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

[chewbacca5000]

So I taken these carbs apart and cleaned them several times and only 1 carb out of 4 responds when I turn the air screw.  I have another set where the air screws work and I am tired of taking this set apart.  Are they toast?  Carb 3 is the only one that seems to work right.

Thanks!



Main 110
Slow 40
Clip 2nd position from bottom.
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[harisuluv]

That circuit could be clogged.  Hondaman in his book even says that the air screw being dirty or the circuit clogged accounts for a very high percentage of these carbs not functioning correctly.  I won't quote the number off the top of my head but it was quite high. 

I would go ahead and check out the passages and go from there.

[chewbacca5000]

That circuit could be clogged.  Hondaman in his book even says that the air screw being dirty or the circuit clogged accounts for a very high percentage of these carbs not functioning correctly.  I won't quote the number off the top of my head but it was quite high. 

I would go ahead and check out the passages and go from there.

I will give it one more more go then these carbs are getting switched out for a different set that works 100%

Thanks!

[lucky]

That circuit could be clogged.  Hondaman in his book even says that the air screw being dirty or the circuit clogged accounts for a very high percentage of these carbs not functioning correctly.  I won't quote the number off the top of my head but it was quite high. 

I would go ahead and check out the passages and go from there.

I will give it one more more go then these carbs are getting switched out for a different set that works 100%

Thanks!

What year is the bike?
Please stay on this thread.
Need to get all of the facts to help you.

CARBS ARE NEVER toast unless a train ran over them.

These are the facts so far.
CB 750. year?
Type of exhaust?
Type of intake?

Main #110mm.
Slow #40mm.
Needle 4th groove from the top.

[chewbacca5000]

That circuit could be clogged.  Hondaman in his book even says that the air screw being dirty or the circuit clogged accounts for a very high percentage of these carbs not functioning correctly.  I won't quote the number off the top of my head but it was quite high. 

I would go ahead and check out the passages and go from there.

I will give it one more more go then these carbs are getting switched out for a different set that works 100%

Thanks!

What year is the bike?
Please stay on this thread.
Need to get all of the facts to help you.

CARBS ARE NEVER toast unless a train ran over them.

These are the facts so far.
CB 750. year?
Type of exhaust?
Type of intake?

Main #110mm.
Slow #40mm.
Needle 4th groove from the top.

1973 CB750
Intake stock air box w/ K & N filter
stock exhaust 4 into 4.
Needle 3 rd from top

Only carb 3 acts right and responds to air screw adjustment.

I have had them apart and cleaned them a couple times to no avail.  I have other carbs and they work just fine.

[harisuluv]

Well if it works fine with the other set of carbs, I think it's safe to say the current set of carbs is the issue.

Did you check that air screw circuit yet?

[chewbacca5000]

Well if it works fine with the other set of carbs, I think it's safe to say the current set of carbs is the issue.

Did you check that air screw circuit yet?

I thought I checked everything last time I had them apart.  Maybe I need a magnifying glass and will check again.  Didn't have to pull them yesterday hopefully I will get them pulled today.

Thanks!

[chewbacca5000]

Well if it works fine with the other set of carbs, I think it's safe to say the current set of carbs is the issue.

Did you check that air screw circuit yet?

What should I look for with the air screw circut?  Is it possible that the seat is just plain worn out from PO overtightening?  1 work the other don't and they all got the same cleaning.  Is there different size air screws?  Maybe I can get 1 size larger.

[iron_worker]

I suppose it's possible the seat could be worn causing too much air to get by at idle. I would think that the brass would be the first to get mashed though since it's softer.

Can you tell if they are allowing too much air or too too little? Are those cylinders lean or rich?

IW

[chewbacca5000]

I suppose it's possible the seat could be worn causing too much air to get by at idle. I would think that the brass would be the first to get mashed though since it's softer.

Can you tell if they are allowing too much air or too too little? Are those cylinders lean or rich?

IW

IW,

I don't think it is completely worn since turning it out changes the color of the plugs to tan from white, but they just don't tune well.  The one that works changes RPM right away.  The 3 others just sit there.  I can take them all the way out and it still runs the same.  I've cleaned all the white powder out too.

If I could find 1 or 2 size larger air screws I bet it would do the trick.  For the weekend I think I am going to run a different set.

Thanks!

PS: Sweet pipes.  If you are going to sell the Inside left I got a buyer for you.

[chewbacca5000]

I think I got this one almost solved and you'll never believe what caused the problem - over doing it with the Ultrasonic cleaner.  See the attached pic of a cb750 carb and the little brass ball that is a plug for the slow jet passageway.  I discovered it was loose by shooting WD-40 in the slow jet and blowing out with compressed air and you can see the WD-40 coming out the brass ball.

So I am thinking this leak messes up the pilot circuit rendering the air screws useless.  The first time I had a problem with the ultrasonic cleaner it did rattled a float tube loose.

What do you guys think?  Is there something to this?

[kpier883]

Subscribing, can't wait to see how this turns out.  Best of luck!

[chewbacca5000]

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?

[samIam]

I'm new to the motorcycle scene but I would try soldering it.  I used JBWeld in my bowls for a cracked stand pipe and it can off and clogged a jet after just 11 miles of driving. 

