Problem with Honda CM400

[Bimmere21320]

Hi there, my friend just recently picked up a 1980 Honda CM400T. For the most part the bike runs pretty good, but it is a little sluggish from the start until about 4500rpm. When it reaches around 4500rpm it takes off like crazy, but the low end bog has got us mystified. Any help would be appreciated, thanks!

[jeffcoslacker]

The slide diaphrams are most likely on an old one like that. The slides raise the fuel metering needles when you open the throttle. They work by pressure differential, when you open the throttle pressure above the slide is lower and sucks the diaphragm upward, raising the needles and allowing more fuel...when they get holed the carbs are unresponsive, engine speed builds slowly until pressure overcomes the leaks in the diaphragms, and they suddenly lift and it takes off like a bat outta hell...

[jeffcoslacker]

http://www.southernhonda.com/fiche_section_detail.asp

Here's a link to parts fiche for that bike...

That one used a different type of slide, that doesn't use a diaphragm, but the principle is the same...if it has weak vacuum, it won't lift the needles. You can take the tops off the carbs and pull the pistons, springs and needles out (pay attention to order and orientation), make sure they aren't sticky and the needles/jets are clean. If the gaskets on the covers leak, it wont lift the needles as I described.

You don't think it's only running on one cylinder until it opens up do you? That's a different problem, one circuit in one carb is plugged up if that's the case...easy to tell, one pipe will be cold when idling....

[Bimmere21320]

Its running on both cylinders alright, but you say it could be a problem with the diaphragms in the carbs? My friend and I really only have a basic understanding of how engines/carbs work. But if I am thinking what you are telling me; are the diaphragms made of rubber and are kind of a red color?

[jeffcoslacker]

The part under the top cover, #8 is the slide. If this listing is correct, yours doesn't have the rubber diaphragms. If it did, they would attach to the top of the slide and would seat under the cover.

This type is closely machined and doesn't need a diapragm.

#7 is a diaphragm for the accelerator pump, they go bad too sometimes, but usually will cause stumble when you bust the throttle suddenly, not bogging.

There should be a small hole through the slide that needs to be clear or it won't work right.