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Discussion Starter #1
Hello everyone, and thanks in advance for your help.

TL;DR - Bike has no power, won't respond to throttle very well, can't exceed 10mph going downhill with full sail and a rowing team.

I picked up a 1979 Honda CM400A from a friend who didn't want it anymore. It's been sitting for about 3 years and I'm trying to get it up and running.

I contacted a couple shops, no one but a local place was willing to do work on it and even then they'd only do tires, battery, chain, etc. The easy stuff. So i had them do that, and now I'm working on the carbs/engine

I wanted to be lazy and pay a guy to work on it but no shops around me are willing to touch this because it's "too old". Well, I guess you don't get my money then guys. So I have to do this myself.

I already took out the carbs and cleaned them up, replaced the float bowl gaskets (they were trash), and checked to make sure the floats float. They do. I put everything back together and into the bike.

It idles, it seems to run pretty smoothly (put put put put put put), but when I put it in gear without revving a smidge it stalls. Once I've gotten a rolling start, it'll run but I can't get the engine to increase in RPM. It won't respond to throttle very much at all. I can't get going unless I'm already going.

However, when I put it back in neutral, it'll rev all the way up and back down again. It responds quite well too (I think, it's not snappy but it'll follow my lead so to speak).

Another thing I noticed is after I shut it off the right side exhaust spews just a little bit of smoke. :coffee:

Any ideas guys? I'm not sure what to do. Do I have to take the carbs back out and do some adjustments? Did the guys who did my tires and chain mess something up?

Your help is greatly appreciated, I hope I can start riding this summer.

:71baldboy:
 

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Gone
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Check the compression first. While you have the plugs out, see if the fuel mixture is too rich or lean.

You might need to go through the carbs again. Where they soaked in a dip the first time?
 

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The hondamatic has an extra safety feature that limits the revs to about 2000 rpm (by changing the resistance of one of the inputs to the CDI unit) if it is in gear and either the side stand is down, or if the rear brake lock (where the clutch lever should be) is engaged. It may be that with the age of the bike either of the two switches are stuck. Worth checking out with a meter.
 

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Discussion Starter #7
Thanks for the input everyone. Payday is nigh, it's time to get some tools and supplies. I'll be working on it this weekend, I'll report back with what I learn.
 

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Discussion Starter #8
Update!

So I bought some tools off of amazon and got to work on it today. I had to wait because some of the tools didn't have fast shipping. Anyways, I synced the carbs, checked the compression (seems fine), adjusted the idle with a tach, and tried to take it for a spin.

I could now go 20mph where before I could only go 10. Progress! Something is still wrong obviously.

I noticed that when I pulled the right side plug out it was fouled all to hell. Obviously it was running really rich. So I've got a fuel/air problem. Resigning myself to the future hell I was about to enter into, I removed all the fairings, tank, and seat, took out the carbs, and did a much MUCH more thorough cleaning on them this time. (Soaking in cleaner as this post is being written).

I discovered part of my problem during this process: The slow and fast jets were switched on that side, and in addition to that the vacuum port with the diaphragm on the side was plugged chock full of garbage. I had to douse it in P'Blaster to get it out, it wouldn't budge at all.

I have high hopes now that when I put it all back together with new plugs and everything adjusted that it will run like a dream.

I'll post again when I've got it back together and report on how it runs.
 

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Sounds like you are on the right track now. :thumbsup:
 

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Discussion Starter #10
It's alive! I took it for a quick spin around the parking lot, and I can accelerate normally now. However, new and interesting problems(?) presented themselves:

1.) The throttle felt super tight. No big deal, I'll adjust it once I swing back around and park in my garage. Nope! I heard a snap noise and now there's a lot of play in the throttle. My bike also wouldn't idle. After checking the throttle pull cable, I see that there's some slack in it that wasn't there before, but it still works. The return cable also appears to be working. What the hell happened here?

I was forced to compensate by cranking the idle screw up a couple turns. Now it idles again. My cheap chinese tach doesn't work anymore for some reason though, so I had to do it by ear. Seems fine-ish though.

2.) It's hard to describe, but when I'm accelerating from a low RPM sometimes the engine very slightly stops/hesitates/stutters (put put put put - - put put put - put - put put put put put - - put put put). What new and exciting struggle does this entail? I can only hear it, it doesn't seem to effect it's ability to accelerate very much. Maybe I can record it and put it on youtube so you guys can hear it.
 
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