Topic: CB400F wont start

[seabird]

This _must_ have been covered sometime - but I can't find any references to the problem in any of the search criteria, so please excuse this age-old question:

MY '75 CB400F resto has now reached the point where I need to start it to synch the carbs.
However, the 'bike will not start, either on the starter or with the Kickstart - She has a Boyer-Bransden Electronic ignition system which produces a big fat blue spark and all four pots, and after frenzied kicking this morning fired for a couple of beats, several times, the last of which produced a "spit back" through one of the carbs (couldn't tell which one, as I was too bloody exhausted from kicking).

The carbs were cleaned and re-assembled, but not fitted until all the rest of the resto had been completed

My feeling is that the carburation is at fault, but until I can get it to fire up and idle, I'm stuffed (apart from being absolutely exhausted, my right thigh feels about the size of an elephant's leg).

Any offers from the Carb gurus would be seized upon happily!

Cheers!

Seabird

[GroovieGhoulie]

Is your electric start out?

Have you "bench-synced" the carbs?  This is where you use drill-bits or other similar-sized pieces of bar stock to get a rough sync on the carbs.  They will still be all over the place, but it should be good enough to get it started at least.

If the carbs are not even benh-synced, then you are not going to have much luck.

What about timing?  Is it set right?  Even if the carbs are way the hell off, it should at least fire up, even poorly.  It could be your timing is way off.

[seabird]

Hey GG, thanks for the thoughts:

My Electric start turns the engine over, but the cranking speed seems somewhat slow, even with a new battery.
The Carbs were cleaned and bench-synched by my local motorcycle engineering company
The timing (according to the instructions) is dead simple and as far as I can see, everything checks out.
To explain:  the advance/retard function is controlled by the digital "black box", the ignition doesn't use the bobweights at all - they are removed and the points setups are also redundant, now there are two magnets on a plate at the end of the c/shaft which spin past two posts as the crank revolves - this sends the impulses to the box which then adjusts the timing to suit engine conditions. Manual advance retard is done the usual way by rotation the backplate (which is a printed circuit board forward or backward for "fine tuning".
I should tell you that if the battery voltage drops below 9 volts, the electronic system closes down to save the battery.

I've been using the k/s, the engine turns over for a couple of beats when I get a good kick at it, and checking the plugs on the cylinder heads, all four show a good strong spark.  Hopefully if I can get it to idle I can use my strobe light to further improve matters timing - wise.

Just before I came indoors to type this, I got a couple of beats out of the engine, followed by a very healthy backfire - but that's all.
In the back of my mind (FWIW)!, I seem to remember that's a sign of too retarded, correct?

Sorry about the rambling !

Seabird
(Terry)

[GroovieGhoulie]

Generally too retarded, but backfiring can also mean lean.

[seabird]

Thanks Groovie - I'll check both out tomorrow - when my hands have thawed...

[crazypj]

Is the Boyer assembled correctly? (did you have it running with Boyer before rebuild?) Its possible timing is 180 degrees out.
Plug wires are correct? 1-4 and 2-3?

[seabird]

Is the Boyer assembled correctly? (did you have it running with Boyer before rebuild?) Its possible timing is 180 degrees out.
Plug wires are correct? 1-4 and 2-3?

HI Crazypi,

before the Boyer I had a very old Pirhana ignition in place which ran the engine for a few seconds before dying - so I replaced it with the Boyer.
Believe me, I've read the print right off the page to sort out theis problem - I can now confirm I have sparks ( big blue ones that easily jump an 1/8th inch gap) so that;'s progress of a sort (poor comnnection corrected).

This afternoon at the last despairing kick, she let go with a (very) loud backfire, so I guess I'll check the static timing as GrouvieGhoulie suggested before setting about the carburettors with an Irish screwdriver (A large hammer)!

Terry
Seabird

[crazypj]

You use the original ATD (advancer) and its possible to fit the rotor 180 degrees out. You could try just switching the wires at the coil

[oldbiker]

If you've got a friend nearby, put your bike into second gear and try push starting! This gives a good spin on the motor without taxing the battery. This will often help if it's the carburation thats out but not if it's the ignition timing thats wrong.

[seabird]

Thanks for the ideas guys - I rechecked the valve clearances this morning, to find they had all closed up, but after re-gapping the valves I still only get the occasional "catch" and no start-up.
I also checked the Ignition seetings for the electronic ignition and advanced it very slightly, with the same results
The rotor is exactly right - as far as I can see, it's impossible to fit it 180 degrees out - as long as the two magnets are positioned correctly in relation to the printed circuit backplate, which it is, with a very healthy spark on all four pots.
I drew plugs 1 and 4 after a session of frenzied kicking, but they show very little sign of being wet - doesn't seem to matter if the choke is fully open or fully closed or any postion in between, although most of the (occasional) firing comes when the choke/cold start lever is almost fully closed (richest?)

