Topic: CB400F wont start

[malcolmgb]

Hey Guys...
Not sure that last post will make sense...but...
When 1 and 4 are at the top of the stroke...lets say 1 is on compression...then 4 will be at the top of exhaust stroke.
Normally this is when plugs 1 and 4 fire...on #1 it lights off the fuel under compression...#4 just fires harmlessly into the exhaust mixture. At the same time 2 and 3 are at the bottom of the stroke...depending on cam timing one of them will be at the bottom of the power stroke, but the other will be at the bottom of the Intake stroke sitting there with a fresh load of fuel air...albeit not compressed...
If all the plug fires then...most likely you would get a nice belch through the open intake valve into the carb. there is also a bit of "lead" (ignition advance) so the piston will still be on the downstroke and the intake valve will be open on that cylinder.

I agree hcritz, it seems to me if rotor is on 180degs out you would possibly get the backfire as it is trying to fire on the inlet stroke, so why not swap the yellow/blue wires over as shown in the drawing, this would be same as moving rotor 180 degs.
Hope this makes sense to you.

[crazypj]

1  fires, 4 overlap (both valves open) 2 on exhaust, 3 on intake. With a digital system its possible to be 180degrees out. Dont know if the Hall effect coils are wired together (++/- -, or + -/- +) That would make a difference as the ignition would be programmed to fire 1243 and spitting back through carb could be firing on intake ( trying to run backwards)
PJ

[howardstapley]

Don’t quote me but most engines run two and two (or three and three for six cylinders, the bastards) so 1+4 in and 2-3 out SHOULD be how it runs, if its one over one than these little motors would rip themselves apart.

[malcolmgb]

If you check drawing the stator still has 2 trigger coils to fire each ignition coil, so it is possible to be 180 degs out. Refer to my previous post, I still believe if 1 pair of blue/yellow wires are swapped then it may point us in the right direction. I believe these ignition units date back to the 70's so not as advanced as todays with regard programming.

[seabird]

Well, just to confound everyone, about an hour ago the beast roared into life having almost fully retarded the ignition after which it ran very roughly for the first 10 seconds or so, then a blissfull 30 seconds of 400F humming, then she stumbled and died.
This repeated for four attempts - each time the same. (BTW - I tried swapping the two leads on the backplate, with no success so swapped them back, after which it showed the above signs of life.
It fired, I think initially on two, then on all four, but on checking the pipes, 3 and 4 were hand hot, 1 and 2 were cold.
the choke was almost closed, and while it was running reasonably well I Tried to open the choke a little, at which point it stumbled and died.I ended the session by advancing the ignition just a hair - it catches, but still stutters and dies after a half minute or so.

Where shall I look now - the carbs?  What's Left to fumble with?

Thanks guys!

Seabird

[crazypj]

Don’t quote me but most engines run two and two (or three and three for six cylinders, the bastards) so 1+4 in and 2-3 out SHOULD be how it runs, if its one over one than these little motors would rip themselves apart.

1&4 and 2&3  go up and down together but dont fire in that order. To get 'even' firing impulses (every 180 degrees, two crank rotations to every firing stroke on 4/s motor)) they usually fire 1243 or 1342 but the second induces a more severe twist onto the crank.
I could write out complete process but my typing is too slow. (2 finger peck)
PJ

[750goes]

Clean your idle jets, and main jets, check for fuel flow, use fresh fuel, try again.... its a slow process.