Author Topic: CB750 Idle issues  (Read 36748 times)

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Offline lucky

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Re: CB750 Idle issues
« Reply #25 on: July 24, 2012, 11:24:38 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.

Idle screw 1 turn out on all 4 cylinders, right now.

It is a mixture screw not idle screw.
Each carb has a mixture screw but there is just ONE screw that controls the idle on all four carbs except the 1969-70 carbs.



Offline brycegp

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Re: CB750 Idle issues
« Reply #26 on: July 25, 2012, 09:02:16 AM »
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.

Idle screw 1 turn out on all 4 cylinders, right now.
It is a mixture screw not idle screw.
Each carb has a mixture screw but there is just ONE screw that controls the idle on all four carbs except the 1969-70 carbs.

that's what I meant.  but not what I said... ;)
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Offline lucky

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Re: CB750 Idle issues
« Reply #27 on: July 25, 2012, 03:11:08 PM »
I still think you will need that #42 idle jet with the velocity stacks.
The #40 was just not enough.

Mixture screw out 3/4 turn or maybe 7/8 turn.
« Last Edit: July 25, 2012, 03:13:17 PM by lucky »

Offline brycegp

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Re: CB750 Idle issues
« Reply #28 on: July 25, 2012, 08:11:06 PM »
I still think you will need that #42 idle jet with the velocity stacks.
The #40 was just not enough.

Mixture screw out 3/4 turn or maybe 7/8 turn.

I had #42's in at first thinking the same thing.  and it needed 2+ turns out on the mixture screw.  So I went back to the #40's...
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Offline lucky

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Re: CB750 Idle issues
« Reply #29 on: July 26, 2012, 04:40:23 PM »
What does it mean "points look good"?

They were shinny???? Lol..lol

Offline phil71

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Re: CB750 Idle issues
« Reply #30 on: July 28, 2012, 11:03:05 AM »
these bikes fire in twos.. .so if one cylinder doesn't fire, but its mate does, you've still got a carb issue, or low compression.

Offline aperry

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CB750 Idle issues
« Reply #31 on: July 28, 2012, 11:59:17 AM »
these bikes fire in twos.. .so if one cylinder doesn't fire, but its mate does, you've still got a carb issue, or low compression.

Or spark.  My stock plug caps were hard to keep tight, and became pretty unreliable. 


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bollingball

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Re: CB750 Idle issues
« Reply #32 on: July 28, 2012, 12:00:20 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



This is what gets people mixed up Your picture does not show float height or level. What it shows is fuel level in the bowl. They are not the same thing. They are both valuable information that can be used to solve a problem. Once I get one of my floats set perfect then I use the clear tube method to make sure the other three are the same. Through trial & error I have found on my PD42b carbs on a 78 750K stock motor 3 to 4 mm below the bottom of the bowl gasket works well at all throttle positions. If someone thinks this is not a good way to do this then I am open for suggestions.

Ken

Offline brycegp

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Re: CB750 Idle issues
« Reply #33 on: July 28, 2012, 02:31:58 PM »
This is what gets people mixed up Your picture does not show float height or level. What it shows is fuel level in the bowl. They are not the same thing. They are both valuable information that can be used to solve a problem. Once I get one of my floats set perfect then I use the clear tube method to make sure the other three are the same. Through trial & error I have found on my PD42b carbs on a 78 750K stock motor 3 to 4 mm below the bottom of the bowl gasket works well at all throttle positions. If someone thinks this is not a good way to do this then I am open for suggestions.

Ken

This was already addressed and remedied in earlier posts.  I set all floats to 26mm per stock specs.

What I could really use now are BASELINE SPECS FOR 657B CARBS WITH VELOCITY STACKS.  I know there are no two engines alike and that each needs individual tuning.  But a safe starting point for:

Main Needle clip setting (5 slots)
Slide height at closed throttle (fully closed or any opening at all?)
Mixture Screw setting

And suggestions on jet sizes, if #120 and #40 aren't good jet sizes to start with.  But I think those sizes are a good starting point.
1971 CB750K/F - El Toro
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Offline w1sa

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Re: CB750 Idle issues
« Reply #34 on: July 28, 2012, 08:08:26 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.
Did you get #2 firing ok ?

You originally stated that the bike starts and revs fine, but won't idle below 2000rpm...........Is that still the situation ?

People have asked about plug condition etc and you haven't provided much feedback.......

