CB400F Jetting & Test Spin
Year, Make, Model
1975 Honda CB400F with K&N air filter pods, Dyna S ignition & 3 ohm coils, & Kerker exhaust.
Service Manual
See this post for a FREE download in pdf format.
Symptoms
The engine sputtered/breaking up at higher RPM’s in the 6-10k range.
Diagnosis & Fix
After checking compression on all cylinders (140psi), I checked the timing with a timing light at full advance, which was set correctly. The valve clearance was set to .002” IN/EX.
The last piece of the puzzle (see my post on 3 basics principles in order for an engine to run) is the carburetors. Here is a list of findings that contribute to the symptoms:
- Vacuum leak on carb caps (located on top above slides)
- Carbs were dirty, especially the jet tube which often gets overlooked.
- Float level was slightly off
- Wide open throttle stop was out of adjustment
- Choke fast idle was out of adjustment
- Float needle and seat showing signs of wear
- Jet tube and needle showing signs of wear
- Slide linkage was binding allowing one slide not to return to idle position
- Jetting not ideal.
Once all the brass and slides were removed from the carbs, I gave them a good cleaning using Berryman Chem Dip & my DIY soda blaster. The soda blaster worked great on rust & white deposits that the Chem dip would not take care of.
At this point, it’s good practice to compare jetting to what is listed in the service manual & searching forums to see what others with similiar mods are running for jetting.
Carbs were synched after carb clean and rebuild.
Stock CB400F Jetting:
#40 pilot jets
Air screw: 2.0 ±1/2 turns out
D22 needle – 3th position from top
#75 main jet
Float level: 21mm
As received jetting:
#40 pilot jet
Air screw: 2.5 turns out
D22 needle – 4th position from top
#72 main jet
Float level: ?
Re-jetted it to:
#38 pilot jet
Air screw: 1.5 turns out
D22 needle – 4th position from top
#80 main jet
I would have to say the sputtering was due to a lean condition. The vacuum leak, lean main jet, & dirty jet tubes all played a role in causing poor performance.
Tools
Motion Pro 08-0022 (this tool makes synching carbs so much easier)
Motion Pro Carb synch tool
Parts List
Full carburetor rebuild kit with jet tubes (Keyster KH-1213). This is the only kit I was able to find with the jet tube, which is discontinued from Honda.
Molded float bowl gaskets. Included in above kit, however if you are looking for gaskets only, don’t bother with the o-ring style as they are a PITA to install. Use Vaseline or superglue in corners to aid in installation.
Main jet kit. I did not purchase this kit however may be required in the future. The main jets are usually sold up to #80 and then jump up to #98 with nothing in between. I’ll have the owner send me pics of the plugs after a plug chop, to see if we need to go richer on the mains. I have read that some use #85-90 mains.
D8EA/D9EA spark plugs. Stock is D8EA, however with the hotter ignition and road race high RPM use, a cooler set of plugs may be needed. The plugs will be pulled after the first event to see how the plug strap looks. I also opened up the plug gap from ~.028” to .035” since equipped with hotter ignition.
Comments
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My uncle give me a cb750 custom 1982 storing for 6 years.The bike start but the idel is not good and after 2-3 minute the bike stop.Y Start THE BIKE AGAIN AND THE RPM GO UP TO 2000-3000 RPM AND BACK FIRE.The friend ove my uncle played with the throttle stop screw and mixture adjusting screw.My question is:do you ave a recipe formula for adjustment.Thank you for your time.
Frederic,
If it has been stored for 6 years, I would take apart the carburetors and clean them out. You probably want to consider changing the rubber carb boots as well. You should start with stock settings and go from there.
Matthew, can you tell me how the 1981 Yamaha xs650 shifting works. is it up or down first for the first gear or down. 5 gear or 4 gear? please thanks
Lito
Lito, the XS650 is a 5 speed. First gear is down, the rest are up.
Hi, I have the exact same model (1975 Honda CB400f) and I can’t reach top speed. I’ve heard it can do around 110mhp I could not go any faster than 60mph with the wind on my back! No crisp throttle and power band tend to flat out at 6000rpm. The bike is stock. I have no pods and no aftermarket exhaust or anything. Where should I start? Thanks
Alex,
Start by checking compression on each cylinder. Then, make sure point gap and ignition timing are in spec. Lastly, make sure the carbs are clean and synched.
Report compression numbers.
Hello, I have my old mans CB400F from ’76. Looks still mostly original, but with a nice Marshall Deeptone. After almost 20 years in a garage I got it running again rode it a bit until I fitted a set of K&N filters. Obviously the bike ran like shit after that. Thanks to your advice on the 38 slow, the 80 main and the lift on the needles. They were spot on for my setup, it goes like hell again (relatively speaking… my other bikes are Ducati’s and a 1200cc BMW, which I normally call slow…). Anyway, thanks for the write-up. It helped me a lot!
No problem!
Having carb problems myself. currently running k&n pod filters and outex exhaust. for some reason my bike is running very very rich now but that doesnt sound right. what was your final tuning resting point on your cb400f ?
What is your current jetting?