Topic: cb400f carb problem

[jcoryea1]

Hello all.   
 I've checked the relevant posts
, but can't quite find what I need..
 77 CB400f  with 14K  doesn't want to run on outer two carbs(right side sitting on bike) pipes cold.
Carbs were cleaned and rebuilt .. flushed tank ,but didn't put a filter  in line,so my mechanic drained the bowls and flushed them. I installed an in line filter..Ran on all four yesterday, now back to same two cold pipes and two carbs.Am I getting just more debris? The bike sat inside since 1986, but the tank was not rusted..I see no debris in the filter now. Should the two carbs be removed again  and heated in the Yamaha  carb cleaner as one post said? Is there something else I should do..other than clean and blow out?  I have the shop manual..Should we  follow the settings exactly or do some carb settings work better? Everything is stock,except for a supertrapp. My mechanic is a Yamaha dealer mechanic and does the serious work on my other six bikes.  I love this bike more than my 76 Bonneville 750!  Thanks in advance.
  John Coryea









 

[MoMo]

John, that sounds more like you are losing spark as those cylinders are controlled by the same set of points.  Use a point file and contact cleaner then blow off with air.  Wasn't it you that I sent the muffler bracket tracing to?...Larry

[jcoryea1]

Larry- Thanks for the info..I believe you offered to send me the dimensions for the bracket, but I ended up making one ..
  Thanks,
 John

[jcoryea1]

Thanks for the advice..I'll talk to my mechanic with your recommendations. have a nice holiday!
  John

[Bodi]

Confused. Outer two carbs (1 + 4), or two on right (3 + 4)?
1 + 4 use the same points and coil. 3+4 don't.
Check for fuel... really easy, just have something to catch and gasoline (a trough made from foil leading to a cup is good) and unscrew the float bowl drain screw. With the petcock off, 50ml or so of gas should come out to empty the bowl. Then turn the petcock on and gas should dribble out, it isn't a huge flow but some must dribble out.
If you have fuel flow the floats could still be badly adjusted but look for ignition trouble before removing the carbs. Check plugs for spark when kicking the engine over. You don't have to pull the plugs, use an old or new plug on the cable.The plug threaded end must touch metal engine or frame to get a spark.

[jcoryea1]

Hello to all-
  Still having problems with the 400f..so we,re all on the same page, I'm calling the cy#'s from left to right,as seated on the bike, #1,2,3,4. previously, the bike was running on all four, then it went to #1 and #2. pipes 3,4 were cold. Today, I tried again,and I have #2 and 3  pipes hot and #1 and 4 cold!! The bike starts right uo, has Iridium plugs and the carbs have been rebuilt,after soaking for a week in Yam carb cleaner. Then the carbs were removed again and that's where I'm at now.I checked for spark on #1 and # 4(today's cold ones) and I did have spark..Any ideas ? Thanks in advance..hate to remove the carbs if there is an ignition problem as well. Also need to address a tinny sound in the front,lower case..I suspect  at 14K ,it's the camchain? We tried to loosten the adj. nut,but it wouldn't budge and didn't want to shear it.Am I putting the engine in harm's way?..also read ,that will happen if carbs aren't right?  Such a sweet bike, I want to ride it!! I thought it gave me the LOOK when I passed the garage on the 76 Bonnie!
  All the best,
  John Coryea
   1977 cb400f

[jcoryea1]

Larry- Thanks for the info..I believe you offered to send me the dimensions for the bracket, but I ended up making one ..
  Thanks,
 John

Larry. Yes I do remember now, you did send me the tracing,and I used it to fabricate a bracket..thanks

[RickB]

John,

I had a similarly frustrating problem with my 400F. Rough idle, plugs fouled, weak purple spark, running short on cylinders. In the end, it was the length of my suppression core spark plug leads.

See my thread battling it:

http://forums.sohc4.net/index.php?topic=87505.0

Not sure if it's relevant, but thought it might help.

Rick.

[MoMo]

John,  make sure all the plug caps are tight. Take each off and trim the plug wire a bit(not too much or the wire will not reach) then rescrew the cap back on.  Make sure the points are clean as posted earlier.  That plug wire contact is a common problem with all Hondas...Larry

[Bodi]

Use some kind of small cup that fits under the carb bowl drains, and check that you have good fuel flow into each carb: each individually should dribble out pretty good with the petcock open (after first draining completely). You can get air locks in the fuel feed tube, possibly exacerbated by an inline filter. Fuel starvation will hit the carbs with the poorest flow first: usually the outer two but there are many mysteries in the carbs and anything can happen. Dirty gas usually causes ovefilling when crud jams in the float valve and it leaks. Make sure the petcock itself is flowing fuel well.

[jcoryea1]

Larry.how do I shorten the wires exactly..? looks like I have two different caps on #3,4.. the wires/coils are original,should I just replace them? if so, what brand for stock? Bodi- what do you use to access the float bowl drain screw? Mine are hard to access,and very tight.petcock flows..
   in line filter installed. , today #2,3 pipes hot, yesterday, it was #1 and 2  hot. Thanks,
  John

[MoMo]

John,  the caps actually screw off and on.  Once off, check to see if the inner spark plug wire is there. If not, use wire clippers to  clip a very small amount of the wire off until you see the inner wire and then screw the caps back on. Also, make sure the wires to the points are tight and the points are clean...Larry

[Psychonaut]

If you can get the lock nut and adjustment nut loose on the cam chain, you can fix the rattle. I have 30k and had to remove the top bolt about 6" above the adj bolt. With it running you can push down until the rattle stops. It doesn't take much.

Were the carbs synced? Are the air screws set to 2 turns out from bottom?

[jcoryea1]

OK guys-
  I shortened the wires,cleaned the plug receptacles,files the points, and it still runs on #2 and 3..I have spark on #1 and 4,but cold pipes..what's next?  My favorite bike is quickly becoming a pita.can anyone in Northern NY (as in Malone,on the Canadian border ) fix this bike?Thanks
  John

[RickB]

John,

That's disappointing to hear you're still having troubles. To me, it really sounds like an electrical/spark issue. I wouldn't play with your carbs until you can be sure it's not electrical. Fuel, air, spark. Three things you need for an engine to fire. Let's start with spark.

When you have the plugs out of the engine and turn it over, do all the plugs fire with a thick white spark? If not, can you tell me more about your spark plug leads? Are they the original copper core ones, or the newer graphite ones?

If they are firing with thick white spark, then it's fuel. Start the bike, pull the plugs from the suspect cylinders. Are they wet and smelling of fuel? It will be very obvious. If they are, then you're back to spark (perhaps your timing is way off). What do the plugs look like?

If they're not covered in fuel or burnt carbon (ie, they look brand new), it means the carbs aren't delivering fuel. Which means it could be air.

Are you sure your choke butterflys are opening? What air system do you have? Stock? Pods? Something else?

Take a methodical approach, don't adjust everything then try and trouble shoot. One system at a time is the way to approach these things.

Report back and let me know,
Rick.

[MoMo]

John, it still sounds electrical to me. What about the connections at the coils? You should have 12v at the double black/white connector, and tight, clean connections at the blue and yellow.  Blue would be for 1/4 and yellow 2/3...Larry

[jcoryea1]

OK guys-
 I just returned from my college reunion(1969..am I that old?), when I get a chance, I will try your ideas, and report back.. I've been getting an orange/yellow spark at the wires ,have spark on the cold cyls also. ,(wires are metal,not graphite).  I cleaned  the wires on top of the coil and checked the connectors for tightness, Cables  going into the coils are tight.. I will have my mechanic,do the tests, check the coils.I'll update when I get some results. Thanks,
  John