Topic: carb jets and needle position

[kbart1]

hey guys

so my 70 CB750KO is back together and running but not great. i have a question re the carbs. keep in mind i am running a newer carb set, possibly 71-73 version. i am looking to find out a few things re the carbs. what main jets to use? what needle position to use? how many turns on the air screw? my bike is set up as follows:

836 kit installed
kerker 4 into 1 installed
K&N air filter in stock airbox, not pods.

as of right now i think its running rich. it will start without using the choke. it runs good when your on the throttle however when you get off it to round a corner for example the motor stumbles or falls off. i am hoping with your help i can get the carbs to a good baseline for this setup then have them sync'd. i attached a couple pics of progress.

thanks a lot!
kent

[mrbreeze]

Hey Kent, Your bike is looking good man. I see you live in Canada. What elevation are you at? Setting up your carbs depends on that as well as intake and exhaust set ups. The Kerker exhaust and stock airbox shouldn't be much of a change from stock carb settings but the altitude will effect how far from stock. Are the needles stock or aftermarket? The more info you can provide the better.

[kbart1]

i am in vancouver so pretty much sea level or a hundred feet above if that. the needles are what came with the bike so i don't know if they're stock or not? is there a way to tell. the air box is stock except i am now running a k&n filter in it. and the 836 kit. thinking stock settings but maybe up one size on the jets?

thx
kent

[mrbreeze]

I'm running a 4to1 Kerker without baffle......just the end cone but I have K&N pods. I'm in California at 89 ft. elevation. I can't remember if I have 120 or 125 mains (I have both) but I know my needles are OEM and clipped at 2nd from top (one step lean from the middle).It goes smooth from idle on up....no hiccups. You should probably see what clip setting you're at and show a picture of your needle (close up). I'm not sure about markings but I do know aftermarkets have different tapers. Its hard to tell what has been changed out in 40 years time if you aren't the original owner. Should be able to see a stamped # on the mains though. Lots of different combinations out there at different elevations so the more info you provide the better. So far you have Kerker, stock K&N, 836 kit, running at 100 ft.

[Davez134]

120 main and 40 slow jets should get you damn close, as well as needle clip position being in the 4th notch from top. Stock factory needles will have the keihin logo followed by a number (27021, or 27071 I believe, not 100%sure) all aftermarket needles I have seen are just plain. I have a similar setup and my air screws are 1 1/8 turn out. Make SURE float heights are correct too!

[mrbreeze]

4th notch from the top is almost all the way rich isn't it? When I did mine (around 2005-2006) I started out in the middle. It had the hiccups so I went next step rich and that was worse. Then I went back to the lean side of the middle and it transitions through the throttle smoothly.

[Davez134]

4th notch from the top is almost all the way rich isn't it? When I did mine (around 2005-2006) I started out in the middle. It had the hiccups so I went next step rich and that was worse. Then I went back to the lean side of the middle and it transitions through the throttle smoothly.

I started there because it was what the factory specs called for on my 72. I am running all factory brass. Haven't had any issues, plug chops at all positions look great and bike runs great.  I'm at about 2000 ft elevation. 836cc, webcam, no number 4-4 exhaust, ported head, airbox.

[mrbreeze]

I guess you just have to experiment till you get it running good and you're not too lean or too rich throughout the throttle range. I'm happy with mine .

[Jerry Rxman Griffin aka MuthaF'er]

No additional input on jets but a set of badges will really make it pop!!

[mrbreeze]

Plus 1 on the badges Jerry and one thing to add to Davez. You said make sure about float height but this should be verified with the clear tube measurement. I found my #3 low compared to the others awhile back. It made a big difference getting them all on the same page.

[kbart1]

A+ on the badges. all the little bits are on hold until i get the motor dialed in. but yes have a long list of bits still to go to get back to stock.

thx all!
kent

[kbart1]

hey guys

so i finally found time to pull my carbs. here is what i have: 125 mains, 40 slow jet, needle is a keihin 27201 which i believe is stock. it is clipped in what appears to be second to bottom spot of 5. if 1 is the top of the needle and 5 is the bottom of the needle it is in 4. i think i need to move the clip up a spot or two? the carbs / airbox always seem to be wet with fuel and as mentioned it stumbles when i get on and off the gas at low RPM. also, i don't need to use any choke even when starting it bone cold. think i have a rich condition right now? what do you think. pics attached.

thx
kent

[mrbreeze]

Mine are 2nd from top so if you wanna try that you need to go up 2 notches. The lower notches are rich and the uppers are lean. Like I said....I started in the middle and went down one making it worse. Went up one from the middle and all is well. Your jets would seem to be good. Most people don't change from the 40 slows. If you run wide open for a while and get black plug deposits you need to try 120 mains.