Topic: 1975 CB400F Tuning Help

[carlinojoe]

Hey Guys, I'm new to this forum and to the Honda CB bikes. This is my problem and any help would be great!  I have a 1975 CB400F.  The pipe is gutted but I put a small baffle in. I also have 4 cone filters on the carbs.  When i got the bike i was jetted and tuned but recently started running weird when i hit 4 and 5th gear. So this is the problem... When i hammer through all the gears 1,2,3,4,5 it pulls really well. When i hammer to 1,2,3 then cruise then roll into 4 and 5 mellow style i get a weird heisitation in the bike. like it wants to stop and not go then catchs then heisitates.  I also replaces the plugs, spark plug caps, and fuel filter. Any ideas what I can do next? thanks so much guys!

[1timduke]

Open the fuel filler cap, could be a vacuum problem, then ride, see if it improves.

Welcome to the forum!

-Tim

[carlinojoe]

like the fill cap on the top of my tank? right now I have the vacum line with nothing on the end. it was on conected to my airbox but thats gone. Where do your vacume lines go?

[w1sa]

Yes, 1timduke is referring to the tank filler lid/cap. As your fuel level drops, air passes via a lid vent into the tank to prevent vacuum lock which would restrict/prevent fuel flowing under gravity to the carbs.
Any restriction in adequate fuel flow from the tank to the carburettor bowls will cause some degree of bad running (in differing situations) where the maintained level of fuel in the bowl(s) is not correct.
In your case it sounds like full throttle application is starving the bowls. When you ease off the throttle there is insufficient fuel level in the bowls to continue running smoothly under the light throttle, until the bowl is sufficiently replenished.

[carlinojoe]

Hey guys, so i just rode to work, opened up the gas cap on a long straight away. Seemed like it might have helped but not 100%. its weird the bike feels great with the new plugs and caps just cruising, its just past half throttle in 4th and 5th. how can i clean my fuel tank vent to cancel that out of the equation? thanks so much.