offline lucky

  • Really Old Timer ...
  • *******
  • Posts: 6,717
Re: CB750 Idle issues
« Reply #21 on: July 23, 2012, 10:24:06 pm »
both sets, current and spare, are 657B and 4 clip positions...

I haven't pulled the carbs yet.  The reason I want to is that I relied on someone else to clean and rebuild them.  After all I've done with this bike, other people always seem to fail me.  And now I just want to verify that the carbs are perfectly clean and rebuilt as they should have been.  I've seen evidence already that proves the contrary...blocked air passages to be specific.  So...I don't trust they are performing as they should until I install a set that I can verify.

I did install the Pamco Ignition.  I haven't started it up with it tho.  Update coming...



Nice ignition unit!!

Offline andy750

  • Really Old Timer ...
  • *******
  • Posts: 7,889
Re: CB750 Idle issues
« Reply #22 on: July 24, 2012, 05:17:54 am »
Good info Lucky. It will also help me so thanks for posting.

cheers
Andy
Current bikes
1. CB750K4: Long distance bike, 17 countries and counting...2001 - Trans-USA-Mexico, 2003 - European Tour, 2004 - SOHC Easy Rider Trip , 2008 - Adirondack Tour 2-up , 2013 - Tail of the Dragon Tour , 2017: 836 kit install and bottom end rebuild. And rebirth: http://forums.sohc4.net/index.php/topic,173213.msg2029836.html#msg2029836
2. CB750/810cc K2  - road racer with JMR worked head 71 hp
3. Yamaha Tenere T700 2022

Where did you go on your bike today? - http://forums.sohc4.net/index.php?topic=45183.2350

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
Re: CB750 Idle issues
« Reply #23 on: July 24, 2012, 11:31:33 am »
I have no experience with 657A carbs...so I don't know what of HM's info is relevant to my situation...  :(
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0

Offline brycegp

  • Hot Shot
  • ***
  • Posts: 532
    • 1971 CB750 "Mut-N-Chop" Cafe Project
Re: CB750 Idle issues
« Reply #24 on: July 24, 2012, 07:17:44 pm »
So I started the bike first try after installing the Pamco Ignition. But cylinder #2 still isn't firing.  But I haven't put the timing light on it yet either.

Mixture screw 1 turn out on all 4 cylinders, right now.
« Last Edit: July 25, 2012, 09:07:19 am by brycegp »
1971 CB750K/F - El Toro
El Toro Build Thread - http://forums.sohc4.net/index.php?topic=73552.0