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Discussion Starter · #1 ·
This bike is a 1981 CM400C that I'm putting back together, the electrical system seems unmolested, but the bike had been parked for a few years before I got it for reasons unknown. I've cleaned and dielectric greased every connection and tested every switch, it all seems good.

Today I connected up a battery and went about the process of verifying the functionality of all of the lights, indicators and switches. Eventually everything seemed to cooperate, but I still don't have a working headlight. The switch switches and all of the wiring seems to be connected correctly when I verify each run with an ohm-meter. The bulb is new and I've verified that it's good.

Do I need to have the bike running to operate the headlight, or should I keep searching for another cause? Carbs go back in tomorrow so I'll find out soon enough I guess.

Any pointers are appreciated.
-Alan

Here's my build thread if anybody's interested: http://nehondaguys.info/forums/index.php?topic=576.0
 

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Re: CM400C Headlight Question

The starter and headlight "share" incoming power (perhaps trade would be more accurate, power only goes to one OR the other at any given moment) .... Could be a bad contact within the starter switch..... When you depress the start button, it DISCONNECTS the headlight, and uses that power to activate the solenoid.....The headlight terminals/contacts SHOULD reconnect when you release the button, but OFTEN don't....
 

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Re: CM400C Headlight Question

The bike doesn't have to be running for the headlight to be on, but like Steve said it could be a bad switch. I had problems with the Hi-Low beam switch on my CB400T and had to disassemble to clean and fix.
 

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Discussion Starter · #4 ·
Re: CM400C Headlight Question

HerrDeacon said:
I had problems with the Hi-Low beam switch on my CB400T and had to disassemble to clean and fix.
You nailed it Man! Switch contacts were clean but the 'rail that keeps the contacts touching needed tweaking.

Here's my full story in case anybody's bored enough...

After another 3 hours trying to figure this headlight out, and even consulting the wiring diagram...it's finally working. It turns out that it was the dimmer switch all along. I'd get power coming into the switch from the fuse through a black wire with a yellow stripe (to ground), but no low or high beam power at the white or blue wires (to ground).

Finally I decided to check the dimmer switch contacts, I popped the switch's contact/rocker part out, it's got one big contact on one side and a spring loaded ball bearing on the other side. it had some wear on it but nothing too bad. The ball bearing side had a 'rail' for lack of a better term that was held on by two screws (that I couldn't budge) I figured the contacts just weren't being poshed together enough to make contact, so with a thin flat blade screwdriver I tweaked the rail toward the contacts about half a millimeter.
I had to compless the spring a little to get the contact assembly to pop back in (thats a good sign right?) I bolted the switch pod back onto the handlebar and flipped it a few times from low to hi beam, quite a difference in feel now. Stuck my meter probes into the headlight socket white and green, turned the key and got 12 volts. Flipped to Hi-beam and it dropped to zero, probed the blue wire and found my 12v again, Sweet! For giggles I plugged in the bulb and flipped the headlight low to hi a few times, then did the dance of joy.
Now to get that cheapo plastic on plastic sub-radio shack brand replacement horn button swapped out for a genuine radio shack brand switch...
Alan
 

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Discussion Starter · #5 ·
Re: CM400C Dirty Carb issues... was Headlight Question

I'm searching, honestly I am.

I put a battery in my 81 CM400C today, fueled it up and after a few seconds I had fuel running out the drain on the left carb, tightened the drain screw and it stopped... Cracked the drain screw on the right carb and drained a little fuel, OK so both carbs are getting fuel.

Turned the key and pressed the starter button and the right cylinder fired right up, left didn't seem to fire.
I pulled the plug and yes left has spark.

I pulled the left float bowl just to be sure and it was full, float was moving freely.
Then I pulled the top off the left carb, lifted out the slide and gave everything a squirt of carb cleaner and chased out a few easy to reach passages with some electronics solder.

Put it all back together and gave it another try. Soon the left pipe seemed a little warmer than before, and the exhaust blowing out the end of the muffler was warm against my hand (like the right cylinder) The right head pipe was blazing hot, the left was merely too hot to touch.

The engine would start easier with choke, and I noticed that I could rev the engine by pulling the choke open wider.
It would rev to 3 grand on full choke and idle at 1100 without it.