[phil71]

carbs are never toast huh?

[chewbacca5000]

carbs are never toast huh?

Come on Phil no need to rub it in.  I can probably stick a set screw in there or a BB.  Leave Carbgyver alone.  Thow in a rubber bank and tin can and I'll have a home made turbo set up.

Just kidding Lucky.

Think I will try solder as it sound more permanent than glue.  Work good on pipes right?
----------------------------------------------------------------------------------------------
Update - Don't try solder it won't stick you will also melt the remaining pliobond  holding the brass ball into place.  Instead I am using a punch to try to reset the brass ball and the cover with pliobond.

Pliobond is a type of contact cement that was originally used to seal the brass balls in.  Interesting enough the only thing that will get rid of pliobond is MEK winch is found in some carb cleaners.

Maybe I'll find the right size set screw and drill the brass balls out then I'll never have to worry about it again.

[chewbacca5000]

The pliobond worked out great.  Before my carbs would not act right now there are running perfect!  All the air screws respond as they should and you can hear and see the changes in RPM when they are turned in or out.

All I did was use a punch and reset the brass balls in the pilot circuit and reseal with gas / oil proof contact cement.  Look for gas / oil on the label, and if it contains MEK it will hold up to gas alcohol no problem.  Redcoat uses MEK as the solvent and is used to coat gas tanks.  I might try JB weld white for aluminum gas and oil formula next time.  Thanks Hondaman for the pliobond tip.   It's what Honda originally used to set these brass ball.

The symptoms of this are air screws don't change RPM and the bike won't start without throttle twist.  The reason is that the fuel is not getting pushed up the the pilot jet the way it should it is leaking back out through those brass ball plugs.  Throttle twist engage a different circuit to get the bike started.

Here some pics.  I will do a proper write up latter as I want to do some ridding today. Just throwing this up now in case someone else is struggling with the same problem today.

Thanks to all for the help especially Lucky.  If it wasn't for your encouragement I would not have gotten to the root source of my carb problems.  Makes me want to rename this thread "Epic Carb Battle of 2012".

Thanks Again!

[lucky]

carbs are never toast huh?

If it was me I would drill and tap that hole where the brass ball was and put in a tiny screw. Like maybe a #440 screw something like that.

Still no TOAST for you!!

BTW...The 657A carb stock set up was needle clip on the 4th groove from the top. NOT 3 groove from the top. That could make it run lean.

ALSO:  People have left carb bodies in those gallon cans of carb dip overnight and that will unglue internal passageway plugs as well.

[TwoTired]

FYI:  Those aren't brass balls.  They are shaped in the form of a dome.  As you mash the center it expands the outer diameter to seal against the carb body drilled hole.

I didn't know there was any sealer there at all.  They really should seal just from expanding the outer diameter of the plug.

I'd wager that corrosion eroded the metal enough to loosen the plug.

If you flatten an arc, the base points move outward from the center.

[lucky]

I just want to applaud Chewbacca5000 for sticking with the problem and really learning something instead of just getting another set of carbs.
So he taught all of us something in the process I think.

So he saved a set of carbs AND learned to pay attention to details when doing mechanic work.

I was a jet engine mechanic at Hamilton AFB, California, when it was discovered one day that a lead pencil mark on a turbine wheel would cause  a crack by drawing heat to the area.  Details,,,,details!!

[Picasoo]

I've got same problem but only with carb No.4...
I'm gonna check this with brass ball...
Thanks for good post!

[chewbacca5000]

FYI:  Those aren't brass balls.  They are shaped in the form of a dome.  As you mash the center it expands the outer diameter to seal against the carb body drilled hole.

I didn't know there was any sealer there at all.  They really should seal just from expanding the outer diameter of the plug.

I'd wager that corrosion eroded the metal enough to loosen the plug.

If you flatten an arc, the base points move outward from the center.

Your probably right in that corrosion had a role to play in the plugs coming loose.  Guess I should not be surprised when there is a ton of white powder all over the place.  I am just happy it worked and my carbs are running strong like I expect them to.  Thanks for the explanation.  Is this the same idea as a Welch plug?

[chewbacca5000]

I just want to applaud Chewbacca5000 for sticking with the problem and really learning something instead of just getting another set of carbs.
So he taught all of us something in the process I think.

So he saved a set of carbs AND learned to pay attention to details when doing mechanic work.

I was a jet engine mechanic at Hamilton AFB, California, when it was discovered one day that a lead pencil mark on a turbine wheel would cause  a crack by drawing heat to the area.  Details,,,,details!!

Thanks Lucky!  I can't tell you how good this feels especially since I have rebuilt many set of carbs mostly Mukuni without any problems. They were about 10 to 15 years newer though.  I can really appreciate your turbine example with the pencil mark as the problems are not always obvious.

I will be sure to check these on every set of carbs going forward.  It might be best to give the caps one gentle tap with a punch and I don't see where clear sealant hurts anything either.  If corrosion was present at some time the cap could be loose and this sounds like it may be a good preventive measure.

Thanks Again!

[TwoTired]

Is this the same idea as a Welch plug?

Yes, exactly that.

[Picasoo]

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...

[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.