Topic: Odd running problem

[jav409]

Let me start this by saying I have used search to try to find an answer and thus being unsuccessful have come to ask of your combined wisdom. I have 2 CB750’s both Built up from basket cases, the first being an 1977 F2 with all the 1977 parts except the engine is a 1971 K motor. /the late model carb's have the accelerator pump and stock air box with a Macks 2 into 2  turnout system. After much trial I am using 110 main Jets and the timing is slightly advanced burning 92 octane fuel. The motor runs like a sewing machine. Plug life is very good , burning a light tan and motor pulls hard with K gearing, this is my cruiser, 60% around town and 40% highway staying with my Harley buddy’s, and I can all day long. No overheat or coloring to the pipes and smooth all the way thru the throttle no matter what the position.
The second is a 1974 Denver’s chopper, with a Sugar bear Springer. The motor is a 1978 K motor with very low mileage ( you could still see the crosshatching thru the spark plug holes ) and a breadbox air filter. The carbs are the early 1974 models. I have Sealed the top and sides of the breadbox the stock intake tubes are used. I can’t seem to Find the number on these carbs to make it purr. Is it strong you bet , pulls hard thru 7000 Rpm ( as fast As I need to go ). Gets good fuel mileage, I have changed the Low jets to 35’s to get rid of a loading up problem it had with the stock 40’s and the needles are one Groove from the top from the center groove stock. The main jets are 110’s.Pipes are open full length past the rear wheel. It pulls hard with no hesitation but here is the problem, between 2000 and 3000 rpm if the throttle Is held steady it misses, till you start to accelerate and then it smooths up, I don’t believe it’s a lean problem cause closing the choke some does not help it and in the morning when cold it seems to run smoother than when warmed up. Above 3000 rpm is fine. Plugs are light tan with a fluffy carbon ring.
Do I need to go leaner to the top notch on the needles and then play with main jets or is this missing normal? It would seem from the 78 K carb specks this motor is built to run on the lean side. But that is with the late model carb’s like the F has. Any help at all is greatly appreciated. I know these things take time, it was 3 or 4 try's on the F before every thing fell into place. But if anyone has had this situation and found a solution it would save me
A lot of time.
Thanks  Jav

[lucky]

If you do a Google search most of all of the SOHC forum entries about your inquiry will come up.
If the slide needle has the clip all the way at the top something seems off.
You are right about the stock F model it came with 105 mains and #38 idle jets.

When you say you "sealed" the bread box air cleaner what does that mean?
When you say "tubes" does that mean the rubber velocity stacks?

FACTS:

1978 CB750K engine.
Breadbox air filter.
Exhausts open straight pipes.
Carbs 1974 CB750
Main jets #110mm.
Idle jets #35mm.
Mixture screws?

I would do a plug chop on this right at the rpm that you say it is acting up.
Also the fact that the slide needles have the clip at the next to the top notch is peculiar, but the 1974 carbs might be richer that the 1978 carbs that had the accelerator pump feature.

If I did any changes to this bike I believe I would start with a mixture screw adjustment.

I would like to know how many turns out the mixture screws are.

[jav409]

Thanks for the quick response! The needles are one notch from the top. The Idle screws are 1 full turn out from seated. Idle is solid with no missing, Yes the rubber velocity stacks. I Used a metal strip from the inside to cover the holes in the breadbox starting from the bottom around the top and around the other side and sealed the edges. I left the bottom unsealed it has a foam/oil air cleaner. I found little difference at low speed but when it was open at high speed I could change RPM by moving my legs in or out. now it is stable at high speed and I have noticed no loss of power. I have experimented with the idle screw setting but this is where it idles the best and yet the miss persists between 2000-3000 rpm. the carb set needles were originally in the middle and way to rich ( they came from death valley ) I moved them to the current notch. the 78 specks show them in the very top notch but that is for the carb's with the accelerator pump.

[lucky]

Thanks for the quick response! The needles are one notch from the top. The Idle screws are 1 full turn out from seated. Idle is solid with no missing, Yes the rubber velocity stacks. I Used a metal strip from the inside to cover the holes in the breadbox starting from the bottom around the top and around the other side and sealed the edges. I left the bottom unsealed it has a foam/oil air cleaner. I found little difference at low speed but when it was open at high speed I could change RPM by moving my legs in or out. now it is stable at high speed and I have noticed no loss of power. I have experimented with the idle screw setting but this is where it idles the best and yet the miss persists between 2000-3000 rpm. the carb set needles were originally in the middle and way to rich ( they came from death valley ) I moved them to the current notch. the 78 specks show them in the very top notch but that is for the carb's with the accelerator pump.

there is some kind of miscommunication.
QUOTE:

"78 specks (should be specs as in specifications), show them in the very top notch but that is for the carb's with the accelerator pump."

No stock bike comes with needles with the clip in the very top notch.

Where did you get that information? just asking.

[BobbyR]

The 78K carbs do not have adjustable needles. 

[jav409]

Sorry I ment the 74-75 carbs , I am realy messing this up, mine are now 1 notch from the top notch, The #4 slot on the not pointy end.
657A & 657B < the ones I have , Type 1 Float 26 main 105 slow40  needle 4    turns 1
I was on lunch break at work.  ooops :/
my 77's don't have an adjustable needle but the stop looks to be where the needle at the#5 position would be. I guess I could pull one and measure it.

mine are Type 1 Float 26 main 110 slow 35  needle 4 turns 1
I am at 2300 feet.