Blipping the throttle had little effect, I'd have to slowly coax the throttle open and wait for the revs would catch up.

The needle on the left carb has significant wear, I might have another in a box of parts I got with a spare engine, should that be my first stop or should I pull the carbs out and soak them in something that'll actually clean them?

I've been pretty lucky with carbs on other bikes and aerosol carb cleaner has always done the trick, until now....so I feel like a real newbie today.
Can anyone point me in the right direction?
-Alan F.
 

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Re: CM400C Dirty Carb issues...was Headlight Question

Sounds to me like you're going to have to take the carbs off and give them a good cleaning. You're almost there but something seems to be blocking fuel when reving. Take them off and clean all the jets and passages.

Also, have you checked/adjusted the valve clearances? Compression good?
 

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Discussion Starter · #7 ·
Re: CM400C Dirty Carb issues...was Headlight Question

I haven't yet checked/adjusted the valve clearances good plan. Compression does seem good...but I've got a cheap HF automotive comp tester... at least they read consistantly if not acurately.

I checked my parts stash and I've got a pair of needles that only have 'a little wear' and should be worlds better than the grooved needles that are in there now.

Thanks HerrDeacon, I'll post back when I've got the clearances set, the needles swapped and have given it a try.
I'll save removing/recleaning the carbs for after that.
Alan F.
 

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Discussion Starter · #8 ·
Re: CM400C Dirty Carb issues...was Headlight Question

I've made some progress today! Last night I dug out the pair of needles I thought I had and polished them up, they have just a little wear but nothing I'd call 'significant'.

I swapped them out for the old needles and again added fuel.
The bike fired right up without choke, and it revved like a bike should rev. Brilliant.

The old needles both had wear rings, the right needle was MUCH newer than the left, it looks like someone incredibly frugal replaced one needle only...
so I have one potential spare needle.

The left pipe is still not heating up as much as the right...but the right seems cooler than it did yesterday.

Revving past 4K causes a backfire through the left carb, or a backfire out the right exhaust...and top end noises are horrible when spinning fast too.
Valve clearances are my next project for sure.

I'll put off pulling the carbs for now, maybe they'll be fine after all.

Alan
 

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Discussion Starter · #9 ·
Re: CM400C Dirty Carb issues...was Headlight Question

Progress report: It revs to redline without any popping or backfiring! WOOHOO!

First I took off the cam cover and retorqued the 8 head bolts to 29 ft/lbs, then another round loosening each 1/8 turn and torquing to 31 ft/lbs (a few took a half turn at 29!)

Second I loosened the camchain tensioner about halfway and gave the front of the tensioner a pry to pull tension and retightened. Then turned the engine a half dozen times so that each cylinder could get to TDC a few times (360 degree twin = a full rev between #1 TDC and #2 TDC) Then I loosened the tensioner bolt, gave it another pry and retightened for the last time.

Third I set valve clearances cold at the bottom of the spec, then popped the cam cover back on, plugs in and ran it for a good 10 minutes until it was good and hot. Pulled the plugs and cam cover and rotated back to TDC and checked clearances on both, they were still within spec, so I'm happy enough.

Buttoned it all back up again and fired up again, gave it a rev and now instead of the dreaded left carb backfire (pop) and right exhaust backfire (pop) at about 4500 rpm (where the ignition advance kicks in) it just revs! At about 6500 there's a popping, but it seems all exhaust this time, and it seems like the left side.

Messing around with the air screws: I noticed when I was cleaning the carbs that the left air screw would go 1/2 turn tighter than the right one would, so I turned the left one until it disengaged the threads, turned it to the point where threads caught and counted turns in from there, it maxed out at 8 turns, so I backed it out to 4. Then I turned out the right airscrew completely and counted turns in until it matched the other at 4 turns in. Fired it back up and revved it to 4500 where the popping reappeared, shut down and gave them another 1/2 turn in, fired up revved, shut down, another 1/2 turn in and tried again. I did this a few times until I was able to rev to redline without any popping anywhere.
Then I tried setting the idle, I settled on the lowest I could adjust it, an indicated 1400rpm, spec is 1200.

