Topic: Help determining carb condition.

[Mitch]

Hey gang,

I've been looking through the achives but can't find the answer I'm looking for. After pulling the carbs for the millionth time I need to ask for help.

I just finished rebuilding a CB750K5. She is a hard starter, I have to kick it at least 6 times to get her going (choke doesn't seem to have any effect). I have to twist the throttle 1/8 open for her to fire up. Once she's started and warm she idles fairly well. She has good acceleration (strong pull, no stumble) from 0 - 1/4 throttle but then hits a flat spot between 1/4 and 3/4. After 3/4 throttle she starts to pull hard again.

Here's my set up:

stock engine w/Keihin carbs - (so clean you could eat off them) air screw: 1.25 turns, jet needle at the 2nd notch from top, 120 mains, 40 slow. Floats are at the correct heighth. Carbs have been balanced.

I'm running pod filters and a 4 into 1 exhaust.

She has been tuned (timing, valve clearance, correct (new) plugs and gaps, coils and wires are good, points correctly gapped).

Plug chop results: 1 and 4 light tan, 2 and 3 lighter tan.

My uneducated conclusion: she's running lean and I need to move the needle to the third notch from the top.

I'm very close to getting these suckers dialed in, can anyone help?

Thanks!

[theunrulychef]

Definately not an expert here, but from what you say, it sounds like you're in the throttle range where the needle position has it's greatest effect.  Try doing a plug chop with the throttle in the 1/4 - 3/4 range & see how it comes out.  Then move your needle position based on that.  Also, since your plugs are different colors corresponding to the coils/points setup, you might also want to double / triple check all of that including the wiring to the coils & grounds etc. 

I'm curious to see what some of the more experienced posters have to say as I'm trying to get through my own carb/ingnition tweaking right now.

[Jonesy]

I've been told by a few old-timers that with a 4-to-1 exhaust the two inner cylinders can run a bit leaner than the outer two. I have no idea why this happens. I concur that you might want to try raising the needles up one notch to richen up the midrange.

[Mitch]

Thanks guys! I'm headed out to the garage. I just wanted to get others opinions before once again pulling the carbs. I'm getting really good at the process. I'll follow up and let you know the results.

Mitch

[dusterdude]

mitch,i bet you aint as good as bob wessner when it comes to pulling them carbs.

[Jonesy]

mitch,i bet you aint as good as bob wessner when it comes to pulling them carbs.

Actually, this is one area where Bob would have it easy. With a K0 you can just pull the carb tops off and adjust the needles individually (being cable-operated), rather than having to pull the whole bank and removing them from the stay plate. On a K0, you're looking at a 20 minute job. On a K1-on you're talking at least an hour!

[Mitch]

Well it worked! Moving the needles to the third notch did the trick. She pulled hard from 0 - full throttle. Still need to go back and re-balance the carbs but I'm happy.

Dusterdude, I couldn't agree more. Pulling the carbs from my K5 is a lot of work and after a dozen times it get real old real quick! But it was all worth it.

Now on to the frontend. Shes got a really bad hop. Man, it never ends huh?

See ya!

[dusterdude]

mitch,i hear ya,i hate them stupid things

[mrbreeze]

I just had to reply to this post because it is so close to my problem.I have a K5 750 that I changed over to 4 to 1, pods, and Dyna ignition.Elevation is about 90 ft. temps around 90 degrees. I started off by rebuilding carbs.I went with 120 mains and needles at middle clip.Didn't like that so went to 137.5 mains. I liked that better but still was notgetting clean mid throttle response.By the way,I hadn't synched the carbs up to that point(Besides a bench synch).I bought a Morgan Carb tune set and while it did smooth things out considerably,I still wasn't happy with the mid range.I decided to pull the carbs and move the needles but I didn't really think things through and moved the clips down(raising the needles--Richer).At this point, now I am noticably running too rich so I decide to go back to my 120 mains.Better but still not right.At this setting, I go on a 350 mile weekend run and discover that it runs just fine gettin on it on the freeway but still #$%* around town.Thats when it hit me that I should have moved the needles down instead of up!Sooooo...... this  past weekend I pulled The carbs (3rd time now) and put the clips 2nd from top and resynched.Now it is the crispest it has ever been but I noticed that it breaks up a bit when I nail it hard.It didn't do that before with the other combination. I am definetly leaving the needle at 2nd from top but am going to go bigger main again.Would you start low and go up or up and come back down? What I mean is go from 120 to 125 and then 130 or go 120 to 130 and then down to 125 if needed? It's $28 each time I buy a set and I am not crazy about having every jet available.I just want it to run good through all throttle settings.

[Mitch]

MrBreeze,

I'm not sure what you mean by "breaking up". Is it stumbling when you twist throttle hard? What position is your throttle when it starts to "break up"? 0 to 1/4, 1/4 to 3/4, or 3/4 to full throttle?

Mitch

[mrbreeze]

Hey Mitch,I mean my mid range is right on now but when I hit it, it stumbles. I just got some 125 mains and hope that will fix it.If not, I'll probably try some 130"s.You know...I acually haven't tried the 137.5's with the new clip positon but I'm thinking that the 137.5's are too fat anyways. I got a buddy coming over this weekend for a putt and we are going up into the mountains(Angels Camp & Sonora).I don't know the elevation but it will just be another experiment with my carbs. My bud just got a brand spankin' new BMW something or other so I'm hoping my 30 year old carbster will at least keep up with his high dollar European scoot!I'll report back and keep you all tuned in. Later.......The Breeze