CB400T crazy idle!

[gregbenson]

Hi all, I'm Greg, 21, from Plymouth. There's a lot of knowledge and experience on this forum, maybe somebody will be interested by this?

My bike is a 1978 CB400T Dream, and it's never run properly in the year I've had it. About 1000 miles ago it got a new Motad 2-1 exhaust. In the last 100 miles it's had a new ignition coil, HT leads, plug-caps and plugs, a good inline petrol filter put in, the carbs cleaned, new float needles (one had seized), float heights set, mixture screws returned to standard setting. Following that, it runs really nicely, starts first kick, and reading the plugs tells me the mixture is pretty much right.

The puzzling thing is that although it will usually tick-over properly, it often doesn't like to tick-over once it is properly hot, or more specifically, once it has warmed up, and then been turned off and on again. My commute involves a 7 minute ferry ride, during which the bike is turned off, and when i start it up again, generally it pops and misses like mad. Sometimes it settles down, sometimes it doesn't. I took it to a good local mechanic to see if it would be worth synchronizing the carbs. Predictably, when he started the bike it ran perfectly and he asked why I wanted anything done.

Have I missed something really obvious? I'd really appreciate any thoughts.
Thanks

[AshimotoK0]

Welcome!

My friend had a CB400N Superdream with only a few thousand miles on it but left unused for years. It would never run properly even though he thoroughly cleaned the carbs and in the end he got some more carbs from a breaker and it then still  worked fine. We then got  an ultrasonic clear, where we worked, and he cleaned the original carbs in it, refitted on the bike and the bike still ran fine. When you say you cleaned the carbs, did you ultrasonic them?

See this site : http://cb750carbs.com/

Cheers ... AshD

[gregbenson]

Cheers mate, interesting website.

My bike was left unused for years before I bought it too, with only 12,000 miles indicated on the clock. Like your mate, I thought of getting some breakers carbs, thought they might have lot of grief inside. No I haven't had the carbs ultrasonically cleaned, just a can of carb cleaner. It got most of the muck out but there was some tarnishing I had to leave because the only way to get it off would been to use something abrasive.

I'm sure somewhere local does ultrasonic cleaning.

Thanks very much

Greg

[tom400f]

It does sound like they need a proper clean/overhaul but also ensure there are no air leaks on the engine side of the carbs.

Aside: In spite of bing a 400F fan and the obvious sacrilege of replacing it with the 400T, I do quite like them. I preferred the bulbous styling over the later "Euro styling" of the 400N. Maroon, not blue. Just sayin'

[bike-pusher]

Oddly, I liked the dumpy styling of the original Dreams as well....

[gregbenson]

"Dumpy Styling" haha! The thing letting the 400T down is the revolting comstar wheels. They might have looked flash in 1978 but they look dated now. And the useless flap with the pretend lock that covers the petrol cap, it seems to just be there to collect moisture and crud!

Thanks for the advice guys, there's someone local that does ultrasonic cleaning, so will be arranging that quite soon.

[AshimotoK0]

Guy wanted to P/ex a bulbous 400 one with my KZ650B1 telling me that the 400 was a future classic.. Told him where to go but still sold him the Z for cash only.

Ash

[Bryanj]

250 and 400 were notorious for carbs blocking and only 50% of the time does ultrasonic worke

[gregbenson]

Quickly bench synchronized them yesterday and they haven't played up since. Hopefully that was what I'd missed.