Author Topic: CB400 low end troubles  (Read 671 times)

0 Members and 3 Guests are viewing this topic.

Offline timmangan

  • Enthusiast
  • **
  • Posts: 176
CB400 low end troubles
« on: July 24, 2014, 02:51:15 PM »
I have a 1978 CB400T that runs super sweet pod filters from Hong Kong, the highest of quality the assured me.

When I run under 5000rpm the bike isn't that responsive at all, not just not quick, but bogs down with too much throttle, has a delayed response, etc.

I added some pics of the bike, not because it would help, but just 'cause i think it looks cool. Dont judge me! :)
Any thoughts?

Offline Tempast

  • Full Member
  • *
  • Posts: 97
Re: CB400 low end troubles
« Reply #1 on: July 24, 2014, 08:18:48 PM »
More in, more out. Maybe you're bogging because it's not getting enough fuel. Maybe ride around under 4k for a bit and pull the plugs. See what they look like.

I'm having a similar issue with my 550F and it's boggy under load until about 6k where it screams like a bat out of hell.
But I have a UniFilter and a "straight through" exhaust with no backpressure.

Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 21,036
  • I refuse...
Re: CB400 low end troubles
« Reply #2 on: July 25, 2014, 04:11:11 AM »
You are no doubt super lean on your carb settings. Without any data regarding the jets and screws and the exhaust, simply adding pods and having problems with "bogging down" is evidence of an improperly tuned bike.

No judgement on you, just declaration. If you modify the induction and exhaust system of an engine, you must modify to complement all the components in the system. An internal combustion engine is an air pump essentially. A measured amount of air going in, a fuel mixture at a targeted ratio, ignited by spark, then spent and exhausted through a designed amount of back pressure.

It's kind of like removing every third joist in a wall and expecting structural stability...

There is a tendency to misunderstand "pods" on carbureted motorcycles. They are not like adding "cold air intakes" on your modern car. Those ram air induction systems are then immediately adapted to by modern electronic fuel injection systems, which are computer controlled and monitored by O2 sensors. Your bike doesn't have that. So if you thought, as many do, adding these would be a performance enhancement, welcome to the misery-

You can get your bike to run well but it will take a decent amount of tinkering to get it there. If you have limited tools and experience, then best to take your bike to a proper shop that has a Dyno, and has experience with vintage carbs. Let them run some tests, examine the exhaust gas (for air-fuel ratios) and make some carb setting modifications to get you "close". It won't be perfect, but it will be a LOT better.

Lastly, two quick points: these carbs don't like to be "whacked" to wide open. You need to roll them open. A sudden long twist will bog them down. They aren't modern pumper carbs. Also, riding under 4,000 RPMs is bogging the engine. Ride above 4,000 from 2nd gear on. It's better for the motor, it will perform more reliably, and your carbs will also be smoother. And, your electrical charging system will operate better above 3,500 RPMs.

Hope that makes some sense-
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis