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Discussion Starter · #1 ·
So I've got my '81 CM400T running very well. Starts right up, idles steady at 1200-1250 rpm, and accelerates well. Seems like it's happy place is around 5000 rpm; plenty of power and torque there. I spend a lot of time in the 4500 to 6500 range; never approaching 10K redline. I have, however, noticed somewhat of a flat spot below 4K. Sitting in the garage running the throttle through the full range it's fine, but if I let it fall below 4k running down the road thats when I notice a flat or rough spot. Just wondering if that's normal for this bike. FYI - bike is stock down to original tires and tool kit.
 

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The carburation maybe at a transition point between the low speed and high speed jets where your noticing the flat spot. You may also be lugging the engine in the wrong gear say at 3500 RPM if your in a higher gear and hit the throttle it will seem flat. Do you notice any surging or unevenness in the bikes running at a steady RPM where the flat spot appears to be located?
Honda motors like to rev, so if your in 5th or 6th gear at 3000 RPM unless your on flat terrain just cruising along nice and easy and no demand for the motor to be accelerating your probably in too high a gear ratio.
Revving the bike in the garage and one under load in real world riding conditions are two different scenarios, which won't help you identify any issues in the garage setting.

Bye the bye... hopefully you are not running on tires from 1981 when you say original tires on the bike? We use those as hockey pucks up here, as they are just about the right hardness of rubber!:eek:
 

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I find I've got a little bit of that flat spot as well especially when cold. I don't really think it's normal but I've replaced all the rubber in the carbs, cleaned using ultrasonic tank + compressed air and it's still there. I haven't managed to get a DIY carb synch done yet but I'm sure that is quite important. My current guess is that it's something to do with old springs on the vaccuum pistons not providing quite the resistance they were meant to (mine don't even push the caps off anymore with the screws undone).

Also original tires on a trailer queen car might be something to be proud of but if you actually want to ride this bike you should take the above advice seriously and not risk riding on old rubber that has had 35+ years for all the chemicals keeping it functional as a tire to evaporate.
 

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Discussion Starter · #4 ·
ya the thought of driving around on 40 year old weather checked tires has been lingering in the back of my mind. I currently have a set of Dunlops being installed on my Softail; I've got that shop pricing me a set of Kenda K657 for this bike (100/90/18 and 120/90/16). As far as the "flat spot", I don't think I'm going to put much more effort into it. I've been through the carbs numerous times and the overall performance of the bike is good. Just wondering if the 5K-7k "sweet spot" was typical.
 

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These bikes came jetted somewhat lean which contributes to what you are experiencing (quite normal). All else left stock it is generally recommended to go up one size on the primary main jets, in your case from 72 to 75. Just something you may consider next time you have the carbs off.
 
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