Topic: my 400/4 is actually a 300/3

[cmorgan47]

so i determined last night--by the finger burning method--that my cb400f is only firing on 3 cylinders.  that explains a lot.  didn't have time to go much further in the testing, but tonight plan to check for spark first.....both coils must be good as i'm getting spark to 3, and i just changed all the plugs.  anything i'm missing there other than the wires to the plugs (which i believe are hard wired to the coil)?

also, any good way to test for fuel getting to that carb?  i believe there is, and i think that may explain the "popcorn" sound and black smoke from the exaust even when it feels like it's running lean.

any advice?  do i seem off base with any of the assumptions?

thanks, this board rocks.
it is to bikes what the northern brewer forum is to brewing.

[mb3000gt]

I'd start by checking for spark, maybe its just a broken wire. If not open the drain screw on the carb and see if any gas comes out. If not you'd prob have to pull the carb and see whats up. Does the plug smell like gas? Have you checked the compression?

[cmorgan47]

yep, like i said, spark test is the first thing i'm doing when i get home.

as far as the gas, a couple of days ago i drained all 4 carbs and all had gas.

[cmorgan47]

ok, spark tested and cylinder 3 consistantly does not spark--plug tested good, end tested good, wire not removable from coil.

since the other 3 cylinders are sparking fine, and there's only 2 coils and 2 contact breakers, is the wire for that cylinder the likely culprit?
am i correct that the wire itself cannot be replaced?

is there anything else to test here, or just order a new coil?

[supersport_CB400F]

you can take the plug cap off and test the HT wire from the coi,l just hold it close to the engine but be careful how you hold it   

[cmorgan47]

what would that tell me?
i put the cap from a good cylinder on the bad one and got no spark...and put the cap from the bad cylinder on a good one and did get spark.  wouldn't that test the same?  albeit with less excitement.

[crazypj]

Do you park on mainstand or sidestand? fuel inlet between 2&3. if sidestand, no 2 usually gets blocked/stuck shutoff valve, mainstand, no 3.
PJ

[supersport_CB400F]

what would that tell me?
i put the cap from a good cylinder on the bad one and got no spark...and put the cap from the bad cylinder on a good one and did get spark.  wouldn't that test the same?  albeit with less excitement.

You could do it that way .......I just dont mess around with ones that are working good and just mess around with the  one with the problem.

[mb3000gt]

sounds like you narrowed it down to the wire. the wire is molded into the coil. there was a recent post about repairing the coil, but its prob less trouble to just get another one.

[cmorgan47]

well, i ran home on lunch and checked resistance on each of the secondaries.  both showed 50 ohms (1-4 then 2-3) which seems correct.

now i'm baffled, i was really expecting 2-3 to show open.


any ideas?  never messed with breakers before but they don't look worn or corroded.  what should i check there?

maybe tonight i'll recheck all the plug ends.

[cmorgan47]

another quick question:

do cylinders on the same coil fire at the same time?
1&4 together, 2&3 together?

[TwoTired]

Yes, operates on a wasted spark principle.  2&3 fire together, and 1&4 fire together.

Cheers,

[cmorgan47]

twotired:
had to look up "wasted spark principal."  if i'm understanding what i read, each coil simultaneously fires both of its plugs twice per cycle.  ie, for cylinders 1&4, when 1 is on its compression, 4 is on exaust.  it fires 4 anyways and that's the "wasted" spark.  then when 4 is on compression it "wastes" a spark to 1.

am i getting that?

[supersport_CB400F]

twotired:
had to look up "wasted spark principal."  if i'm understanding what i read, each coil simultaneously fires both of its plugs twice per cycle.  ie, for cylinders 1&4, when 1 is on its compression, 4 is on exaust.  it fires 4 anyways and that's the "wasted" spark.  then when 4 is on compression it "wastes" a spark to 1.

am i getting that?

You got it right, it sparks but it's not needed on the stroke for the waste cylinder...but it's not wasted it's a 400F spark 

[GroovieGhoulie]

Set of 400 Coils on eBay:

http://cgi.ebay.com/ebaymotors/Honda-CB400-cb-400-coils-w-wires-1976_W0QQitemZ300003418034QQihZ020QQcategoryZ35595QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Might wanna ask him if they work, but for 11 bucks, you almost can't go wrong.

[malcolmgb]

you got me interested there    but they don't look good 

[TwoTired]

am i getting that?

You got it!

 

[cmorgan47]

NOBODY BID ON THE EBAY ONES!!!!!!!

that guy lives about 6 houses down from me....he's the one i bought the bike off of..... weird.

GroovieGhoulie:  thanks for the heads up
TwoTired:  thanks for the education
malcolmgb:   seriosly, let me have them please!

[malcolmgb]

all yours, I'm not bidding.

[Max Power]

After getting debris in my carbs for the last three years in a row, I discovered that my in tank filter snapped off. 300/3 is definitely not enough for these bikes.