It won't make a lot of sense changing from the present set-up/specs until you can successfully start, warm idle and run the engine without basic tuning faults/ problems......stick with the #40, #120, existing (middle?) needle position, airscrew 1 turn out. for now.

Try and get the idle adjustment/slide heights uniform at the lowest adjustable level (closed/fully down). The idle stop screw should be turned out enough so that the slides can be lowered all the way down. Then adjust each slide up so that a uniform minimum gap/clearance exists at the rear (engine side) of the slide on each carb. or, until each slide just begins to indicate lift.   Then adjust your idle stop screw as necessary to get/maintain idle speed............... Fine tune the sync with gauges once its firing/idling uniformly on all cylinders.

Then you can start to properly address the jetting requirements and any other issues affecting proper running.

Offline brycegp

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Re: CB750 Idle issues
« Reply #35 on: July 29, 2012, 09:27:14 AM »
Did you get #2 firing ok ?

You originally stated that the bike starts and revs fine, but won't idle below 2000rpm...........Is that still the situation ?

People have asked about plug condition etc and you haven't provided much feedback.......

It won't make a lot of sense changing from the present set-up/specs until you can successfully start, warm idle and run the engine without basic tuning faults/ problems...

So let me preface this by saying "thank you guys for baring with me on this."  This is my first time tuning a bike to this extent and I am still learning all the variables and ways to check symptoms.  I'll try to be more thorough with my posts so you have all the critical details.  Below, I answered your questions in order you asked them.

1.  Cylinder #2 stopped firing after I started making mixture adjustments, specifically after I sprayed carb cleaner through the idle passages to make sure they were clear. Last time i started it up, #2 still wasn't firing, properly, i.e. #2 header was warm but not hot.

2.  The last time I started the bike, with new electronic ignition, it ran pretty much the same as before.  Started up stronger...but it immediately struggled at idle (wanted to die but I kept it alive with the throttle) and revved ok after about a minute.  Mix screws 1 1/4 turns out.

3.  After that, I pulled the plugs and all 4 are very BLACK.  Running rich, its seems.  Which is not what I was expecting.

4.  So when I first started this thread, I was still very unconfident in going in as deep as I have now.  Once I started getting into the carbs by inspecting passages and adjusting mixture, I realized that the carbs I thought had been thoroughly cleaned and rebuilt (by someone else) had blocked idle mixtures passages.  So this changes the game for me.  After that, I can not be confident the carbs are functioning properly.  This is the reason I decided to build a spare set MYSELF and will eventually replace the current carbs with this set that I can verify as clean, rebuilt and properly set up.



1971 CB750K/F - El Toro
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Offline lucky

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Re: CB750 Idle issues
« Reply #36 on: July 29, 2012, 12:29:53 PM »
I still think you will need that #42 idle jet with the velocity stacks.
The #40 was just not enough.

Mixture screw out 3/4 turn or maybe 7/8 turn.

I had #42's in at first thinking the same thing.  and it needed 2+ turns out on the mixture screw.  So I went back to the #40's...

Thanks for that report back on the idle jet.
#40 seems like it worked better in this F model.

Offline lucky

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Re: CB750 Idle issues
« Reply #37 on: July 29, 2012, 12:38:31 PM »
This is what gets people mixed up Your picture does not show float height or level. What it shows is fuel level in the bowl. They are not the same thing. They are both valuable information that can be used to solve a problem. Once I get one of my floats set perfect then I use the clear tube method to make sure the other three are the same. Through trial & error I have found on my PD42b carbs on a 78 750K stock motor 3 to 4 mm below the bottom of the bowl gasket works well at all throttle positions. If someone thinks this is not a good way to do this then I am open for suggestions.

Ken

This was already addressed and remedied in earlier posts.  I set all floats to 26mm per stock specs.

What I could really use now are BASELINE SPECS FOR 657B CARBS WITH VELOCITY STACKS.  I know there are no two engines alike and that each needs individual tuning.  But a safe starting point for:

Main Needle clip setting (5 slots)
Slide height at closed throttle (fully closed or any opening at all?)
Mixture Screw setting

And suggestions on jet sizes, if #120 and #40 aren't good jet sizes to start with.  But I think those sizes are a good starting point.

Yes that has been my focus all along get you close. But now you can start to check your plug reading and get it much closer.

The real problem is that I have been collecting information on many forum members bikes to try to help all of the forum members to get their bikes jetted properly that have non stock exhaust and intakes, but then once they get close I never hear from them.
If we share information we all benefit.
So thanks for sharing all of your hard work and information with all of us.

