Topic: Cb750k2 carbs setting problems

[olasek18_18@o2.pl]

Hello, I was finish my Cb 750k2 project,
Rebuild engine and carbs,
New pistons +1,00 OEM style
Carbs Keihin 657A
Oryginal airbox and new filter inside
Mac exhaust 4in1
Slow jet 40
Main jet 122,5
Pilot 1 turn
And sttill too por, The question is how much should I increase the jets?
Does enyone similar settings?
Please check the photo of spadł Plugs.
Regards

[smee]

What carb style? For a K2 with OEM carbs they would be stamped B750A, 7A, 657A, 657B, or 086A. The PDs I am familiar with are lower numbers (ie 42B,43A, 44A), but those would be on later model CB750s.

I do agree, your plugs look lean to me, but we know nothing about how you got that. Was this at idle? How long? Plug chop? What throttle position? Do they all look like this? Could you provide us more info.With the high performance exhaust, getting it dialed in may be a bit more challenging then with stock pipes.

Also, it appears your timing is a bit off as well when you look at the plug strap (IMHO). You want the mark to be more centered on the bend so may require some adjustment there as well.

[willbird]

I am messing with things along these lines myself.

But basics are IMHO..........

Make sure no air is leaking in.

Confirm float levels.

Carb sync can make some cyl look weird vs others again IMHO.

Not sure what riding conditions your plug condition is from ? To do it "right" we need to ride a condition, say cruise, then pull in clutch, hit kill switch, and coast to a stop and pull plug(s) to look at. Any idling is said to ruin a plug reading.

Wish I had a Dyno LOL :-),  or even an airport runway :-).

Also who made the jets matters too.

FWIW I have Mac 4:1, old school K&N pods (bigger than modern ones) and I have 130 Kehin main jets, 40 Kehin pilot jets, my needles are raised one notch, and idle screws at 1 turn. My setup seems a touch lean IMHO so my next step is to work into aftermarket mains which some say are about 2 numbers lean from Kehin mains. So I might give a set of 140 aftermarket mains a spin and see what that feels like.

Bill

[PeWe]

Needle setting?
Stock clip setting is middle, 3rd from top.

If that 4-1 system flow better than HM341 4-4, more like Yamiya No numbers 4-4 or HM300. Raise needles 1 notch to 4th from top.

Main jets probably 115.

That is how my K2 has with Yamiya no numbers 4-4, stock airbox, now with KN filter.
- Pilot 40
- Air screws 3/4 (better take offs)
- Needles stock but raised 1 notch to 4th from top.
- Main jet 115

It has Cruzinimage +0.50mm pistons, 392 type  (K7/K8) making higher compression.

(Using pods will probably need bigger main jets.)

[olasek18_18@o2.pl]

What carb style? For a K2 with OEM carbs they would be stamped B750A, 7A, 657A, 657B, or 086A. The PDs I am familiar with are lower numbers (ie 42B,43A, 44A), but those would be on later model CB750s.

I do agree, your plugs look lean to me, but we know nothing about how you got that. Was this at idle? How long? Plug chop? What throttle position? Do they all look like this? Could you provide us more info.With the high performance exhaust, getting it dialed in may be a bit more challenging then with stock pipes.

Also, it appears your timing is a bit off as well when you look at the plug strap (IMHO). You want the mark to be more centered on the bend so may require some adjustment there as well.

Sorry 657A its a carb serial number
Iddle on 3(middle position)
Spark plugs after 50 miles ride.

[willbird]

What carb style? For a K2 with OEM carbs they would be stamped B750A, 7A, 657A, 657B, or 086A. The PDs I am familiar with are lower numbers (ie 42B,43A, 44A), but those would be on later model CB750s.

I do agree, your plugs look lean to me, but we know nothing about how you got that. Was this at idle? How long? Plug chop? What throttle position? Do they all look like this? Could you provide us more info.With the high performance exhaust, getting it dialed in may be a bit more challenging then with stock pipes.

Also, it appears your timing is a bit off as well when you look at the plug strap (IMHO). You want the mark to be more centered on the bend so may require some adjustment there as well.

Sorry 657A its a carb serial number
Iddle on 3(middle position)
Spark plugs after 50 miles ride.

From messing with mine today "just" 1/4 turn in on the Idle screws took it from whiter than that and running poorly 4200 rpm 60 mph cruise to running better and getting some light brown after aprox 8 mile ride with little or no idling. I did idle 100 feet up the driveway.

Many recommend adjusting idle screw 1/8 turn increments but in my case I was taking it from 1-1/4 back to the 1 turn I always run.

[smee]

Stock settings for 657A--
Float: 26 mm
Main: 110
Pilot: 40
Needle position: 4 (top position is 5)
1 turn out on the air screw

OEM Honda jets?

How does it ride? Do you have throttle positions where it surges or falls flat?
Does it idle well? Get worse or better as it warms up?

[olasek18_18@o2.pl]

the motorcycle is going well, I would even say very well, but before that I had 120 jets and replacing it with 122.5 gave it more power.
power in the full range, but I feel that there is still potential in the upper part!

Also it runs good on 950/1050 rpm

[olasek18_18@o2.pl]

I raised the needle 1 notch
position 4 from up.

power became available throughout the range, especially a noticeable increase in the 3/4 range

The main jet is currently 127.5

And the spark plugs looks too lean

[PeWe]

If still lean indicating use of  pods or no filter at all, right?

[olasek18_18@o2.pl]

have oryginal airbox with new oem filter,but maybe airbox is not hermetish... need to check it.

[PeWe]

Needle setting?
Stock clip setting is middle, 3rd from top.

If that 4-1 system flow better than HM341 4-4, more like Yamiya No numbers 4-4 or HM300. Raise needles 1 notch to 4th from top.

Main jets probably 115.

That is how my K2 has with Yamiya no numbers 4-4, stock airbox, now with KN filter.
- Pilot 40
- Air screws 3/4 (better take offs)
- Needles stock but raised 1 notch to 4th from top.
- Main jet 115

It has Cruzinimage +0.50mm pistons, 392 type  (K7/K8) making higher compression.

(Using pods will probably need bigger main jets.)

Update:
My carbs above are K6 -76 carbs, 086A.

That bike has now got another set, 657A.
I had to set the needles to middle. 4th from top too rich.

Otherwise same.

I'll try smaller main jet, now 115, next 110. (Plug color)

The later 086A carbs must have been designed to run leaner.