Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Pauarc on July 31, 2023, 03:20:17 PM
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I have been reading up on the main carb jet for my bike they have been fitted with 120
When looking at info I think it should have 110 the carb are 657b all the plugs are on the rich side could this be the course.The float are set at 26mm slow jets 40 air screw 1and1/2 standard air filter HM341 exhaust Middle slot on needle
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All my CB750 K2's 1972, 1973, have had 657A carbs with 110 main and 40 slow. Yours is a later K2 so, 657B carbs are the original ones ;)
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Why would someone fit 120 when they should be 110 I have now fitted 110 but have not been for run yet
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Maybe some one changed the needle's to suit a 120 jet. I had K271307 needles with 110 jets and it had trouble on tick over.
Changed to K27201 and it ran fine ;)
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I didn’t think the needle would effect the tick over all I no they’re on middle slots it rev from tick over ok sits at about 1000 to 1100 will have to see what its like when I get out for a run
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Found this useful, may need to paste the address into your browser, as it wont paste as a URL. Shows the combinations of Honda carbs, by model / year with float heights, jet sizes and needles:
content://com.sec.android.app.sbrowser/readinglist/0801085831.mhtml
👍
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Can’t open link
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Just copy it and paste it into your Internet browser.
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I tried a copy and paste. Doesn't take me anywhere.
This is for USA Bikes >> http://www.hondachopper.com/garage/carb_specs/carb_specs.html
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Strange, still works for me. Don't know how to fix it.
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Found this info my air screws have the holes in end
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This as well
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Found this info my air screws have the holes in end
There is a difference between the USA market on carbs and the UK. The USA kept dropping sizes as they had the problem with USA regulations on emissions. UK K2's run on 110 main jets.
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Is there a good place to look for uk info and specs on uk bikes
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Is there a good place to look for uk info and specs on uk bikes
Unfortunately not.
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Is it possible to check needles on the bike or is it a carb strip off bike
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Is it possible to check needles on the bike or is it a carb strip off bike
Carb strip ;)
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I have put the 110 main jets in I think I will see how it runs before I strip carbs and see how the plugs look. Thank you Trigger
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I've done needles on the bike on a 500/550, tricky but can be done. Might not be possible on the 750 though due to differences in carbs but TBH they look the same type of arrangement.
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Can not be done of later 750 carbs as the lifter is on top of the carb. Can be done on early 750 carbs with cables to each carb ;)
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110 jets in Been for run for about 30 miles runs ok tick over a little rough air screws now 1 turn out. was 1 and half going to look at plugs tomorrow morning
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110 jets in Been for run for about 30 miles runs ok tick over a little rough air screws now 1 turn out. was 1 and half going to look at plugs tomorrow morning
Certainly interesting as the use of airscrew resetting when using the smaller mains shows that it's really asking to run with the needle lifted one notch (needle up, clip down one) to get it in the right place now.
This would bring the initial phase of needle taper and richer mixture down towards the idle circuit flow to stop it dropping into a bit of a lean flat spot about third throttle opening, then leave the idle back where it was to enhance that smoother running when they were set slightly further out.
Subtle difference, but places the elements more accurately.
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1 turn out on the airscrew is around where the service manual suggests. I wondered if carbs on and off and jet changes, just affected the balance? Maybe re-balance the carbs, to see if that improves the idle?
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All plugs are now running a bit on rich side
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All air screws are working correctly all set to best tick over
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All plugs are now running a bit on rich side
I always set mine to run very slight on the rich side rather than the lean side.
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All plugs are now running a bit on rich side
What that is showing is the richer mixture of idle circuit only.
Changing from 120 to 110 main jet, you'll see at wide open throttle specifically. You have to run them absolutely wide open and pulling maximum torque to assess this, nothing else will show the effect.
Its classic "plug chop" routine. Warmmup, then getting it pulling "balls out" and hauling it's maximum torque in something like 3rd or 4th gear, pull clutch, switch off, then exhamine the plugs to see what the maximum demand is telling you. Nothing else will fully inform you about accuracy of main jet mixture than that, even if you never want to ride it like that :)
Main jet size is about peak flow through that with the needle fully retracted (wide open) the taper controlling all the rest of intermediate mixture below that.
If it goes flat as you come up on the throttle in low speed with 110 installed, that's telling you it's going lean in the region where idle hands over to needle taper, hence the response you get when you turn the airscrew inward, it just brings idle mixture slope to a more steep gradient in getting the two to attach.
Now it'll show rich at low rpm use, because of this.
Raise the needle, leaves the maximum alone at full throttle broadly, but "fattens" the mid to low range slightly, and avoids the need to run idle too rich.
It's "mapping" but with a carburettor.
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How the plug’s looks now
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The colour doesn't look too far out from those plugs as far as I can see.
The ceramic collar around the central electrode is what I'd use for primary judgement, it'll probably take more lengthy run at load to see accumulated view from the outer surround near the threads.
Picture is a little blue bias, masking the yellow content (opposite to blue and significant part of brown hue on plug carbon ) to judge more accurately.
Was wondering what drives the decision to turn the idle circuit more rich initially ?
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It idle better about 1turn and rev up ok and good response up to 65 to 70 from stand still. It just the tic over I would like to get better
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Striped carbs needles are 27201 they are on slot 4 from the top
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Were in the middle as a factory set ;)
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Would this make it run rich what do you set them at
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Would this make it run rich what do you set them at
position 3, middle ;)
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Would this make it run rich what do you set them at
The position you have effectively raises the needle, thereby allowing more fuel in the mix, and as you surmise, richer.
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I think I will change it to slot 3 and I think this should put carb back to standard fingers crossed
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When out today the bike running the best it has done
Setting on carb are 110 main 40 slow 1 turn air screws, needle slot on 3
Hm341 exhaust, carbs are 657b, Dynas ds1-2kit electric ignition, standard coils
D8EA plugs and sync with carbtune pro,will check the plugs after a good run
thanks for all your help
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Glad you got it sorted with a factory spec ;)
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Had a good run out today when down to Mersea and back will have a look at the plugs later running a lot better now
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Had a good run out today when down to Mersea and back will have a look at the plugs later running a lot better now
Never understood why people get paranoid about plugs, as long as the bike is going well and the plugs are not fouled, i can not see a problem with a old engine ;)
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Had a good run out today when down to Mersea and back will have a look at the plugs later running a lot better now
I was riding the other way to Maldon. That was me nodding to you at the T-junction by the Peldon Rose. Small world! 👍
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Hi Martin
Yes I did see you is it dark red if you are passing through Totham and want a pit stop
if I’m in stop for a chat give me a message
Paul
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