Topic: 76' CB400F leaning on top end, stock

[yellow400f]

Hi Guys,

I purchased my first motorcycle last Thanksgiving, a yellow 76' CB400F. She had been sitting in a garage for a very long time considering the registration had expired in 83'. Fortunately the garage was always heated and the bike is in pretty good condition given its age and neglect. I rebuilt the carburetors, replaced the ignition coils, and did some work to brakes, tires, wheels, and the rest of the fuel system etc. She starts right up now and runs good, however the bike falls on its face right as it starts to come into the powerband around 5-6k. I feel like she's running lean but I'm not sure how/why that is happening with clean carbs, original jetting, airbox, air filter etc. I have attached a picture of one of the plugs.

Thanks,

[strynboen]

in line fuel filter kills the flow..you must remove it ..and be shure the tank/inside filter and all is klean..before doing more..

[Don R]

One usual suspect if the gas tank vent in the cap. Put a fuel line from the petcock into  a known size container and clock the flow. I've also had a problem with an inline filter but you need them on some bikes.

[yellow400f]

I've removed the inline fuel filter and took out the entire gasket/ gas vent to see what would happen. I didn't notice any improvement. I did notice however that at wide open throttle it would actually accelerate some past 7-7.5k if I closed the throttle some. I'm reasoning that this is further proof that it is running lean wide open.

[Bodi]

Explain what you did when rebuilding the carbs.

[yellow400f]

The carbs were very nasty, so I had to soak them in one of those carb cleaning buckets just to separate components without breaking them. From there I replaced the float needles and pins, cleaned the jets/emulsion tubes/floats and then reassembled with new gaskets. I set float height at factory spec.

[przjohn]

Did you check the points and timing?

[MoMo]

stock clean air filter? 

[Don R]

 Just random thoughts, you may have already checked, rust or other debris in tank re-clogging jets? Actual float level checked by clear tube? Partially plugged exhaust? (this one got me once! usually a mouse nest, mine was the po wrapped fiberglass over the end of the baffle.) Bad plug caps or wires,

[flybox1]

Does the bike continue to behave like this at higher rpm?
What jets DO you have in there? verify the PO didnt put in some shiit aftermarket jets. 
They should be stamped with the Keihin 'K" logo and size

[vames]

Some of these have already been mentioned, but......


1) Clean your carbs again. Poke wire through all holes in the pilot and the emulsion tubes. If you don't know the PO, check that it's got the right jets.  Make sure your float heights are correct and that the floats move easily and aren't installed upside-down. Bench synch them. Make sure the choke butterflies are all moving correctly. Make sure the carbs seal well with the intake rubber when you install them. Make sure the airbox rubbers are sealing well too.

2) I personally feel like the comment to take the in-line filter off was premature. I've run every flavor of CB over the years, each with an in-line filter and have never had an issue with fuel flow even hour-on-our of highway riding. Doesn't mean you shouldn't maintain a clean tank, but it's good insurance to make sure you don't get crud in the carbs.

3) Set your timing and check that your timing advance cam springs are functioning. Lube the whole thing.

4) Check/set your valve clearances.

5) Make sure you are getting spark on all cylinders and that each wire/boot connection is solid and clean.

6) New air filter element.

7) Check that exhaust is flowing.

[yellow400f]

I've been riding the bike to work every day and it definitely is still missing the top end, otherwise it runs great. The previous owner was not a gear head and so I'm pretty sure the carbs weren't ever apart until I got it. I know the aftermarket jets have a bad rap so I reused the old ones after cleaning. The air filter is brand new and I'm using the stock airbox. Looking through the jets they seem clear, and I can also see through all the holes in the emulsion tubes. Floats are at factory spec and I've varied the heights with no change in results. I'll look into resetting the valves and timing after Thanksgiving.

[yellow400f]

I finally have time to work on the bike again. I set the valves, and am currently checking timing. The advance mechanism seems to be working properly but overall 1/4 need to be advanced while 2/3 slightly retarded. The problem is the base plate has already been set to max advance. What can I change the point gap to in order to allow me to reset the base plate where I can have more adjustment?

[flybox1]

Actual float level checked by clear tube?

This ^^^
Perform the clear tube test to ensure you have the proper fuel level in the bowls.

[Redline it]

Just random thoughts, you may have already checked, rust or other debris in tank re-clogging jets? Actual float level checked by clear tube? Partially plugged exhaust? (this one got me once! usually a mouse nest, mine was the po wrapped fiberglass over the end of the baffle.) Bad plug caps or wires,

The snipe hunt of the fuel delivery is a nightmare. Little tiny specks that partially plug a pilot jet or a line, orifice, is so critical. And the fuel today is not helping. I've never checked plug caps, of course now knowing they have to do more than insulate a spark, they'll probably fail, or have been shot. Carburetor sensitivity is so delicate, these Japanese engineers were masterminds ahead of their time. If you can get these old cb400f's to run very well in only a range, pick the higher one. Let the lower ones work themselves out. As if it's lean on the bottom to midrange, don't spend your riding time on those as much. WFO and OFF. When you reach 20 or more tries at consistent or uniform carb performance, fuel levels, vacuum levels and you haven't stripped any threads in the carb body, you're doing pretty good. Patience.