Topic: cb400f, misfire under load, coils?

[kajtek]

Hi all,

we are having some very good weather here in england (27 degrees), so of course.. my bike stops working.

rode to work fine yesterday morning, on the way home2 mins down the road it starts to misfire, and fall to pieces under load. cant really get over 6k and when i roll off the throttle it backfires lots, more engine braking too. seems to idle fairly ok and i can sortof bimble around if i need to.

the misfiring is quite jerky and inconsistent, which leads me to think its electrical not fuel starvation. it feels a lot like when i had problems woth rain stopping play.

i pulled my plugs last night and all looked ok, a couple were slighlty lean but nothing significant. checked fuel and fuel lines, undid fuel cap (incase breather hole was blocked), checked air filter. all ok.
checked points using timing light and they were a bit out, ive nearly run out of adjustment so im having to close the gap a little to keep the timing on. i noticed though that the light was skipping about on 2,3 as i it was firing sometimes advanced sometimes retarded. basically all over the place.

so im thinking coils could be bad. obviously need new points (gonna go with pamco when i get paid), david silvers is out of stock but ive found these two on ebay:

http://www.ebay.co.uk/itm/160796295309?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

http://www.ebay.co.uk/itm/190495947136?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

would these be ok?

any other suggestions or ideas please, just incase im missing something..

ashley

[iron_worker]

My guess would be points. You've possibly closed the gap too much and the arc is not extinguishing completely when the points open. This could cause an inconsistent spark.

I can't see a reason why a bad coil would sometimes fire early and sometimes fire late. The coil must be charged and the points circuit opened in order to fire.

Anyway, I'm no expert but that is my take. Sounds like you need points anyway so I would go for a nice set of genuine Honda points if possible. (trust me the Daiichi points are no fun to set up!)

IW

[OneWheelDrive]

I was having a very similar problem and thought it was my 2/3 coil.  It was in fact something as simple as the very small nut holding the blue wire to the points plate.  Tightened it up and solved the problem.

[iron_worker]

Good point. Check your connections!

IW

[OneWheelDrive]

It's amazing how it can be something that simple.  Check blue and yellow connections at the coils and points, and pay special attention to the black/white wire.  It supplies power to the coils from the ignition/kill switch and could come loose at the coils, or in the headlight bucket.

[Psychonaut]

What's your plug gap? Too wide a gap will cause WOT miss.

[dave500]

check the plug cap resistance,if rain upsets it easily the caps are probably too old and gone high resistance.

[kajtek]

Thanks for the suggestions guys, tank off and checking all connections tomorrow!

Will check plug caps but I replaced the due to the terrible originals shorting out.

[kajtek]

Also, checked plug gap, 0.7mm across the board.

[iron_worker]

Points gap?

[dave500]

ok if the caps were new theyll be ok,if they were used id look at them.

[Drewster]

+1 on the "keep it simple".  I've been trying to get right my '77K, with similar problems as yours and tried a lot of fixes.  Changing coils, cleaning the carbs, setting and resetting points.  Came down to the new $5.00 Daiwa  #2-3 condenser was bad.  Running like a top now.
Good luck

[TwoTired]

checked points using timing light and they were a bit out, ive nearly run out of adjustment so im having to close the gap a little to keep the timing on.

There it is! 
Gap the points and time it correctly.  The points control the charge and discharge of the coils.  Coils will "look" bad if the points aren't working as they should.  That's why they are part of the routine 3000 mile tune up.

Cheers,

[kajtek]

ive got a spare set of condensors so will try those out, also will see if i can get a little more life out of my points. its a honda set with the better fitting tek plate, i have an aftermarket one but spent ages trying to make it work right, was at max adjustment and still coudnt time it. either im an idiot or its junk!

any tips on how to extend the life of some points that are running out of adjustment? just need it to work a bit longer until i can afford a pamco..

ashley

[TwoTired]

Have you tried...
http://forums.sohc4.net/index.php?topic=5752.msg47539#msg47539

Also, if you peen (mashing it between hammer and anvil at the fitment places) a loose fitting plate can be made to fit snugly within the mount posts.  Personally, I'd find out which position the shim gets the point adjustment in range first, and then peen the plate there.

Are you putting lube on the points cam?  (Keeps the rubbing block from wearing down quickly on the cam.)

[kajtek]

All fixed! Switched out the daichi points for sim no name ones I had, cleaned and checked all the connections, re timed and works great!

I add a bit of oil to the pad that ribs against the cam, should I be using some sort of grease instead?

[TwoTired]

Oil is better than nothing.  Grease would be better.
I recall when changing points in cars that new ones would come with little pill capsule of grease to put on the cam.  It is placed where the rubbing block hits the cam on the counter clockwise side, so the cam can pick up grease and move it under the rubbing block.

[kajtek]

Thanks will try find some grease that can take the high temperatures

[tory b]

   I am not familier with a 1978 400f.
Do you have pictures of it?