Topic: 064A carbs on a K2 750

[Johnny340]

This '72 CB750 is honed with new rings and a fully rebuilt head.  Has a stock airbox and filter element.  The exhaust is a Cycle-X four into two.  The carbs are clean 064A carbs from a '75 F0 and have the following jetting:
40 Pilot
120 Main
Floats at 26mm
271307 needles
Clip in the middle position
Mixture screw at 1 & 1/4 T out
Ethanol free 94 octane gas.
The bike starts, idles and runs perfectly at lower RPM.  Past half throttle it doesn't rev cleanly.  I'm thinking those 120 mains are too fat for this setup at sea level and current weather.
I believe those carbs came with either 105 or 110 main jets and the clip in position 4 (from the top??).
I have both the Keihin brand 105's and 110's available to install.  What is the recommendation here?

Also, there is much debate as to the clip position count starting from the blunt top end vs the pointy bottom end and wondered what position is recommended to start with here.  I know I shouldn't do two changes at once and there are lots of other factors that go into jetting but what was the stock position referring to for clip position?
Many thanks in advance!

[seanbarney41]

I have used the 064's a lot.  I think the needle profile is very flexible and forgiving, so that clip position is not super sensitive or critical.  I would try the 110's first.  Hell, it only takes 10 or 15 minutes to swap main jets.

Also, why the 94 octane?  I have found it just makes for fluffy black plugs with stock compression.  Try some 89 or 87...or better yet some of that Motorsport 90 no ethanol if you can get it in your area.

[Johnny340]

Also, why the 94 octane?  I have found it just makes for fluffy black plugs with stock compression.

Thanks for the reply.  I'll try the 110's.  As for the gas, I thought Chevron 94 was the only non-ethanol option around here but since you asked, I looked it up and now see there are other options (Shell, Esso & Costco) that offer 91 octane without it.  Cheers.

[scottly]

Also, don't rely on the mechanical float level measurement; the "clear tube" method will show the true level. For now, try the smaller mains, and if that doesn't seem to improve things, check the fuel level in the bowls.

[Doobie]

The 064A carbs originally came with #105 and #40 jets, needle in third or fourth slot.

[Johnny340]

The 064A carbs originally came with #105 and #40 jets, needle in third or fourth slot.

Thank you for the info.  Was that 4th slot from the top (blunt) end?

Also, don't rely on the mechanical float level measurement; the "clear tube" method will show the true level. For now, try the smaller mains, and if that doesn't seem to improve things, check the fuel level in the bowls.

Smaller mains are in and it runs good in the shop but I need a dry day to test it properly.  Good idea with the clear tube method!  One float was slightly bent (which I don't remember seeing when I set it) and that bowl happens to continually leak..

[Doobie]

Yes, 4 slots counting top (blunt end) down.

[Johnny340]

So I took it out today and it ran way better but I still have a huge hesitation if I quickly open the throttle.  Actually, it dies and doesn't recover unless I back off the throttle. If I wind the throttle up normally, it revs up fine and runs cleanly at higher rpm.  I have the needles in the middle position which should be leaner than the factory setting.

So, it starts easily, idles perfectly, and runs great unless I get too aggressive with the throttle.  Keep in mind I'm breaking in the top end and valves, etc. plus the float valve on carb #3 sticks intermittently.

I will do the clear tube method again tomorrow.  Today's attempt made no sense because that fuel valve was sticking.

[Johnny340]

Should I richen up the middle by dropping the clip (raising the needle) one position to the factory position?  I'm wondering if it still can't handle a quick transition to a relatively richer full throttle setting.

[PeWe]

My CB750 K2 got my stock K6 carbs.
For many years ago it ran with:
40 pilot,
needle middle (3rd notch from top)
Main 105 with stock HM341 4-4
Stock air filter
Old leaded fuel

Now on my K2 using better flowing Yamiya no numbers 4-4 like HM300 and modern fuel max 5% ethanol as we have here.

40 pilot,
air screw 1 turn out, the area is 1 +/- 1/8 .
Needle 1 notch  richer (4th from top)
Main 115  (120 might be rich)
Stock air filter

[Johnny340]

That stock jetting is similar with the exception of the needle clip. I don't have ethanol to contend with. 
I have slight popping on decel which might support the lean low / middle theory. I have the leaky float valve fixed now.  A couple float bowls show 5mm below the top of the bowl using the clear tube method. A little low?

 

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[Don R]

 I'd think it may be a little low. That would support it being lean too. My F1 liked the 110 jets, after much work and anguish we realized the stock 105's were drilled to who knows how big. I have pin gauges now.