81 CM400 carb issues

Clayman98

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Joined
Jul 3, 2026
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Location
Ohio
So recently I had my carbs rebuilt after siting all winter after getting them back I put them on the bike and had a difficult time re-syncing them I don’t know why they actually ran better without the air filter but I know you need the air filter to get proper air flow to open the carbs fully so I took it out on a little ride I got the bike up too 60 and it ran great for about 5 minutes after that it started breaking up and the right side quickly sputtered out and the whole bike shut off so I brought it home and took the bowl off and fuel flowed great so I am at a lost if it’s cause I took out the air filter or is the right bowl sucking fuel too quickly any input is appreciated just lost on what the problem could be and don’t wanna get miles from home and have it break down on me
 
I noticed you said "So recently I had my carbs rebuilt" and I am wondering if you rebuilt them yourself or had the work done at a shop? No problem either way but you may have to ask some questions if someone else did the work. Also you said that you rode about 5 minutes and the the bike stalled which sounds like it could be an electronic ignition that fails when it got hot. Would the bike start back up after it cooled down?
 
RobMan said:
I noticed you said "So recently I had my carbs rebuilt" and I am wondering if you rebuilt them yourself or had the work done at a shop? No problem either way but you may have to ask some questions if someone else did the work. Also you said that you rode about 5 minutes and the the bike stalled which sounds like it could be an electronic ignition that fails when it got hot. Would the bike start back up after it cooled down?
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So I helped my brother rebuild them since he’s rebuilt vacuum carbs before and i was able to restart it after a few second of the bike being off which is why I am assuming the carbs just aren’t filling fast enough
 
ancientdad said:
Moved to SOHC Twins Carburetion section.

Did you make the cardinal sin of changing float needles with kit parts?
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I did change the floats and float needles because the original float needles were starting to show signs of wear
 
Clayman98 said:
I did change the floats and float needles because the original float needles were starting to show signs of wear
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Unless you bought OEM Honda parts, I guess you didn't realize how bad aftermarket kits and many replacement carb parts are. There are often inaccuracies in float needle lengths, jet sizes, etc. Take a look at this article by an industry professional.

 
If you got the black adjustable floats, then you might be able to adjust the to make the aftermarket needles work.

Running the bowls too low due to low fuel flow to the bowls is definitely a possibility, i have done it my self. Usually happens on a long uphill section of road where you are really give alot of throttle.

Check the flow from the tank, and keep in mind that the fuel filler cap is suppose to be vented, so flow can be good at first then as it starts to draw a vacuum, the fuel flow slows down. If thats the case when you let it sit for a while did you open the cap?
 
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