Topic: Big issue with the carbs

[Patgaret]

Hi every body, I have a Honda cb 750 super sport 1975. The problem is the following: the motorcycle starts very well on the choke. the engine revs at 2000 rpm choke on. As soon as I cut the choke, the engine revs up to 4000 rpm. If I put the choke back on, the engine speed will drop to 2000 rpm. I did the test with another carburetor ramp and no problem at all. I know the problem is not with the boots. I disassembled the ramp three times, checked the slow jet, screw air (1 turn), the height of the sleeves, the height of the floats (26 mm), main jet is 105. I also check the cable to be sure that it doesn’t pull the ramp. I put the idle screw out and remove the speed cable, but same problem. I don't know what to think or where to look anymore. If you could help me, that would be much appreciated. Thanks in advance.

[ekpent]

If the large knurled head idle screw on the carb rack is backed out all the way and the idle is still too high then the carb slides are set to high or something is causing them not to close all the way. Did you completely disassemble the carbs at some point and put them back together. The slide/synch adjusters on roundtop carbs are at the top with a large nut and a smaller lock nut.

[Bodi]

This is rather common when the carbs are taken apart with the bodies removed from the rack.
The slide adjustments, the ones used for syncing, have to be set where the slides can fully close with the idle screw a turn in from where they start moving the cross rod.
Then do your bench sync by turning the idle screw in so your tool (1/6 drill maybe?) goes under the lowest slide, and adjust the other three slides to match.
If you then do a vacuum sync, again leave that slide alone and adjust the other three.
I don't know if your rack has the choke throttle advance cam - I don't think the 750 carbs have it. If it does, adjust it so it's not contacting at choke off. You can then adjust that throttle advance to where you like. I have it (on my 400F) so about 1/2 choke gives roughly 2500 RPM.

[Patgaret]

If the large knurled head idle screw on the carb rack is backed out all the way and the idle is still too high then the carb slides are set to high or something is causing them not to close all the way. Did you completely disassemble the carbs at some point and put them back together. The slide/synch adjusters on roundtop carbs are at the top with a large nut and a smaller lock nut.

Yes, I did it three times

[Patgaret]

This is rather common when the carbs are taken apart with the bodies removed from the rack.
The slide adjustments, the ones used for syncing, have to be set where the slides can fully close with the idle screw a turn in from where they start moving the cross rod.
Then do your bench sync by turning the idle screw in so your tool (1/6 drill maybe?) goes under the lowest slide, and adjust the other three slides to match.
If you then do a vacuum sync, again leave that slide alone and adjust the other three.
I don't know if your rack has the choke throttle advance cam - I don't think the 750 carbs have it. If it does, adjust it so it's not contacting at choke off. You can then adjust that throttle advance to where you like. I have it (on my 400F) so about 1/2 choke gives roughly 2500 RPM.

When you say '' the ones used for syncing '', does it have a particuliar carb to set the slide and then set the other three. Because I observe when i start to set one, it disturbs the other ones.

[TwoTired]

You must assign or pick one to be master carb.  With the big idle knob backed out,  you make certain your master carb slide can reach all the way to the floor of the carb body.  Never adjust that carb position again.  Make all the others match your master carb.  And use the big idle knob to set your idle speed.

[newday777]

If the large knurled head idle screw on the carb rack is backed out all the way and the idle is still too high then the carb slides are set to high or something is causing them not to close all the way. Did you completely disassemble the carbs at some point and put them back together. The slide/synch adjusters on roundtop carbs are at the top with a large nut and a smaller lock nut.

Yes, I did it three times

Did you ultrasonic clean the carb bodies or dead soak clean them? If not you probably have a plugged fuel port in the carb bodies. That gives a lean condition with high idle.

[david 750f]

There is no fast idle cam on those carbs. Choke on or off should not affect idle by 2000 rpm. Are you sure you are operating the choke lever properly? Lever up (horizontal) is choke on. Lever pushed down is choke off. You would see those symptoms if you are operating the choke backwards and the idle speed is set too high.

[ekpent]

Most of the time on these primitive carbs the bike will hardly run right on full choke once started. I use it for first start and then feather it back and forth a little as needed but mostly off and use my right wrist as a 'fast idle cam'. 

[Patgaret]

If the large knurled head idle screw on the carb rack is backed out all the way and the idle is still too high then the carb slides are set to high or something is causing them not to close all the way. Did you completely disassemble the carbs at some point and put them back together. The slide/synch adjusters on roundtop carbs are at the top with a large nut and a smaller lock nut.

