You are here:

Motorcycle Repair/1980 honda cb400


QUESTION: She starts easily, has good pickup and power, has been popping on deceleration for the last few months, and occasionally missing a beat going down the road at constant speed or idling, but otherwise running fine until today when it began stalling, running rough, which I could attribute to fuel or ignition but the weird part is when restarting with the starter it would turn briefly and then stop as though it was jammed. After a couple of tries the starter worked, it ran rough but fine acceleration all the way  home.  It's hard to consider there is something wrong with the starter coincidentally with the other symptoms. What I would like to know is what about ignition or fuel could cause the starter to act like possibly the timing was off and then work fine again?  Also curious what your take is on the earlier problem with popping on decel--ignition or lean mixture?

ANSWER: Clem, the Honda 400-450 series bikes have dual air-cut valves to control emissions and they have a function as anti-backfire valves. I imagine that one or both diaphragms have failed. You can get the repair kits from K&L distributors for less $$ than at a Honda shop. The one on the left carb is easy to service, but the right side is a challenge, without removal of the carburetor rack.

The ignition systems are CDI, so even with a dead/dying battery or charging system, the bike should still run. I would check to see that the battery is fully charged and can pass a load test, though.   #11

Do know that there is a balancer shaft and chain adjustment for these twins, that needs to be checked. If the chain is left loose or breaks, there will be problems.

Bill Silver

---------- FOLLOW-UP ----------

QUESTION: Thanks.  I guess the diaphragm you are speaking of is behind he cover on the side of the carb.  I am unfamiliar with the balancer shaft location, guessing it is somewhere in the engine itself.  The bike only has 23k on it and as tot he battery it is only a year old and cranks fine.  What puzzles me is the coincidence ot the poor running and the starter locking up intermittently after a low rpm stall.  Would these diaphragms cause back pressure so the starter would begin to run and then just jam before starting, but then perform normally after doing that one or two times permitting me to get home with it trying to stall at low rpms but otherwise running well under acceleration?

Clem, the starter issue shouldn't cause engine running problems, unless the starter clutch is coming apart and causing a drag on the motor.

#6, 7, 11

Honda has had problems in the past with the kickstarter pinions hanging up causing the k/s function to jam.

The balancer adjustment is under the right side engine cover. There are access holes to remove and make the adjustment. The slotted adjustment arm will only go so far before it runs out of travel. If that happens (especially when it hasn't been adjusted), then you have to remove the cover and reset the arm on the splined shaft, so the adjustment bolt is in the middle of the slot again.  #12  #12

The air cut diaphragms will cause an air leak, lean running on the affected side and backfiring issues.

You might want to buy a manual to better explain the maintenance procedures.

Bill Silver  

Motorcycle Repair

All Answers

Answers by Expert:

Ask Experts


Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

©2016 All rights reserved.