Topic: carb tuning question

[The other Derek]

I apologize if this is out there already but I didn't find my answer with the couple of searches that I did.

I've got a cb750 k6 and I recently cleaned/rebuilt my carbs- and I'll be the first to admit that probably wasn't qualified to do it.  Part of this experience is supposed to be me learning to maintain and service this bike and I know I've still got LOTS to learn- and I want to learn it.

Prior to the cleaning and rebuilding, the bike was running pretty good- pulled very strongly through the power band.  It was tough to start at times and as it was new to me I figured I'd clean the carbs and go through them as best I could to try to increase my understanding of how they work, and what kind of shape they were in.

The carbs were cleaned in diluted Simple Green in an ultrasonic cleaner, rinsed in water and blown out with a compressor.  Bowls were cleaned with carb cleaner and a toothbrush.  They were rebuilt with Keyster kits.  40 slows.  110 jets.  Floats set at 26mm (although I haven't checked them with the bowls on).  I moved the needle back to the center gap (which raised the clip one step from the previous setting) and the air screws are set at 1 full turn out vs the 1/2 turn they were at when I pulled them apart.

The issue is bogging at WOT.  It pulls very well at 1/2 throttle but as soon as I pass 2/3 or so it starts to bog down.  If I back it off it will stop bogging.

My understanding is that I did 2 things that moved the carbs to the lean side- turned out air screws 1/2 turn and moved needles down one position.  Prior to the rebuild I could smell the gas pretty strongly at idle.  I think I need to move the needles rather than turn the screws, but I'm hoping to get a little advice before I start fooling around with the carbs again.

FWIW, I did try adjusting the choke to see if that would alleviate the problem, but if it did it was marginal at best.  I'd probably still be out there running up and down the road if it wasn't Sunday night.  Shoulda gotten to this earlier in the day, but there's always tomorrow night.

Also, I did not have clamps on my air boots previously and two of them were very loose in the air filter box.  I put some clamps on and taped up the loose boots as I was hoping that would solve the problem.

[Markcb750]

Sounds like float levels if you have them set a little low it will starve for fuel and bog at high throttle settings.  Several methods  to set floats discussed here if you weed thru the search function and there was a carb manual and service manual available on the main page of the Website.

I had to tweak the float height down from the factory setting to make my K6 run at WOT, just a mm or 2.  did not figure out why.

[The other Derek]

Thanks Mark.  Not exactly the answer I was hoping for, but it's getting easier to pull the carbs each time.  I was really hoping to avoid pulling them again, but your answer makes sense.  I was pretty careful in setting the floats, although I haven't verified them yet.

For sh!ts and giggles, I'll probably turn the screws in 1/4 turn before I ride into work tomorrow just to see if that makes a difference.  My commute is pretty short and in morning traffic I probably won't get a chance to push it too hard, but I can always go for a ride at lunch and see how she does.

[lucky]

What are the numbers on the carbs???

Quote
"  I moved the needle back to the center gap (which raised the clip one step from the previous setting) and the air screws are set at 1 full turn out vs the 1/2 turn they were at when I pulled them apart."

My question is did you raise the needle or the clip?

Can you just say what groove the clip is on, counting from the top?
Why would you raise the clip on a set of carbs that was running very well??
You also turned the mixture screws.
Big mistake because you made two changes at once.
Now you do not know which one of those two changes made a  result you do not like.
Makes no sense.

The final question. Which number carbs are these, and where IS the clip on the slide needle, (counting from the top?)

[The other Derek]

I raised the clip (lowered the needle).  The needle clip was at 4, but it is now at 3.  I've see conflicting info on the stock needle setting.  Maybe it was there already?  I realize my mistake in making multiple adjustments, but a few other things that I've found in going through the bike have led me to the mentality to take things back to stock and start from there. 

   bracket to hold the centerstand down rather than adjust a very loose chain
   loosened caliper bolts to deal with seized caliper
   huge balls of electrical tape at connections
   big wad of brown wires in headlight bucket that took a bit to sort through to figure out why I had no rear running light

The bike ran well when once I got it started (which could be a bear).  At idle the exhaust smelled strongly of gas and the plugs were fouling.  Pulled hard at the power band- particularly after I put a couple of hundred miles on it.  I figured the best approach was to start from stock.  Oops.

I'll look for a number in the AM, but here's a pic of my carbs:

[The other Derek]

The 1/4 turn in (to 3/4 out) of the air screws seemed to make it worse.  It was pretty cold this morning (35) although I'm sure I was feeling it a lot more than the bike was.

I'll turn the screws back out and raise the needles next.

[72_350_FOUR]

Have you synched your carbs after cleaning ?  Bench synch, then vaccum synch.  This changes the way my bike runs completely.  Also you may have some air leaks from what you described on your carb boots.   let the bike idle and spray some wd 40 or something around the boots and see if the RPMs increase, if so you have a vaccum leak and would need to fix that first..   just my 2 cents.

[72_350_FOUR]

Also i used simple green to clean my carbs, and rinsed them also, and upon re-assembly it ran like Sh**  I found that my emulsion tubes were plugged with white residue from the simple green, i had to pull them and clean each hole with a toothbrush bristle.   

[The other Derek]

The carbs have been bench sync'd but not vacuum sync'd.  I put together a manometer, and tried to use mig tips as vacuum port adaptors, but they wouldn't seal to the tubing and I gave up on trying to get clamps on the adaptors for 2 and 3.  I ordered some adaptors and am hoping they'll be here in a couple of days.  After all that BS I wish I'd just ordered a 4 gauge set.  At least it'll be more fun to lend out the manometer.

I've seen a bit of white residue on the outsides, so it's probably a good idea to pull the carbs and remove any residue and ensure all the passages are clear.  That'll give me a chance to re-check the floats and (maybe) move the needle clips.

Thanks

[iron_worker]

You've set your carb settings to stock but the big question is ... Does your bike run stock exhaust and stock airbox? If it doesn't you will need to do some custom tuning.

IW

[The other Derek]

I moved the needle clips back to the #4 position.  Floats still checked out OK.

DAMHIKT, but apparently the jets will vibrate loose and possibly be laying in the bowls if they aren't sufficiently torqued. 

I visually bench sync'd the carbs again and adjusted the choke linkages as there were minor differences across the four.  It was pretty late by the time I got everything back in place and it was tough to not start it last night.

But, it started pretty easily this morning and on the ride into work I had no bogging or flat spots.  It felt maybe a touch sluggish overall, but a HUGE improvement over yesterday.  Pretty excited for the vacuum adaptors to show up so I can finally vacuum sync the carbs,

One thing that I'm not sure is an issue is the adjustments that I need to make to the idle screw to get the bike started when it's cold.  She needs a little throttle to get started and idles very low (and rough) unless I adjust the idle screw.  Once it warms up, the revs increase and I need to back the screw out to get the idle back down to 1200-1500.  It still idles a little rough and has a hard time running slower than that.  Maybe I should revisit the timing before I try to sync the carbs.

[The other Derek]

BTW thanks for the advice, support, and kick in the butt (Lucky).