Topic: 1976 CB400F JETTING

[mdart]

My 76 CB400F  has K&N style air filters and a Kerker muffler, other than that it's stock. Can someone tell me what the optimum jet sizes would be?

Thanks

[Triffecpa]

depends upon elevation, but probably an 80 or an 85.

stock is 75.

If the bike won't pull easily all the way to readline, then it's probably too lean.  If it will pull all the way to redline, but at redline when you crack the choke lever a tiny bit and it picks up speed, then you're too rich.

Tracy

[mdart]

The bike starts and idles fine but during acceleration it really boggs until I crack the choke about 1/4. It will then pull through the RPMs better. Another symtom is at idle, if I quickly open the throttle it will stumble then rev up.

[Triffecpa]

Are the idle jets and the passages behind the jets clean?  Sounds like they might be plugged.  Remove the idle jets and spray carb cleaner thru the hole where the jet was.  It should leak out the brass tube in the carb throat on the filter side of the carb.

What jets are in the bike now?

[mdart]

I've got the carbs on the bench to re-jet.

[mdart]

The main jets are 75s

[Triffecpa]

still sounds to me like you have some clogged slow jet passages.  with 75 main jets, it should run fine on the low end and thru the mid-range and just run out of gas before you get all the way to redline.

It's hard to find 400F jets also.  There is a company on Ebay that sells a set of 16 jets (4 each of 4 different sizes) but they're now $50 or so plus shipping. 

BikeBandit.com and some of the other OEM parts sites lilst a 75 and an optional 80, but a friend of mine had problems getting the 80's.

Sudco doesn't sell 80 and 85 mainjets.  I think that they have 78's and then they jump up into the 90's.

I bought some .8mm and .85mm drill bits on line to drill out stock 75 main jets and tehy seemed to work well.

Tracy

[Industrial Rat400f Killer]

Finding the right jet size for my bike seemed impossible so I thought about drilling but I settled for 78's. Where did you find the .8mm and .85mm drill bits? I think it might be worth it to try that sometime before I have my new built engine done.

[Triffecpa]

found the drill bits by going to google and typing in ".8mm drill bit"...

Can't remember the name of the place that I bought them.  Bought two of each just in case I had problems with them.  Also, my drill wouldn't go down small enough to grip the bit, so I clamped the bit in a pair of safety wire pliers and chucked up the jet in my drill and did them that way.

I think that the bits probably drilled a slightly larger hole than they were marked for.  I had 90's in my latest CB350F and it ran too rich, so I drilled a set of 75's out with the .85mm bit.  Still seemed ilke the bike ran about the same, so I drilled another set with the .80 bit and it runs good now.  I was expecting to have to run 85's to get it to run right, so I'm thinking that the bit drilled a tiny bit oversize, so keep that in mind.

TR

[Industrial Rat400f Killer]

Thanks for the tips. I figured the bit would oversize the hole a little. I do a lot of fabricating and drilling of stuff for a living so I was already hip to that even though most of the time I'm drilling wood.

[bistromath]

Not to be really anal or anything, but you really should use a reamer instead of a twist drill for doing jets. Might as well actually KNOW the size of the hole you're making, instead of just guessing, and better a round hole than a tri-lobed one. A 68 wire gauge size is a 78.7 jet. A 67 wire gauge is a 81.2 jet.

http://www.mcmaster.com/#reamers/

[Industrial Rat400f Killer]

I don't think it's anal, I think it's helpful.
Thanks.

[mdart]

I pulled all the jets this morning and discovered that one of them was not properly drilled through. The .75mm hole was only about 3/4 of the way through the jet. There was a .60mm hole the rest of the way. I also found that the o-rings are in bad shape and probably leaking. I can't wait to see what else I find inside these carbs.

[dougandarthur]

I am about to install the same combination. What did you end up using? How does it run? Does anyone have any new recommendations?

[Deltarider]

Interesting enough Honda's Parts List for the CB400F lists a #72 as well as a #78 main jet besides the #75 which is denoted standard.
Parts numbers are:
99202-601-0720
99202-601-0750
99202-601-0780

[Bodi]

Try pilot jets one size smaller. 38 vs 40 if I recall correctly. Sounds crazy but that's what is usually required with pods and a free-flowing exhaust.

[dougandarthur]

Thanks Bodi. That's great! I just happen to have a set of 38 Pilots that I used when I lived in Denver. The consensus seems to be that 80, 82, or 85 Mains wil probably work best...now if I can just find some 85s?
I appreciate the response, I know that this subject has been beaten to death.

[jessezm]

Not to continue beating it to death, but I just want to second Bodi's recommendation on the pilots.  I am running k&n pods and a more open muffler and got great response from the 38 pilots.  I'm running 90 mains but I also went with the 466cc kit and Webcam so I needed the extra breathing.  I just pulled my plugs yesterday and found I am still a bit lean, so I may end up trying the 95 mains (I got that ebay kit mentioned above). 

[camelman]

Your mixture needle adjustment might be off too now that you gave made those changes. With the bike idling, adjust each carb mixture screw in and out to find the best idle. Start with #2, then 3, 1, 4. You are listening for the idle to raise as you adjust each carb. It will be noticeable and may smooth out your idle if it us rough. After you get them set, go back and do them again.

Check to see if you have plastic screw caps on the mixture screws before you adjust them.

If your adjustment is off, then you will notice it just off idle... and at idle.

Camelman