Topic: yet another carb tuning question.

[Lefty337]

I searched for a short while on the forums and didn't come up with an answer to my question (by the way, I don't like the search function on the forums.  I can never seem to find what I'm looking for.). 

I have a mostly stock '76 CB750F.  The only changes are that I put in a K&N Air filter (stock airbox) and the exhaust is a 4-1.  I'm not sure if the exhaust is stock or if it's a Kerker.  I've been told that it sounds like a kerker, but it was on the bike when I got it.

The carbs are fairly freshly rebuilt and I sync'd them and all of that.  I turned the air screws out 1 turn on all 4 carbs and when I was accelerating, the bike would sputter at about 4k rpms then clear up past that.  I pulled the plugs and saw that carbs 1-3 were carbon fouling, so I turned the air screws out another 1/4 turn.  It's running much better now, but I haven't rechecked the plugs to see how it's running.

My question is this:  All 4 carbs were rebuilt with the same kits and adjusted the same, why would 1-3 be running rich while the 4th looks like it's running great?  Did I do something wrong, or is this somewhat normal?

Thanks in advance for any help.

[Smitty057]

Number 2 on my 75f does the same thing. I am going to do some carb work this weekend. I will let you know if I find anyting interesting.  Im leaning towards the air screw for said carb. I have a couple more bugs to iron out, though.

[Grabcon]

You have a fuel delivery issue. Pull the carbs and walk through everything, cleaning all jets, emulsion tubes, all passages, etc. I use Xylol (Xylene) and aerosol brake cleaner. Be careful around rubber with the Xylol it softens it up.

Check the needle jets and make sure that the clips that adjust the height are all the same. I have learned never to trust a PO's work on carbs or anything else. It took me almost two year to get my CB900F squared away by trusting what the PO did to the carbs.

Check all jets to make sure they are the same size.

[bender01]

I agree with Grabcon. Clean them again. I use simple green though. Unlees theyre frozen up which yours arent. Did you pull the emulsion tubes when you fairly freshly cleaned them? I think thats a common skipped item.
 They pull out but take a little effort with  pliers and cloth to not damage the brass.
  Report back with your progress.
 Later.

[Spanner 1]

The low speed and main jet air bleeds must be clear to avoid black, sooty plugs unless you run your bike balls-to-the-wall every time you get on it 
With the bowl removed hold your finger over the main jet tightly and squirt carb cleaner ( holding the straw tightly ) into the right-hand ( carb #1 ) air tube in the carb throat, Cleaner must squirt up thru' main jet around the slide needle.   Do the same for the left air tube with your finger over the slow jet, cleaner must spray out of the very small opening ahead of the slide (motor side ) of the throat..... this is a must do part of a carb clean BTW. You will see that bleeds are reversed for carb 3 and 4  ...... good luck.
Here's the thing a lot of folks don't know/get... your bike needs a feed of air and gas FROTHED by the slow and main air bleeds to run as designed. If the air bleeds ( which few seem to clean/verify ) are blocked ( and they are small passages with 90 deg. turns ) then the vacuum generated by the motor will pull in solid gas thru' the rpm range which makes a rich condition even with correct float height, jet size etc...... froth is much less dense ( rich ) than  liquid gas 

[Lefty337]

Thanks for all of the responses. 

You have a fuel delivery issue.
 . . .
I have learned never to trust a PO's work on carbs or anything else. It took me almost two year to get my CB900F squared away by trusting what the PO did to the carbs.

If there is carbon fouling on the plugs, doesn't that mean that it is getting too much fuel?  I definitely don't trust this PO, but I've done the carbs myself.  I'm no expert either, so I may have missed something.

The low speed and main jet air bleeds must be clear to avoid black, sooty plugs unless you run your bike balls-to-the-wall every time you get on it  . . .

