Help with Carbs!

[theoldbluehouse]

Hello everyone,

I am having a bit of trouble with the carbs on my bike and would really appreciate some advice.
the set up: 72CB, keihin cr29, k&n pods, 4-1 exhaust
The issue:
bike idles low (800rpm) when i give quick throttle it dies
some days it seems to idle and run ok, others not so much
if i run it it has little power on 1st, beyond that any throttle doesn't do a thing or it takes seconds to pick up.
I have 115 mains on it now.
Took apart the carbs and found some little residue on the carb bowl. I clean it, including jets etc. Didn't want to fiddle too much with the floats or needle floats.
Also, when the bike was running (poor but running) if i pushed the carbs down a little, the bike would idle higher. Pushed them up the opposite would happen.
Do the carbs need to be perfectly leveled on the bike?
Also the valves (pods side) have more opening than the valves on the intake side?
is this right or should they match?

I know its a lot but I'm very confused with this carburetors...

 

[TOOLS1]

You have a lean condition going on here. The fact that the RPM changes when you move the carbs indicates a vacuum leak.
TOOLS

[theoldbluehouse]

Thank you tools. I took the boots out and they look fine to me. It is possible that they were not pushed in all the way on the carb side. I will double check once cleaned.

[TOOLS1]

Could be, but your clamps could also be stretched out.
TOOLS

[theoldbluehouse]

How about the throttle valves? do they need to be exact (opening) on both ends?

[TOOLS1]

If it is the slides you are referring to, then yes. The inlet side is cut at an angle on them.
TOOLS

[Re-run]

What else was done to the engine? Any port work, boring, anything. Seems that cr carbs would be a bit overkill for a bike with no engine work. Knowing if work has been done, may help in determining if jet sizes may need to be increased. WHat size is the idle jet? The idle jet does more for the overall rpm range than the main jet does.

[theoldbluehouse]

Re-run
The engine is stock, I have a power arc ignition.
Other than that, 4-1 exhaust and that is it.
My jet settings are:
120 Main jet
240 Air jet
60 Slow jet
YY8 needle 4th spot

hope this helps
thanks

[theoldbluehouse]

Ok guys, took carbs apart, cleaned all parts, jets, bowls adjusted to 14mm etc. changed fuel line and clamps.
Bike started right away but the idle is low again (700 rpm), just the way it was when i took it all off.  
i have to give it throttle to keep it on, otherwise it dies. I adjusted the idle screw to make it stay on and it did. but once i rode it had no power to go beyond 2nd, choking it slightly kills it, opening throttle makes it hesitate and goes nowhere....
what could it be??

[TOOLS1]

Two things. First, a proper fuel line for a motorcycle should not need a clamp. If it does, it is the wrong line. As for the running, it is way too lean. Try lowering the clip on the needle, and larger main jets.
TOOLS

[Re-run]

Too small a fuel line can choke flow off. As for using clamps on a fuel line, every bike i have ever had, has used clamps. My cb has always had them, same with my 84 vf700c, 03 rebel, and 06 fz6. I would never do a bike without hose clamps on the fuel line. It shouldn't leak without the clamp, but the clamp makes sure the line doesn't come off.

[theoldbluehouse]

Tools,Re-Run thanks for the advice.
The clamp was for the carb boots, the fuel line plugs in tightly. I just replaced the fuel cable in case the old was dirty.
The main jets were 125, and the needle (7 ring) in the 4th pos. from top., so on the richer side.
You really think it's running lean? i ask because a slight choke shuts it off, and even a quick twist of throttle kills it too..
I have 3 sets of jets, #115, 120, 125...
this bike is really playing with my head!!

[Re-run]

Have you done a plug chop? Pull the plugs out and clean them up if you can, or put in new ones. Run the bike for a while and try to ride it if you can. Get the rpms up pretty high, if possible. Then kill the engine and pull the clutch. Pull the plugs back out and look at them. They will tell you if things are rich or lean.

[theoldbluehouse]

Measured floats again, put all back together and tried to fire up. It wouldn't hold idle unless I turned idle screw in a lot and kept throttle open a bit. But then I reved it  up and it felt like the something sparked and I saw smoke come out of the carbs... Now before that I had moved the needle to the 2nd position (richer) and it struggled so I changed it again to the 2 position from the top (leaner) and that's when this happened. I left it like that for now.

[TOOLS1]

Is the timing advancer working?
TOOLS

[theoldbluehouse]

Tools I have no idea! how do i check?
Power arc ignition
here are the plugs they look black and dirty

[Re-run]

Holy hell are those rich, or your ignition is not firing right. That is so rich that even iridiums with your ignition are having issues. Ok, now we have a direction to go.
Here is the manual for your ignition, or it should be anyways. http://www.powerarc.com/pdf/c2-sohc-750cb.pdf

Go through that and check things out to make sure your ignition is set right. Once you have your ignition system set according to the manual. Try it again.

If things are the same, we move to carbs. Being different carbs, I don't know what the float height should be, but you should fine the carb model on the side of them and look it up online.  When you did your float height, you held the carbs at a 45 degree angle? You want the float needle to close, but you do NOT want the pin in the end of the needle to push in. If that happens, the height will not be right. So needle closed but pin not pushed in.

I see you have pods, those have a tendency to make tuning harder.

Now, on the carbs, how far are the fuel screws turned out? Stock carbs on later 77/78 cb750 k are 1.5 turns from lightly seated. I doubt you would need much more on these. The stock jets on the 77/78 K are 40/110. Your carbs have 115 mains, but contrary to what many think, the mains don't do much until at least half throttle really. Your idle jets are where it is at from idle to a good way up the throttle range.

Do you have some sort of manual for the carbs, if not, check this out, It may help.
http://www.duncanracing.com/TechCenter/KeihinCarbJetting.pdf

It sounds like your carbs may have air screws, I don't know.

[theoldbluehouse]

Yes, those plugs are extremely rich. I'd have to check the timing, the shop that installed the ignition module (recently) set the timing, of course I am not 100% sure as I haven't checked myself.
Something tells me the floats are wrong, very wrong because I haven't had any changes on the bike for over a month.
But thinking back, there might have been a time when I was riding and the bike was doing some sort of gurgling thing, I could hear it as it was idling. It then came back to normal and I rode on not paying much attention.

I am not sure the 45 angle is right. i know the height should be 14mm, it's in the manual, but not the angle to measure.
Mikunis i think measure at 35 degrees...

[Re-run]

The angle if for holding the carbs so the float closes but the pins do not press in. It isn't an exact angle, as that may be different depending on the float needles. So you are just holding your carb rack at an angle. I don't think most manuals mention that, I could be wrong. But it is something I learned years and years ago and it has given better results.

[theoldbluehouse]