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Discussion Starter · #1 ·
I'm trying to help a friend with a CB400A (1978). when shifting into 1st (or 2nd), the engine stalls under load. Once moving, it seems to run ok. Could this be the CDI or are there other causes? Many thanks for the help

Cas
 

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Im actually leaning more towards a carb issue. I remember LDR told me to drain the float bowls with the little screws at the bottom a few times to get some gunk or water out and it ran great after that
 

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Same here. Set idle speed, mixture adjustments, and sync. Check for vacuum leaks.
 

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Discussion Starter · #4 ·
I'm sorry - maybe I did not describe this correctly. Imagine you are driving a car with automatic transmission. You stop at a red light. The engine stalls just as if it was a manual transmission. It does not matter if you hold the engine open at 4,000 rpm and then shift from neutral into 1st. This will still stall the engine. Same as when shifting from neutral into 1st while stopped. the engine is not dis-engaging from the drive sprocket. Increasing RPM does not really help.
 

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Im still going with it being the carbs. Source being mines done this before although it was a little hard to start the bike and idle as well
 

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Discussion Starter · #6 ·
it is not the carb. even if I set the idle at around 2500rpm it still does it. The engine will not keep running when stopped. Its like stopping a manual motorcycle in gear - the engine stalls because it is still trying to rotate the rear wheel. same when starting. once 1st is engaged, the bike stalls. I have a bit of experience with these woeful carbs. and I have cleaned them several times and rejetted them. they run well at any speed. Trust me on this, it is not the carbs (famous last words I know).
 

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I have a 1981 cm400a. I know practically nothing about these bikes.
Dont they have something called a change over relay that bumps up the idle (advances timing?) when the bikes put in gear and the extra load of the torque converter is placed on the engine?
Not sure if I explained that correctly, but its something like that. Maybe that system has a fault...
 

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As atowmbrg rightly says, when engaging a gear the ignition is advanced a bit. Check the change switch in case it is staying in retarded position.

Then again could be the torque converter is transmitting too much torque at idling revs. Of course at low revs the power to the back wheel should be low enough to hold back with your feet.
Other ideas....................
Oil too thick? Muck in the torque converter vanes? Has anything been done to the bike before this happened?
 

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You could have a bad or have bad wiring on the side stand switch. Yes off the wall but dont rule it out. 2nd thing,I would check is the stall speed on the torque converter. Do you have a full service manual? If converter checks ok then move to the idle stabilizing circuit. You will need a FSM to do these checks.
It could be a bad oil pressure valve but I would do the other checks first. You could also have a problem in the change relay circuit but take it one step at a time.

Note the side stand switch shorts out the CDI if the bike is put in gear with the stand down . If bad or the wire is bad it could stop the motor.

Keep us posted.

Bill
 

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The bike runs while moving, therefore it's not the sidestand switch.

The bike dies when put in gear even when holding the throttle open, ignition advance isn't the problem.

The torque converter is supposed to "slip" at low rpm, sounds like yours isn't doing so.

It would help to know more of the bike's history. Did it spend some time in storage? Was it running well and this suddenly developed? What oil are you using?
 

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Discussion Starter · #12 · (Edited)
thanks for the ideas people.

  • Just a recap: bike has been standing unused for several years and has had a general "freshen-up". The problem was present when last running but not as bad as now.
  • engine stalls when bike is stopped and transmission is in 1 or 2
  • engine stalls under load when shifting from neutral to 1 or 2
  • bike runs normally once moving



  • carbs cleaned and rejetted. Not running perfectly but running ok
  • fresh motorcycle-type oil. I forget the brand but a premium oil. Old oil was in good condition. not black, still good viscocity, no bits floating around.
  • fresh plugs, plug caps, battery
  • sidestand switch works properly

possible causes:
  • torque converter not slipping at low rpm (vanes clogged, check valve or out of spec, or some other thing)
  • change relay
  • change switch
  • bad cdi (I saw that in another thread)
  • idle stabiliser circuit

being a lazy ass wrencher, I would check the easy stuff first so this is what I intend to do in order:
  1. test the change switch. I've already taken it out and checked the wiring. its a mechanical switch and appears to be working correctly. (at least the gear indicator lights match with the actual gear position)
  2. bench test the change relay using the resistance table in the fsm.
  3. bench test the cdi as above
  4. check through the idle stabiliser circuit, clean all connections and apply dielectric grease.
  5. remove and check the torque converter. (this sounds like the most work and is not something I've done before).


does this sound like the correct approach? any advice appreciated.

quick update: looks like the relay is bad. I get no reading on many of the possible test combinations.
 

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I like how you think. List of possible causes and having an order of tests/checks to sort it out.
Once you've changed the bad relay do the easy tests again as a recheck
 

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FSM stated a bad side stand switch or wiring can cause the motor to stall I was just passing it on. Pulling the converter is a lot of work I would do the stall test first to see if you need to.

If it turns out to be the converter I have a good spare.

Bill
 

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Discussion Starter · #15 ·
thanks Bill. It is not the sidestand switch but very helpful suggestion. Hopefully it is the relay and/or the cdi. Interesting thought. CDI units, especially early ones will fail in time. The relay and cdi on these is unique to the model. What is going to happen when they all start to fail? Not many hondamatics were ever sold, so do these bikes become unrepairable? I had the devil's time finding used replacements...
 

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There is a sticky in the 400 elec section that covers the options of replacing the CDI on these models. Power for the ing comes from the stator to the CDI box. Pull the connector from the wires going from the CDI to the coils ,turn the motor over and see what kind of reading you get. If this checks ok then that side of the CDI is working. When the motor is in neutral the starter will work and the motor will run with the side stand up or down and the timing will be at 7.5 degrees advance. When the motor is put in gear the starter will not work, if the side stand is down the motor will not run
and the advance is moved to 15 degrees advance. All these changes are made by the CDI box. If you have power to the coils then the problem my lay between the change relay and the CDI. If the change relay tests are ok then its a good chance the problem is in the CDI. Just my thoughts.

Bill
 
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