I'm pretty sure that all of the passages were clean, but what exactly are the emulsion tubes.  I looked through my Clymer manual and they don't mention them anywhere.  If these are the tubes that the jet screws into, I did remove them and clean them.  I did take compressed air and make sure that everything was clear with each carb.

I've taken the carbs off so many times that I really don't feel like doing it again, but I'll give it a try at some point soon.  At this point it doesn't take very long to get them on and off.

[Grabcon]

Lefty, From my experience when looking at your plugs typically black and wet means ignition, black and dry is fuel. Carbon on the plugs means that that cylinder is running rich. You can sync them all you want but there is something making it run rich. These carbs are simple everything is the same on each carb and when put back together everything should be the same. As I mentioned earlier make sure that the clips on the needles are all the same. You will have to remove the adjustment stems from the slides to check the needles and clips. Make sure that you have a good philips screw driver when doing these. the screw heads strip easily. They are typically very tight.


I use a #2 Philips with a 10" shaft. This provides good leverage to get the screws broke free.

[Spanner 1]

Lefty, the long tube that the main jet screws into with the little cross-drilled holes is the emulsion tube.
The slow jet has the emulsion holes 'built in' in one piece.......... these holes being clear is half of the equation, the other thing is the holes/tubes need an air supply to 'emulsify' ( froth ) the fuel being drawn up the tube by the engine vacuum. The air is supplied thru' the air bleed passages which start in the carb throat ( the 2 little brass tubes you see when you remove the airbox or pods ). These must be clear thru' to the emulsion tubes ( main and slow ). Test for clear by spraying carb cleaner thru' each one while blocking the bottom of the main and slow jet with your finger. Spray must come out around the needle valve ( main jet ) and thru' the slow port ( ahead of the slide, motor side ) for the slow jet......

[Lefty337]

Lefty, From my experience when looking at your plugs typically black and wet means ignition, black and dry is fuel. Carbon on the plugs means that that cylinder is running rich. You can sync them all you want but there is something making it run rich. These carbs are simple everything is the same on each carb and when put back together everything should be the same. As I mentioned earlier make sure that the clips on the needles are all the same. You will have to remove the adjustment stems from the slides to check the needles and clips. Make sure that you have a good philips screw driver when doing these. the screw heads strip easily. They are typically very tight.


I use a #2 Philips with a 10" shaft. This provides good leverage to get the screws broke free.

I've checked these before, but since I'm going to have to take the carbs back off anyway, it can't hurt to check them again.

Lefty, the long tube that the main jet screws into with the little cross-drilled holes is the emulsion tube.
The slow jet has the emulsion holes 'built in' in one piece.......... these holes being clear is half of the equation, the other thing is the holes/tubes need an air supply to 'emulsify' ( froth ) the fuel being drawn up the tube by the engine vacuum. The air is supplied thru' the air bleed passages which start in the carb throat ( the 2 little brass tubes you see when you remove the airbox or pods ). These must be clear thru' to the emulsion tubes ( main and slow ). Test for clear by spraying carb cleaner thru' each one while blocking the bottom of the main and slow jet with your finger. Spray must come out around the needle valve ( main jet ) and thru' the slow port ( ahead of the slide, motor side ) for the slow jet......

That's what I thought the emulsion tubes were, but my manual called them something different.  These were clean, but I'll take a look and make sure that everything is clear.  I'm also going to take a look at my float heights as they have been giving me trouble from the start.

Hopefully it won't rain tonight and I can pull that carbs off of the bike tonight after work.

Thanks for the help and I'll let you know what I discover once I tear into them . . . again.

[Deltarider]

Needle jets is the proper name.

[Lefty337]

I finally rebuilt the carbs over the weekend and it seems to have made a huge difference.  I'm not sure why though since nothing seemed to be blocked.  I guess I knocked something loose.

I'll repost after I ride for a day or two and recheck the plugs to see how they look.  Probably not until the end of the week though since we're supposed to be getting thunderstorms for the next day or two where I am.

Thanks a lot for the help!

Lefty