Topic: 76 CB400F dies under load

[Gibbo]

Diagnosis help please?

1976 CB400F - Engine stock, Airbox stock, Dyna-S ignition, Aftermarket headers and muffler

Carbs cleaned, tank lined, new filters and fluids, new ignition system upgraded.
Jetting unknown but the bike was previously running like clockwork before it sat for a decade. 

+ At idle it will Rev cleanly to the red line if desired.
+ At rides smoothly with very, very light throttle. 
+ A little more throttle and the engine starts to splutter. 
+ Any more than quarter throttle it's like the kill switch has been turned off.
+ Roll the throttle back to just open and the engine rides smoothly again.

Riding with choke on kills the bike.
Riding with Airfilter removed makes no difference.
No vacuum leaks on intake
It's not the new Dyna-S ignition - the symptoms were the same with the 333 points and advancer

Give me some ideas please because I've reach the limit of my diagnostic abillity.

[Nurse Julie]

Check your air/fuel mixture screws. Also check your spark plugs, too back, running rich, too white running lean. Adjust by using the air/ fuel mixture screws.

[minimo]

Thinking it might be possible after leaving your bike to sit for a while, some sticky varnish may have developed around your carb floats causing them to get stuck in a position, likely to starve your system of fuel, creating a lean mixture. Next time you experience any weirdness, give your bowls a good whack with a rubber mallet (or a wood stick against it) and see if that will loosen anything up. In either case, sounds like a carb clean weight be in order.

Have you checked your plugs after a shutdown? That might be telling of the condition.


Sent from my iPhone using Tapatalk

[bryanj]

Could be stuck rings damaged pistons have you checked compression

[robvangulik]

Does the tank cap vent properly?

[Kevin]

What others have said plus a crazy thought.....check the airbox for a mouse nest.

Sent from my SM-N960F using Tapatalk

[jakec]

Every time I've had a 4 cylinder running  on 3 cylinders (350, 400, 500) it would die as soon as you put load on it