Topic: '75 CB400F - Lean condition @ 4-7k rpms

[AMT]

Hey all,
Posting to get some fresh perspective on this issue.

Bike Details
• 1975 CB400F w/ ~40k miles - Located in NYC (sea level)
• Original carbs w/ stock jet sizes, stock airbox & filter, stock 4 into 1 header and replica oem muffler from david silver
• Stock bore, gearing, etc.
• Dyna-S electronic ignition w/ new coils & caps from 4into1 (https://4into1.com/ignition-coils-and-caps-honda-cb350f-400f-500-550/)

Problem
Hesitation / sputtering at various points between 4000 and 7000 RPMs. Not always in exactly the same spot but usually worst around 6000. The bike starts and idles very nicely, low end throttle response is snappy and quick, and acceleration is quick up to ~ 4000. The bike will stutter and hesitate through this range, and then will want to take off again and rip up to redline.
• The issue seems to be consistent whether or not I accelerate slowly through the rpm range with throttle 1/4 to 1/2 open, or if I crank it wide
• WOT seems to be happy once you get up above 7000 rpms
• Symptoms resolve by closing the choke ~1/4 to 1/2

Things I've tried
• Verified timing per the manual and tested advancer
• Tappet adjustment
• Cam chain tensioner adjustment (used the trick of giving it a little help with a screwdriver coming down through the plunger access hole)
• Replaced coils, wires, plug caps, and plugs
• Complete disassembly of carburetors, cleaned in an ultrasonic cleaner
• Tried an aftermarket set of jets, needles, etc - no improvement
• Returned to the stock brass and adjusted needle 1 position higher than stock setting (now in position #2 from the bottom as opposed to the middle position) This made a minor improvement
• Carbs synchronized - noticed there is definitely some variance at idle vs higher in the rpm range.. tried optimizing for both but didn't notice a significant difference either way. Right now the sync is optimized for smoothest / quietest idle.
• Compression test showed all cylinders between 145 and 150psi (between 10 and 15% below stock if the gauge is to be trusted)
• Cleaned tank, replaced internal filter, cleaned petcock in ultrasonic cleaner, new fuel lines, etc.

Considering as next steps:
• Raising the needles 1 more position to their highest (richest) adjustment
• Returning them to the factory position and trying an #80 main jet
• Returning to points & condenser

I'm not really sure why this would be necessary on stock configurations.. except that perhaps compression has waned equally across the board over the years, and the engine is just not operating as efficiently as designed? I know people will recommend a plug chop.. It's not particularly easy to find the space in my neighborhood but I suppose I could do it. The choke seems to tell the story, though, and with the bike seemingly happy in the higher RPMs at WOT I'm a bit less suspect of the main jet and a bit more of the needle. Anyone else running needle in its highest position? Any other ideas based on the specificity of the symptoms?

[newday777]

I'm currently working on the carbs on my 750 and reading the 750K0 thread currently going on and Hondaman referring to his book about today's gas and the need to drill the emulsion tube holes to a larger diameter to add more fuel at low rpms, rather than changing the needle. I  asked Mark in a reply to him if it effects the 400F(and really probably all carbs that were designed for non ethanol gas)
This is his earlier reply so read it to the end(or you can read the whole thread if you desire)
http://forums.sohc4.net/index.php/topic,192296.msg2241393.html#msg2241393

[HondaMan]

I worked on a CB400F in 2010 that acted just like this. It had started suddenly as the owner was riding home from work one day. Before that (and after) it ran fine, had 40k miles on it. I worked on that thing until I would have to walk away from it as it defied diagnosis: when cold it ran fine for about 2 minutes, then acted just like yours.

In the end I had cleaned/refurbished the carbs (only needed some new Orings, were all original), tested compression dry and wet, and installed new coils, plugs, and caps. Then I found a can of freeze spray and discovered this: every time it acted up I could spray some freeze spray on the 2-3 Dyna pickup and get 2 more minutes of good performance from it. Then it acted up again.

That bike received new points, old condensors from my junkbox, a Transistor Ignition (after riding it 50 miles) and the old coils back into it. I still have those new coils around here, having run them in my 750 for one season after that.

[AMT]

That bike received new points, old condensors from my junkbox, a Transistor Ignition (after riding it 50 miles) and the old coils back into it. I still have those new coils around here, having run them in my 750 for one season after that.

Oh wow.. so to be clear it was the dyna electronic ignition that was causing the issue, and return to points & condensers solved the problem? Very interesting! Thanks for the reply.

[AMT]

I guess one thing that boggles me a bit is that if the issue were ignition related at those RPM's... presumably the engine would be in a rich condition (poor combustion). I can't quite work out why running the bike with some choke would resolve a points related issue.

[scottly]

You are correct that applying some choke indicates a lean condition. Have you checked the fuel level in the float bowls with the "clear tube" method? What is the stock main jet size?

[AMT]

You are correct that applying some choke indicates a lean condition. Have you checked the fuel level in the float bowls with the "clear tube" method? What is the stock main jet size?

Scottly,
Stock main size is 75 but I believe i have a set of 80's on hand. That was one of the next steps I've been considering, but again WOT seems to be happy above and below the problem area.

I have not used the clear tube method. I set the float height with a float gauge by the service manual specifications. I did measure the fuel in a graduated cylinder and fuel level was identical across the carbs (granted my method had some margin of error).

[AMT]

Update
This weekend I swapped the Dyna S out for the original points plate. There was no noticeable difference in performance.

Next, I pulled the carbs and raised the needle one more position to its final (richest) position. The sputter around 6k RPM's was nearly eliminated. The sputtering / hesitation was gone. I would still describe that RPM range as a little weak but the engine still felt smooth, not choppy. Unfortunately... no amount of adjustment to the air screws or obsessing over synchronization would resolve a rich condition right at the very start of the throttle opening. Engine bogs leaving from a start and you have to give it more gas than typically necessary to get thru it, and low speed maneuvering was annoying if not dangerous. Plugs came out black as dirt.

So... I think I've proven that the relationship between slow jet and needle were better at least 1 position lower, if not ideally back in the middle (stock) position, but also that a little extra fuel in the high/mid range is helping carry the engine thru that sputter spot around $6k.

I guess the next attempt will be to put the needle's back to the original position and try an 80 main jet.