1981 CM400A stalls shortly after stopping at stop signs or stop lights

Micoviso

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Austintown, Ohio, USA
I recently restored my (wife’s) 1981 CM400A Hondamatic, which included a full carb sonic clean and rebuild. It runs absolutely like new with one minor exception. If I set the idle below a certain point the engine will idle perfectly fine in every other circumstance, but will stall every time I stop. It seems like about a second after I stop, the idle RPM dips slightly, and if I have it set too low, this will stall it. If I turn it up a little higher, I still hear the dip, then it comes back up to normal, but it’s high enough to not stall it. I’m assuming it has something to do with the torque converter disengaging that’s causing the the brief idle dip, but thought I’d post to see if anyone has experienced this and/or knows a fix for the idle dip. I wouldn’t bother with it except that turning the idle up just that little bit causes me to have to hold the bike back just a little, or it will inch forward. Not a strong pull, but it’s there. If I turn the idle down that little bit, there is no pull forward, but then it stalls on each stop.
 
I used an electronic tach to set it initially. At the correct RPM, there is no pull, and it idles flawlessly 99% of the time, but it stalls during that brief reduction in RPM after stopping. I have to keep it slightly above the correct setting for it not to stall. Did you ever notice the brief RPM reduction after stopping on your A model(s)?
 
Is the float height set correctly? Since it is after stopping, the fuel sloshing back and forth in the float bowl could be a contributor.
 
I'm confused. Is the concern that the bike is pulling forward at idle without applying brakes? I never really saw this as a problem because the brakes hold mine in place with light pressure. It is odd holding it in place that way if you're used to pulling the clutch on the other side at stoplights though.
 
When the idle is adjusted to spec, the bike does not pull forward while in gear at idle. But It does stall frequently immediately after stopping. If I increase the idle enough to prevent the stalling after a stop, it does pull forward a little bit. The brakes or my feet easily stop it, but I’d like the best of both worlds. No pulling forward at idle, and no stalling. In my mind the key issue is that after I stop at a light/stop sign, the engine settles in to a nice idle. Then about a second or two after the stop the engine drops a couple hundred RPMs for just a brief second, then returns to a normal idle. It’s during that brief RPM drop that it stalls many times (not always) when the idle is adjusted to spec. My question is, why does it lose that couple hundred RPMs for a second then return to normal idle. One responder suggested float adjustment, but these floats aren’t adjustable and looked perfectly fine when I rebuilt the carbs. Bill suggested a test, which I’ll perform tomorrow and see where that path leads. BTW, I know this isn’t a huge issue. The bike really runs great, but I’m a curious perfectionist. If there is ANY problem, I just enjoy solving it.
 
That description makes me wonder if the neutral switch is sending a signal that the bike is in neutral which retards the timing and will cause the idle speed to drop. Hmmm.
 
I wonder how accurately the carbs are sync'd and balanced...... A variance between them WILL cause similar symptoms.....
 
Im thinking its in the idle circuit which is why I sked him to put it on the center stand and put it in gear . If the idle picks up its working . If it drops off the problem may be in the CDI but will ask a few more questions.

Bill H
 
old timer said:
Im thinking its in the idle circuit which is why I sked him to put it on the center stand and put it in gear . If the idle picks up its working . If it drops off the problem may be in the CDI but will ask a few more questions.

Bill H
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Good point, Bill… lots of fuel/air discussion, maybe it’s electrical?
 
Idle speed in neutral should be about 1250 RPM . Idle speed in gear should be 2000 to 2500 RPM. Bike on center stand and set idle in neutral at 1250. Put it in gear and see what you have. In neutral motor running the change switch is off and the change switch relay is on. The SCR 2 gate is grounded thru the change relay, motor fires at 7.5 degrees at around 1250 at idle. Motor in gear the change switch is on, the change switch relay is off and the motor fires at 15 degrees thru the SCR 2 gate at about 2000 -2500 RPM.. I think maybe the problem is in the CDI. This timing change is separate from the ING timing. If your sure its not a fuel problem then there may be a problem with the idle stabilizing circuit.

The starter circuit is grounded thru the change relay so if the starter works that tells me the change relay is working.

Bill H
 
I finally got back to this issue. I figured to start off, I’d re-check the carb sync. There was about 2 inches difference between the carbs, which is odd because when I set it originally, there was only 1 inch difference after tightening the nut. It took quite a bit of fiddling, but eventually I got them exactly the same after tightening the nut. I wasn’t expecting this to be the issue, but it was. After I finished, I took it for a ride, and the rpm drop after stopping was completely gone. I was able to adjust the idle speed low enough that the bike doesn’t pull while idling, and it no longer stalls. 66Sprint wins the prize, you were correct. Thanks.
 
Door #1 (1) event of good karma redeemable the next time you need a break.
Door #2 A big, sloppy kiss from my 10 month old Husky, a little gross, but definitely raises your spirits.
Door #3 Mystery Prize - word on the street is that this is either a large check, or 3 garbage bags full of shedded fur from that same husky.
 
Very hard choice, but IF I can opt to transfer it to someone more deserving, I'll choose Door # 1....
Otherwise Door # 2 is equally inviting (Nothing better than puppy breath)......
While a check could be handy, large ones tend to be connected to/create other problems, and I since already have an unending supply of hairs shed by my own dogs...Door # 3 is out.........
 
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chefta1 said:
I am having the same exact problem.
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Have you tried syncing the carbs as 66Sprint suggested to the OP in post #9? If so, then you should start a new thread in the SOHC Twins Carburetion section to get more exposure to your situation.
 
Micoviso said:
It WAS the carb sync. It takes a lot of fiddling and a sync tool to get it perfect, but it totally eliminated the problem.
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Thanks for checking back, I wasn't sure if you'd see this since you hadn't been around since late January. :) Now I hope the new member checks back for the confirmation.
 
ancientdad said:
Thanks for checking back, I wasn't sure if you'd see this since you hadn't been around since late January. :) Now I hope the new member checks back for the confirmation.
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No problem. I still get a notification when one of my posts has a response. I sold my little restored twin for an offer I couldn't refuse, but I'm on the hunt for another restoration project.
 
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