I know that those jets sizes are a good starting point.
I also know you may be able to go down to a 115 main jet.
But being a little rich will not damage the engine like being too lean will.
But now make only ONE change at a time and ride it for a couple days to see if it really was better or worse. Keep checking the plugs after the engine has cooled down.


Offline lucky

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Re: CB750 Idle issues
« Reply #38 on: July 29, 2012, 12:52:20 PM »
Soon I will be able to compile all of my jetting info on the CB550, CB500, and CB750,s and publish it online . Then all members can have it for reference.

Offline andy750

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Re: CB750 Idle issues
« Reply #39 on: July 29, 2012, 06:36:20 PM »
Soon I will be able to compile all of my jetting info on the CB550, CB500, and CB750,s and publish it online . Then all members can have it for reference.

Looking forward to that Lucky. Let me know if you need any help.

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

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CB750 Idle issues
« Reply #40 on: July 30, 2012, 10:06:54 PM »
finally got my timing light and set my timing...

1971 CB750K/F - El Toro
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Offline lucky

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Re: CB750 Idle issues
« Reply #41 on: July 31, 2012, 10:35:34 AM »
Good photo!

Offline brycegp

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Re: CB750 Idle issues
« Reply #42 on: August 04, 2012, 12:33:51 PM »
Finally got all the pieces I needed to install the 3ohm coils...







1971 CB750K/F - El Toro
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Offline brycegp

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Re: CB750 Idle issues
« Reply #43 on: August 04, 2012, 12:55:38 PM »
After installing the coils, I started it up...and just happy it ran on the first try.  No reason it shouldn't...but you never know. ;)

I'll get the tank back on and see how it idles with a proper amount of fuel in the bowls...

« Last Edit: August 04, 2012, 01:09:59 PM by brycegp »
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Offline lucky

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Re: CB750 Idle issues
« Reply #44 on: August 04, 2012, 01:33:52 PM »
That sounds like a good idle. was it cold?? OR was it warmed up?

Offline brycegp

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Re: CB750 Idle issues
« Reply #45 on: August 04, 2012, 01:53:19 PM »
It was cold...
That sounds like a good idle. was it cold?? OR was it warmed up?

cold.  and hadn't been started in a couple days.  but it hits that idle you hear and will steadily drop RPM till it dies...
« Last Edit: August 04, 2012, 01:57:06 PM by brycegp »
1971 CB750K/F - El Toro
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Offline brycegp

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Re: CB750 Idle issues
« Reply #46 on: August 04, 2012, 02:30:11 PM »
I just started it cold and again, it wanted to die not idle...#2 isn't firing and #2 carb is spitting at me.  Grrr.....



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Offline lucky

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Re: CB750 Idle issues
« Reply #47 on: August 04, 2012, 03:45:42 PM »
I just started it cold and again, it wanted to die not idle...#2 isn't firing and #2 carb is spitting at me.  Grrr.....

How did you determine #2 is not firing? Just asking.
If #2 (that is not firing), is also spitting back at you, it sure sounds lean for some reason.
 I would wait till it cools down and remove the idle jet and see if you can see daylight through it.

BTW...In the video it started up cold and started well. Then you revved it up and it did not hesitate at all. Cold, Choked. (important)

The weather was warm and you said it will die after a couple minutes.
Sometimes you need to use PARTIAL choke after it gets running. Not full choke.
I would really like to see a video with the tank on, WARMED up,CHOKE off, and then see how good the throttle response and idle is. That would tell me a lot.

And all of this is with needle clips in the stock position with velocity stacks. Right?
« Last Edit: August 04, 2012, 03:56:38 PM by lucky »

Offline lucky

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Re: CB750 Idle issues
« Reply #48 on: August 04, 2012, 03:58:03 PM »
It was cold...
That sounds like a good idle. was it cold?? OR was it warmed up?

cold.  and hadn't been started in a couple days.  but it hits that idle you hear and will steadily drop RPM till it dies...
  But when you hear the rpm start to drop do you take the  choke partly  off?
In the days this bike was designed you needed to get to know when to start to take the choke off or partly off as it warmed up.

Like you knew when you were two blocks from your house you could take the choke from 1/2 on, to completely off . See what I mean?
« Last Edit: August 04, 2012, 04:01:17 PM by lucky »

Offline lucky

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Re: CB750 Idle issues
« Reply #49 on: August 04, 2012, 04:02:12 PM »
This video was VERY helpful. Thanks!