Topic: RE: CB750 657A 3 out 4 Air Screws Don't Work Right - Solved!

[Picasoo]

^^^^Once more thing, when I close with my finger air hole (on the back of carb) that cyl is working better and rising RPM!!!

[chewbacca5000]

^^^^Once more thing, when I close with my finger air hole (on the back of carb) that cyl is working better and rising RPM!!!

Can you post a pic?  I am not quite following you.  Thanks.

[Picasoo]

Yep...


when I block this hole (mark with red circle) with my finger, engine is running better and rises rpm!

[chewbacca5000]

Yep...


when I block this hole (mark with red circle) with my finger, engine is running better and rises rpm!

Thanks I see what you mean.  Did you get your number 4 carb fixed?  What happens when you turn the air screws in.

[Picasoo]

I wrote it on page 1

Still have a problem with carb. No.4...do not response on airscrew at all...
I checked a brass ball and they are on place with no sign of leaking...
spark plug on that cyl. is dry and black, float level is just like on other carbs!!
It's '75 cb750f with Keihin carbs on...

sos-sos-sos...

And in lower RPMs(from 1000 to 2000 rmp) bike is twitches, on higher he do just fine!
I was pm Hondaman and his doubt are on KN filters

[lucky]

That hole where you are putting your finger...That is a vent. If you blow into it with a plastic straw it will come out of the hole on the left side.

Wish we could see a photo of the whole set of carbs.

Also wondering if your floats are set correctly.
It is hard to understand you. "twitches"??

[chewbacca5000]

That hole where you are putting your finger...That is a vent. If you blow into it with a plastic straw it will come out of the hole on the left side.

Wish we could see a photo of the whole set of carbs.

Also wondering if your floats are set correctly.
It is hard to understand you. "twitches"??

I think he means its sluggish or hesitates in the lower RPM or when taking off.  Isn't this normal though?

[Picasoo]

First of all...thanks you for helping me!
Here are the carbs:

That hole where you are putting your finger...That is a vent. If you blow into it with a plastic straw it will come out of the hole on the left side.

Wish we could see a photo of the whole set of carbs.

Also wondering if your floats are set correctly.
It is hard to understand you. "twitches"??

The problem is only with carb No. 4-Carbs No. 1,2,3 running good...and on other carbs when I put finger in that hole, that cyl. is choking, but with carb No.4 is total opposite-he running better!!
Floats are set correct and they are on all carbs on the same level on 26mm...(as I remember)
Sorry on my English...when I ride on low rpm (III gear at 1000-2000 rmps), engine is not running smooth-feel like skipping tact!!

That hole where you are putting your finger...That is a vent. If you blow into it with a plastic straw it will come out of the hole on the left side.

Wish we could see a photo of the whole set of carbs.

Also wondering if your floats are set correctly.
It is hard to understand you. "twitches"??

I think he means its sluggish or hesitates in the lower RPM or when taking off.  Isn't this normal though?

Taking off and fiering up is normal, but during the ride on lower rpm engine is not running good!

[TwoTired]

That hole where you are putting your finger...That is a vent. If you blow into it with a plastic straw it will come out of the hole on the left side.

No, it is not a vent.  No vents originate inside the inlet air duct.
It is an air jet inlet which feeds either the pilot system emulsion tube, or the mains/throttle valve emulsion tube.

If shutting off the air to the emulsion tube improves operation, it suggests an air leak for that system elsewhere in the carb.  I don't have that carb style to check for where the air leak can be.

Below from the  "Honda Motorcycle Carburetion - Tech Series"  booklet.  Click to enlarge.

[Picasoo]

Problem finally solved...
Cyl.4 wasn't react on airscrew because bad intake rubber
Bad engine work solved(caused by tube air filters) with airproof duct tape!



I wish to thanks Hondaman who is help me a lot!!

Regards from Serbia!!

[lucky]

We'll I think this mystery is just about solved.  I looked harder at Carb 1 and noticed that there is no brass ball plug in the pilot circuit!  So no gas was getting sucked in at idle on this carb.  I am not too worried as I have another set.

My real problem is that all my carbs have been through my super powerful ultrasonic cleaner.  I PMed Hondaman and he said the little brass balls are set with Pliobond glue and the ultrasonics may have washed the glue away.  I reset one with a bolt as a punch and the wd40 no longer goes out the brass ball.  I am thinking of regluing or soldering with a drop of silver solder.

Any thoughts on which way to go glue or solder?

Why not tap the hole and put in a small set screw with JB weld.

No need to throw away a good carb.
It is so sad to hear of people throwing away a carb just because they do not have the skills to repair them.

Once they are gone there is no more.