Honda Twins banner

1978 Cb400TII Not Reaching Top Speed

1835 Views 11 Replies 3 Participants Last post by  longdistancerider
Hey guys,

I have a 1978 Cb400TII that I've been working on trying to get it tuned up after it was given to me by a family member. The bike has 24K miles on it and has not been sitting so any long period of time, it was used pretty regularly before I had it. The engine runs well by my standards, this is my first bike so I'm not too experienced in that regard. It will start right up and doesn't really get bogged down through the gears/RPMs. The issue I seem to be having with it is that the speed tops out at about 55-60mph, and from what I've read online this bike should get up to 80-90mph. I did a compression test and it was about 150psi in each cylinder. I also did a leak down test at TDC on each cylinder and found less than 20% leakage. I've seen that the sprockets could be changed out but my front has 16 teeth which I think is stock. There is what I think to be a small leak in the head gasket as there is a small amount of oil that is around that area I just haven't cleaned it off yet to see f its leaking out or not. I'm not really sure where to go from here so hopefully, someone can advise on what to do to bring the speed up.

Other things I've done to this bike since getting it.
changed the oil (Delvac 15w40) and the filter,
changed the brake fluid and adjusted the rear brake
adjusted the clutch
Cam chain adjustment,
cleaned and lubed the chain,
cleaned and reoiled the air filter
cleaned/adjusted and Synced the carbs
replaced the battery (AGM battery)
adjusted the slack in the chain
1 - 1 of 12 Posts
Hi there!

Indeed, your bike should be reaching for 80-90mph or even the ton. I know because I've reached it on the same bike.

The maintenance you did is great, as you've addressed the air flow and the all important brakes.

However, I think you should dig deeper on the fuel and sparks. You can remove the carbs and check if the Pistons are moving freely. They are vacuum actuated so if they stick for whatever reason you won't get enough fuel to make her run hard on the top end.

Change the spark plugs, as they get worn. Once you do that, run with her for a while. Then give her a whack in 1st gear and cut the motor on the top rpm (with your hand on the clutch). Check the spark plugs. They should be tan, not white nor smokey black. The electrode shows the top end of the rpms, while the base shows the bottom end. If white, you are lean. Black, you are rich in the mix.

Also check the petcock. Remove the gas from the tank and change the petcock for a new one, while checking for rust. This is very important, and often overlooked. The petcock should provide enough flow to keep the bowls full when on wide open throttle. Do not use any fuel line filter, as they never have enough flow, and the petcock screen should suffice.

Feel free to chime in if you have any problems.
See less See more
  • Like
Reactions: 1
1 - 1 of 12 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.