Topic: newly acquired cb400f1975 with 10500miles chasing the carb adjustments

[Redline it]

similar to this this old thread http://forums.sohc4.net/index.php/topic,54644.msg587544.html#msg587544
only i'm working with stock carbs air box and pretty much complete, no leaks, timing is spot on, advance is functioning just right. spark plugs are d8ea clean and gapped right. plug caps if i measured right is like #2-10000 or 1000, #3 is 10200 or 1200. caps are stock. measuring resistance on a micronta and set on 10K (it's the lowest that will read) and the needle goes to 10.2 and 10 on the 10,000 scale. ignition is all stock.

this carb i don't know if i changed any adjustments but assembled on the bike with the idle set to run, the choke cam has too much of a gap and doesn't move the throttle cold. when i put it back on acouple times i didn't measure the return cable drum to the cable stay, it looked close enough is that my problem with the choke cam are they related in set up?

the first sync attemps were close except for 1 carb.

the bike had nice responsive power from idling to to 4.5k rpm in any gear. no missing, until 4.5k and it falls on it's face if the choke is pulled half way it'll rev to 7K and climbing that's as far as it has been, plugs not checked for a chop just pulled look pretty nice, a bit lean, as expected. valves are set, it's getting better. but what's the similarities of the link and my carbs? or if it's a million different things then what's the potential problem of the choke cam? i adjusted it last night and the set screw is at the end of it's travel and still is not .002 or whatever the close clearance that the manual shows. fuel bowl levels were set, and are close the float tab is flat and smooth. no overflow leaks. thanks for all the help so far, it's been a real life saver. tanks clean,filters clean gas is clean and new. carb jets are stock, exhause is stock and not plugged i got another carb, should i put that on and check if it's a solution?

[M 750K6]

When I bought my 750, it would only run on full choke. Blocked low speed circuits. Strip and clean, including blowing through with an air gun, got it running. If the problem is at higher throttle (which I guess it must be, over 4500rpm), have you ruled out needle setting and any air leaks?

[HondaMan]

I once fought with a 400F that acted just like yours. It came to me to "fix that" problem. When stone cold it would run to 7k RPM: 30 seconds later it wouldn't run past 4500 RPM without sputtering, missing, bucking and generally trying to throw me off the seat.

After doing carbs, plugs, new points (condensors were fine), air filter (it was parked 10 years before it came to me, for this very problem) it turned out to be: a bad #2-#3 coil.

I installed 2 new Sakura coils from PartsNmore and it ran like the wind after that! PartsNmore didn't even know those coils would fit the bike. For a while (after I told them) they listed them for the 350F/400F, but sometimes they take it down. They fit and work fine for all the SOHC4 bikes except the 650.

[Redline it]

finally got it, i had another set of carbs sitting around that i bought sight unseen from italy years ago, and i figured swapping the coils would put more strain on possibly damaging those fragile parts, i opted to change the carbs. the last problem was midrange faltering, wouldn't run well. and it worked.

it's been over 8 months of trying to get this 400f 1975 running, with 10.5k miles and full of rust frame from sitting out in the elements for 20 or 30 yrs. this site has been instrumental in the help along the way. i have a few of them now, 2 or which run nearly showroom condition power and functions. appearances are not high on my list.

these bike are like cats, it can be multitude of different problems that can have the same symptoms. this site alone can be difficult to travel through by old guys like me. i know i can't explain or even communicate as well as some around here, but like the cat, i could of thrown in the towel and not returned for that reason, but that wouldn't be my best option, on account of riding is basically the reason i'm still hanging around this old age. i apologize to the guys here that seemed to lash out about how i didn't stay on the same thread or why i started a new one. i learned about that, but still can't figure out the details. so newcomers when you get suggestions that range from pin point solutions to process of eliminations by starting at the beginning to open a much bigger can of worms that you might be doing by following that order, especially when engine work like valves are included-stop and think a while before taking it all apart, because as these ex pro mechanics will be giving the correct solutions they'll include specific handling  that they don't think twice about figuring it's what you'll be doing if you do follow their advise such as a valve re-facing or replacing, you have to clean the carbon off of the valve stem before forcing them through the guides if you plan on using the guides again, with an ability to ream them or even reuse them after changing seals. so go real slow in your spare time. the bikes are complex for sure. they'll outlast mechanical errors, exwives, or nuclear wars,  etc.   

[HondaMan]

With just 10.5k miles on it, that should just be hitting it's stride! I've seen the 400F with more than 40k miles on it being used in normal daily commuting, darting thru traffic like a cat. Terrific bikes!

[bryanj]

Stood that long i would also suspect low compression from rusted bores

[Tom R]

My 400f had that problem in reverse. When cold, it would start and idle fine, but would only run properly above 4500 after it warmed up. I put on new carb boots, went thoroughly through ignition, different pilot jets and clip positions to no avail.
What should have been my first step was my last step…. Tearing the carbs down and doing an actual cleaning and rebuild… using all the original brass. She runs better warm than cold now… like it’s supposed to.
I can’t say I found anything obvious or clogged when I was taking them apart…. But something was clogged.

[HondaMan]

I can’t say I found anything obvious or clogged when I was taking them apart…. But something was clogged.

What I've often found is: the jets have buildup inside of them, making them act [erratically] lean. This often comes from having used the bike during the years of the MTBE acid in the fuel: it etched the surface of the brass (and some of the other carb) parts. Then if the bikes sit, moisture collects in the etching when the gas evaporates away, and slowly they collect a tiny bit of white powder crust in the holes of the etch. This makes the passages uneven, and sometimes restricted. So, cleaning them carefully is always a smart move.

[Redline it]

With just 10.5k miles on it, that should just be hitting it's stride! I've seen the 400F with more than 40k miles on it being used in normal daily commuting, darting thru traffic like a cat. Terrific bikes!

it is doing just that, i haven't found it's stride limits yet but where i found is like no end in sight. I got lucky,and thanks for this site's dedicated mechanics to relentlessly push on, even though i don't write so good,  took me almost a year in a dark front room after all the work was done, i put a high power porch security light in there. i got 1 more 400f with the yellow tank to add to the collection if i got another year left. the newest one the 75' with red tank and side plates, and a 76' with red tank and black side plates and the 76 parakeet yellow tank and side plates.