I've come to own a very nice '76 CB400F. My dad owned it for probably 15 years. If I remember, it ran good when he got it but several years ago it started running poorly. He's messed with it on an off since then. He's now retired and downsizing so I bought it from him. Very nice bike mechanically (less than 10k miles) and sharp cosmetically even with the primered tank.
The front brake was sticking when I got it and I made short work of that problem thanks to info I read here. Just needed a good cleaning. On to the carbs. It would start easy but had a fluffy idle, very boggy, wouldn't run clean on the needles. Ran pretty good once up on the mains. Overall seems to be running rich. Plugs all black.
I know Dad had put some ebay-overseas-sourced carb rebuild kits in but I decided to go through them so I'd know what was going on. I did a thorough cleaning. I found the emulsion tubes were cruddy and the choke plates weren't adjusted all the way open on carbs 3 and 4 with the choke off. Dad had lowered the needles on clip. I put everything back to stock. Needle clip in middle groove. Set the idle mix screws to 2 turns out. The carb kit jets weren't marked with sizes but assuming they are stock. Float heights were right on spec.
While I was at it I gave it a thorough check-over:
- checked valve clearances
- checked compression (125-130psi)
- checked timing (used a strobe light) and point gap
- new set of plugs
- new air filter
- fresh fuel
Everything is stock except it has a 3" Supertrapp muffler in place of the original.
After re-installing the carbs, it started right up and ran beautifully... for about a minute or so. It definitely runs better now. Runs great once it's fully up onto the needle and main. Once it warms up though, the idle still seems "fluffy" to me, it won't take throttle cleanly, and revs are slow to come back to idle. When the revs are hanging, adding choke does not bring the rpm down until I essentially have them all the way on then it will kill it. It seemed to run best with the idle mix screws at about 3-3.5 turn out. Other than the hanging idle, which I've always thought was a lean thing, everything is pointing to a RICH condition. Runs much better when cold. Doesn't need hardly any choke to start from cold. Puffs of black smoke when revving. Dry black plugs after only about 10 minutes of running.
It idled well enough that I did a quick check of the sync and I cleaned it up a little but wasn't off much. I also checked all four float/fuel levels using the clear tube method. All four carbs were the same at about 2-3 mm below the float bowl gasket line.
Carbs are going to have to come off again. Here's where you guys come in!!! Anything else I should check before pulling them? What should I be looking for once I open them back up?
My suspicion is that the carb rebuild kits Dad bought could be junk. Who knows what jet sizes they really are since they aren't marked and I don't like the way the main jet o-rings fit. They seem way too tight and may be damaged from forcing them in.
At a minimum I want to put new pilot and main jets and new main jet o-rings in it. I see Sudco sells a rebuild kit, is it a quality kit? Is there a better source or am I better off getting the parts from Honda?
Thanks for any and all suggestions... I'm sure there's something simple I'm missing. Man, this thing is going to be fantastic once I get this sorted out!