I'm leaning more and more toward the carburation being way off - got a couple of spit backs from a carb  during this morning's little tap-dance on the kickstart......

I hate to admit defeat, but it begins to look as if I'll have to send it off to my local M/C Engineering company again...

All this damned kicking isn't doing much for my artificial hips, either!

Seabird

[mack808]

I was reading all your responses and your answers but was wondering if you had pulled the top end off during the restoration or not?  If so maybe start with a couple of things. !st if bike still has center stand and you have access to 2nd bike also w/center stand put them tire to tire and you can use 2nd bike as starter to ease your battery and your legs life. If still no positive response, start with simple compression check on all cylinders, if all not over 90lbs you have problems, if all within 10 % of each other and compression above 90lbs try checking cam timing as well as the ignition advancers center cam lobe. they can be removed and reinstalled 180 out as someone else pointed out, that puts ignition on intake stroke. and also have you drained each carb individually to verifiy each bowl has fuel, and as a final item, this may sound stupid but I have seen this before, Have you verified fuel in tank is gasoline, not kerosene or diesel fuel and that the fuel is fresh or not heavily alcohol based. I have seen more than 1 bike not run after recent fueling, only to learn owner topped tank with either diesle from the wrong pump or kerosene placed in the wrong container......   good luck

[seabird]

mack808 - The top end was professionally restored, valves ground, new seals etc.. the pistons, rings wrist pins and circlips were replaced with newstandard since there was no ,measurable wear in the cylinders.

I don't have anyone with a 'bike anywhere near me, even if I trusted them to run my little darling up with their rear, so that's out!
I did the rough and ready compression check (Thumb over plug hole and turn the engine - all four blew my thumb off the hole (as I expected them to with good valve sealing and new pistons/rings).

I must say I hadn't thought tat the advance/retard unit was a cam - the ignition system doesn't use the bobweights and plate - all the advance/ retard is worked out by the Microdigital box of tricks - no loose connections and big sparks across the electrodes (All cylinders get a spark at the same time, but only the cylinder on the firing stroke goes Bang!)

Fuel-wise, the contents of the tank are fresh, and are definitely NOT Diesel or Kerosene (Parafin over here in UK)  The Carbs were cleaned and setup by the same guys the sorted the cylinder had out.

I'll take a look at the Advance/Retard unit tomorrow morning, when the defeats of today have ceased to dispirit me quite so much.

Thanks for you interest and ideas!

Seabird

[750goes]

have you tried just giving the carbs a good squirt of fuel straight down the throat, close the choke and then hit the electric starter??
if it runs  for a few seconds, it may just be blocked jets, backfiring caused by lots of things, check if all carbs are fitted onto the inlet boots and tightened up.

sounds to me that you may have a fuel blockage or starvation.. If your local engineers did the carb res-sync etc...do yourself a favour and have a loook at them yourself.

check each carb bowl for free flowing fuel..

keep at it

[crazypj]

I've used Pirhana (I think they are still on market as Newtronics?) only ever one Boyer (no experience of the 'new' microdigital) are you sure all 4 fire at the same time? I thought there was only 1 cutaway on the rotor. If there is only 1 cutaway you  can get the timing 180 degrees out. If it has 2 cutaways then all 4 fire at once. It really sounds like this is the problem. If its carburation you shouild get something, even if only a flooded motor.
PJ

[seabird]

HI PJ - thanks for your thoughts - much appreciated.

The rotor in the case of the Boyer-Bransden Microdigital ignition is a very simple device:
Picture a small steel disc about 1.5 inches in diameter - draw a line across the diagonal.
Now place two small magnets (0.25 inches rectangular on that diagonal.
This rotor fits onto the old Advance/Retard shaft, the original bobweights are removed from the shaft and discarded - all advance and retard calculations are now made by the Microdigital "black box" (which is in fact red - but I digress)...
On the new backplate (a printed circuit board) there are two coils with hexagonal metal poles sprouting from each coil - as the rotor spins as the crankshaft rotates, the magnets and coils create a current which is fed to the 703 coils atop the engine and connected to the spark plugs.  The system provides a spark to _all_ cylinders at the same time, but only the cylinder under compression (i.e., with petrol/air mixture in it also compressed) will fire.

As I kick the 'bike over, I get a "cough" from which ever cylinder is firing, but it's not sustained - in other words, it doesn't run.

