why is my bike slow off the mark?

Question

I have an '81 honda cb400. The bike is low on power until it hits about 6000rpm then there's plenty of power. Will adjusting the idle jet on the carbs increase the low rpm power? Could there be a compression problem or would that rob power at all rpm? Does timing affect power differently at different rpm? Any other thoughts on what might be at the root of the low power?

Answer 1

Hi Mark, I would look at the mileage first and then servicing to see whether it has been looked after or not!
How long ago did it have it's major service, ie: plugs, valve clearances, airfilter etc. If all of that is on par then you would look at fueling and then probably compression.
If all of that checks out your gearing might be letting you down and the answer might be to place a larger sprocket on the rear to allow for more pick-up!
If you are riding it a lot you may not realise that your ability is improving and your machine may feel slower, which is a good thing but then you need to get something maybe with a little more performance.

Good luck!

Answer 2

there are several reasons..its old its ,low tech,its worn out stop looking for technical reasons its just not a powerful bike by modern standards playing with the jets or the idle or anything if it runs its probably not far out of tune and the most u could hope for is a tiny better responce and thats it u cant get power out of a bike that never had any

Answer 3

yes to all parts of your question. traditionally, a motor makes peak power between 6500-8500rpm on 4 stroke motorcycle engines.

I used to have a cb 400-four. It had good power from 3000 rpm on upwards.

Compression check is easy and cheap. you should start there.

Answer 4

Tight or leaking valves would cause those symptoms.
May need a valve job to reseal the valves.
You might get away with just a valve adjustment.

Answer 5

No adjustment will give it much more power below the 6,000rpm point
try joining this forum
http://www.sohc4.us/forums/index.php