Yes, I did it three times

Did you ultrasonic clean the carb bodies or dead soak clean them? If not you probably have a plugged fuel port in the carb bodies. That gives a lean condition with high idle.No I didn’t ultrasonic clean the carb bodies. But I clean the fuel port with carb cleaner and I could see the fluid pass through.

[Patgaret]

There is no fast idle cam on those carbs. Choke on or off should not affect idle by 2000 rpm. Are you sure you are operating the choke lever properly? Lever up (horizontal) is choke on. Lever pushed down is choke off. You would see those symptoms if you are operating the choke backwards and the idle speed is set too high.

yes I use the choke lever properly. I could see through the intake carb when the choke is on or off. The butterfly goes down when the choke is on, lever up. It’s like there is too much air, because when the butterflies go up, lever down, the motor revs up. But when I shut down the air, revs goes down to 2000 rpm.

[pjlogue]

With these carbs, it is important that the slides of each carb be bottomed out when the idle adjust screw is backed out all the way. (i.e., tip of idle adjust screw is not contacting anything.)  When this is done, the top of each slide height adjuster rod should have ~1-2 threads showing above the small 10mm lock nut.  If you have even just 3 threads showing then they are too high and the rods need to be lowered.

-P.

[Patgaret]

Try to find the problem. I post a video and as you can see the idle goes down when the choke is ON. I verified everything in the carbs and I canot figure également where is the prob. Help ! https://youtube.com/shorts/4bUQrAygLXE?feature=share

[Bodi]

I'm on really bad internet and can't watch the video atm.
The choke will lower RPM. It adds more restriction in the airflow and makes the mixture crazy rich, burning slower. This is part of the reason for some racks having the idle cam on the choke.
So it appears to me that your minimum idle is high and the choke is choking down the revs.

Have you addressed the multiple suggestions to ensure the carb slides will all bottom out all the way with the idle screw turned out all the way?

[Patgaret]

I'm on really bad internet and can't watch the video atm.
The choke will lower RPM. It adds more restriction in the airflow and makes the mixture crazy rich, burning slower. This is part of the reason for some racks having the idle cam on the choke.
So it appears to me that your minimum idle is high and the choke is choking down the revs.

Have you addressed the multiple suggestions to ensure the carb slides will all bottom out all the way with the idle screw turned out all the way?

yes I did. Although I removed the cable throtle to make sure that it have any resistance and lower the ramp.

[PeWe]

Those stock CB750 carbs have no idle increasing function on choke like Mikuni VM29 Smoothbore or Mikuni TMR have as examples to refer to.
Full choke will choke

[jpdevol]

I suggest that the lifting arms and cable pulleys are not properly "clocked" on the shaft, preventing sufficient cable freeplay and full slide closure.

[PeWe]

Idle screw is turned in for high rpm, choke reduced it?

[Patgaret]

I suggest that the lifting arms and cable pulleys are not properly "clocked" on the shaft, preventing sufficient cable freeplay and full slide closure.

But it’s impossible to adjust or change the position of the lifting arms and the cable pulleys. They have a default setting position which is not adjustable.

[Patgaret]

Idle screw is turned in for high rpm, choke reduced it?

idle screw is completely rmoved.

[Patgaret]

Those stock CB750 carbs have no idle increasing function on choke like Mikuni VM29 Smoothbore or Mikuni TMR have as examples to refer to.
Full choke will choke

I know that. So why I can’t get a normal idle and why when The choke is on the rev decrease ?

[TwoTired]

See reply number 5.

[Don R]

  Be sure the slides are not installed backwards also. Most common on the old K0 4 cable pull carbs.

[jpdevol]

I suggest that the lifting arms and cable pulleys are not properly "clocked" on the shaft, preventing sufficient cable freeplay and full slide closure.

But it’s impossible to adjust or change the position of the lifting arms and the cable pulleys. They have a default setting position which is not adjustable.

Not true, the arms and the pulleys have setting bolts and can be mis aligned. Enlarge attached pic, see bolts circled

[ofreen]

I suggest that the lifting arms and cable pulleys are not properly "clocked" on the shaft, preventing sufficient cable freeplay and full slide closure.

But it’s impossible to adjust or change the position of the lifting arms and the cable pulleys. They have a default setting position which is not adjustable.

Not true, the arms and the pulleys have setting bolts and can be mis aligned. Enlarge attached pic, see bolts circled

The lifters are keyed on the shafts.  The only way that could happen is if they were reassembled without the keys installed.