Motorcycle Repair: Odd problems with Hondamatic, vacuum leak, cb400a

Question
Hi Bob,

    I bought a 1978 Honda CB400A Hondamatic Hawk a while ago. It ran when I bought it, but the carbs were too dirty. I took the carbs apart and cleaned it all nicely and put it back together with new carb kits, taking care to remember every part. The bike runs and drives, but there is a huge problem. It will only go a top speed of 25 mph (it sometimes can't go past 10) and the acceleration is awful. It gets better speed and acceleration when I don't pull back the throttle all the way, but when I do it bogs down and goes slower. I've had people tell me it could be a vacuum leak, diaphragm problem, or unsynced carbs. Right now the floats need to be adjusted as well. The floats are set too high so there is gas coming out of the overflow tubes out of the bottom of the float bowls at a slow but steady pace. When the bike is cold it barely moves at all. It seems to improve after a bit of riding, but sometimes reverts to its normal problematic state. So far I haven't gotten the bike over 30. Could you help me please?

Answer
Your problem could be any of the things your friends have suggested and it would be impossible to diagnose it without seeing it in person. Carbs are precission instruments and need to be adjusted properly in order to work so if you just disassembled & reassembled the carbs without setting them up properly afterwards I would start there. Set the float levels first because a flooding carb can't be synched & balanced properly.

Information on adjusting your carbs should be in your shop manual. If you don't have a shop manual you should get one - your bike is an antique and you will have a hard time finding a shop that will work on it. The sad fact is that a lot of the mechanics working now are too young to remember bikes this old and aren't interested in learning about them so that they can do a proper job since yours may be the only one they see this year. This means that you will have to do most of your own work, and working on a bike without a good shop manual is just not worth the headaches. (My bikes are from the same era - that's why I fix them myself.)

I recommend that you get two manuals - a Factory Shop Manual and a Haynes or Clymer. That way when you don't understand the explanation in one you will probably figure it out when you have read the other too. If you spend some time on Google you might even find a factory manual available for free download.

I also recommend joining a good online forum that specializes in bikes like yours where you can find others who have already learned how to keep them running and are willing to share their knowledge. A quick Google found http://www.hondatwins.net/ If you spend a bit of time searching you may find others.