Carb synch is next, I'm going to build one of those $1.55 manometers and try it later on. ( http://www.powerchutes.com/manometer.asp )
 

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Discussion Starter · #11 ·
Thats a good catch, the 31 was newton/meters which equals 22.86439 ft/lbs.
My 250 inch pound craftsman torque wrench is acurate to 33N/M checked against a buddy's snap-on wrench... I seem to have an easier time converting newton meters to ft/lbs than inch pounds to foot pounds.


So I actually torqued to the middle of the range, thanks for catching that one.
I posted the same update on two other forums and nobody noticed. Funny.

Alan
 

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Discussion Starter · #13 ·
Progress today, the carbs are synchronized, air screws are set happily to stock 2 turns out, idle is set to about 1200 on the bike's tachometer. It revs to redline with a just little popping around 9 grand through the exhaust. My $6 carb balancer worked just fine once I restricted flow with a couple small clamps.
I was able to get the checknut with a 1/4" ratchet and a deep drive 8mm socket, I recommend a 12 point socket ( I only found a 6)
To adjust synch I just used a long screwdriver through the round hole in the right side of the upper motor mount.

Afterward I put on my new clutch cable and adjusted free play at the lever. A stock length clutch cable is a tight fit with the bars I'm using, but it'll be ok.
I wonder of any of the 450 Rebel clutch cables are longer?



The carbs started pretty far out (to the right cylinder) which probably explains the difference in head pipe temps, which seem equal now.
I'll check them another day with an infrared temp gun.

Alan
 

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Ok, it looks like you are having the same issues with your bike, I have a 79 CB400 and she is giving me a fit with the carbs.

How are you attaching the $1.55 manometers to the bike, ( I know where) but how, and with what ?

I really love this bike, and she starts and idles really well, but bogs really bad in the mid range.
I pulled the carbs and gave them a really good cleaning today, put them back on and it started fine, idled fine, just wouldn't run correctly in the 2500 - 6000 RPM range.
Did the sync. kit ( $1.55 ) work, and give me any details you can so I can get back on the road !!!
 

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Discussion Starter · #15 ·
I just bought a few M5 screws, chucked them in my lathe and center drilled through them, then with the lathe still turning I used a file to remove the threads from the area that the tubing would attach to. Not the ideal solution, but it worked.

Be sure to:
1. retorque your cylinder head
2. set camchain tension with the engine stopped, use a screwdriver to pull the tensioner where you want it (the springs in these are crap)
3. set valve clearanes
4. Set air screws to stock 2 turns out, set idle as close to 1200 as you can, then crack loose the checknut from the carb synch screw and give synching the carbs a try. Don't use any lubrication on the adjuster locknut or screw, leave them rusty so that it's easy to lock them down later.

The vacuum pulses at idle are spaced pretty far apart and you'll have a few seconds to hit the kill switch so no iol is sucked up.
Start the engine and watch the oil level to see which way to adjust and shut down the engine, make your adjustment and fire up again. You've either turned the adjustment screw the right direction or not, readjust and try again, I recommend 1/4 turn at a time.

Eventually you'll get to the point where every time you adjust the synch screw your idle will be off again, you're getting closer now! Readjust your idle back towards 1200 and adjust synch again.

After each adjustment I'd crack the throttle open and let it snap back at least one time before starting the engine again.

The oil moved around a little fast with the tubes unrestricted so I placed a small pair of forceps on each tube and locked them on their loosest setting.
Alan
 

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O.K. Cool !! Thank you and I will give it a good shot.
How do you know when your cam tension is correct ?
Thank You !
 

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Discussion Starter · #17 ·
Here's the link to the synch tool, manometer, balance tube or whatever anyone wants to call it. It should work fine on any twin.
http://www.powerchutes.com/manometer.asp

As for the cam chain tension, I just gave the tensioner a pry and tightened down the acorn nut, gave the engine a few turns by hand and poked the cam chain with a screwdriver if it was floppy it was loose. I left it at a point where i could poke the chain with a screwdriver and it would deflect a little with pressure, but wasn't loose enough that i could knock it around....
Cheers and good luck.
Alan

Hey Mods, this thread has evolved a little from electrical to mechanical, is it located in the wrong section now?
 
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