Occasionally I get a "spitting back" from one carburettor as the engine rotates.

Extracting the plugs shows me that the plugs, to all intents and purposes are dry - hence my belief that the carburettors are not supplying sufficient fuel to mix with the air and allow the engine to run.

It's impossible to fit the rotor the wrong way round - there's no difference between the magnets, and there's no cam because there's nothing in the setup that runs off a cam, so it's coming down more and more as a carburation problem.

Hope this gives a clearer picture of the ignition system, and you're right - Piranha was bought by Newtronics some years ago, but essentially it's the same system - nothing appears to have changed.

Seabird

Terry Tyler

[Bodi]

My first impression is that you can't fire all 4 plugs together. When one pair (1-4 or 2-3) is good to fire - ie one is on compression and one is finishing exhaust - the other pair is unsafe, as one pot will have just filled with fresh fuel/air mix. If it sparks it may fire and you'll get a sweet backfire through the probably-still-open intake valve. I've accidentally swapped the points wires to the coils and seen the flames shoot out the carbs. I think any SOHC4 ignition needs to have 2 triggers 180 degrees apart and if so it is possible to have connected them reversed.
Anyway, try shooting a bit of gas or ether starting fluid into the carb barrels. If you have fuel delivery problems then the engine will fire and run a few seconds this way.

[Tim2005]

Er, starting with the obvious, are the fuel tap & pipes actually passing any fuel through? Best way to test is to turn the tap off,  remove drain screw from one float bowl (any), catch the fuel that come out in a spray can cap or something. Then hold that (emptied) cap there and turn the fuel tap back on again. Fuel should start flowing out in a second or 2.

[mb3000gt]

I dont see how all 4 plugs can be safely fired together. You should still see fresh gas on the plugs after trying to start it, so you still have a carb problem. I'd squirt some starter fluid in there-if it runs then you can eliminate the ignition as the source of the problem.

[seabird]

First off - thinking back to when the Piranha system was still in place, the engine ran for a few seconds each time I attempted to start it, so (to my dumb way of thinking) the timing/valve clearances and to some extent, the carburation weren't too far out.
Since then, all I've done is to replace the Piranha with the Boyer-Bransden system, and recheck the valve clearances.  I know the B-B system is set up correctly (with the exception of dynamic timing with my strobe - and I can't do that until the engine will run at the needed idle speed of 4,500rpm).
As for all cylinders having spark at the same time - here's what the "General Data Sheet" says =   

- With this system, All spark plugs spark at the same time, but only the cylinder under compression will fire".
(Remember, the advance/retard processes are now controlled by the CPU in the digital box).  The B-B system is tried and tested in many road bikes, and more to the point, has been used in racing machines for many years, including GP Cars, so it's not a "one shot wonder"!  <G>

My feeling is that it's coming round to it being a Carburation problem - it coughs a couple of times when kicked over with the choke 3/4 closed - it sounds "dry" though (old hands will know what I'm waffling about here), and occasionally spits back (not a backfire).

For the electrically minded among us, here's the wiring diagram of the ignition system installation:

Seabird

[hcritz]

Hello Seabird...
Very interesting setup...the wiring diagram doesn't show up very well, but it a appears that there are two pickup coils and two seperate lead to the coils...doesn't make much sense that all 4 plugs would fire at the same time. If that happens one cylinder will aways be in the intake cycle and would very likely get a backfire through the carb. Hopefully they are just firing two plugs with each of the pickup coils...like the original points did...not sure why else they would have seperat pickups and coil leads?
You should be able to static time to some extent and see if the magnets pass the coils at about the correct point with the firing marks.
If the timing is close...she should fire up and run if you put some fuel down the carbs...might want the fire extingusher handy in case it does spit out the carbs!!! a little spilled fuel can be bad!<G>
Hope you get her running!!!

[dusterdude]

when cylinders 1&4 are on compression stroke,2&3 are on the exhaust stroke,so therefore the fire is on dead cylinders and vice-versa.

[seabird]

when cylinders 1&4 are on compression stroke,2&3 are on the exhaust stroke,so therefore the fire is on dead cylinders and vice-versa.

You've got it DusterDude!  (See?  I'm _not_ crazy)!

Seabird

[dusterdude]

seabird,im glad i could confirm your sanity.

[hcritz]

Hey Guys...
When cly. 1 and 4 are at the top...isn't one on compression and at the top of exhaust!!!
Same with 2 and 3...on each revolution one cyl. will be on intake stroke...
Haven't looked at the crank...but figure it has two power stokes per revolution...and a firing order of something like 1342???

[hcritz]

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.

[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.