Author Topic: B31 spluttering and flashing back through carb  (Read 7730 times)

Offline john11668

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Re: B31 spluttering and flashing back through carb
« Reply #15 on: October 25, 2019, 01:03:53 PM »
Likewise rex .
We made the puller  but with the puller in place in the pinion and the jacking screw backed off a light tap on the end beds the  pinion onto the cone. But how would you suggest to prevent the pinnion frpm turning while you nip it up.
Ours was prettty tight when we first came to pull the pinnion and while we havent "strangled" the nut up since  it is natural to ensure it is tight ,
I have insstructions which say not to hold with a spanner at the contacts end   so jamming the gears would seem to be the only other option???


Offline iansoady

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Re: B31 spluttering and flashing back through carb
« Reply #16 on: October 25, 2019, 02:05:44 PM »
The nut doesn't need to be bar-tight as once nipped up the taper should hold everything together. I agree a sharp tap with a socket usually seats the taper. I would use some sort of sprag on the timing geas rather than holding the contact breaker end as that is feeble and easily damaged as said.
Ian
1952 Norton ES21955
Francis Barnett Cruiser 75

Offline Rex

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Re: B31 spluttering and flashing back through carb
« Reply #17 on: October 25, 2019, 04:31:26 PM »
If the taper is "grabbing" like it should then there should be no need to jam any gears for the tightening, but of you really must then use the time-honoured method of a piece of rag balled into the inward nip of the gears.
I prefer to give the end of the spanner or socket and tommy bar a whack with a dead-blow hammer for the final tightening.

Offline john11668

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Re: B31 spluttering and flashing back through carb
« Reply #18 on: October 25, 2019, 05:06:44 PM »
Thanks all
Will let you know the outcome

Offline chaterlea25

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Re: B31 spluttering and flashing back through carb
« Reply #19 on: October 25, 2019, 10:10:41 PM »
Hi ALl
I would not and do not use anything to wedge the gears
You already said you have a spanner to hold the crank, Uses It!! or top gear and apply the brake
The mag nut does not need  a long spanner or socket/ratchet to tighten it
A short ring spanner and hand tight is enough
Degrease the tapers with some brake cleaner or acetone, some sprinkle a tiny amount of chalk dust on the taper
if it tends to slip while tightening

John

Offline R

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Re: B31 spluttering and flashing back through carb
« Reply #20 on: October 26, 2019, 05:38:50 AM »
Have you removed the kickstart mechanism entirely though ?

Being able to easily start an old single is a good yardstick of testing if its in good tune.
Leccy starters often hide such details.

The ole pencil-down-the-plughole is also a simple yardstick of finding where the ignition timing is at,
and finding the tdc mark should likewise not be too difficult.

Offline john11668

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Re: B31 spluttering and flashing back through carb
« Reply #21 on: October 26, 2019, 10:26:38 AM »
We still have the kickstart and it does function.  Will give it a kick when we get it timed properly

Offline john11668

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Re: B31 spluttering and flashing back through carb
« Reply #22 on: October 26, 2019, 07:28:23 PM »
Right !
So found TDC and marked it on the sprag wheel with a scribed line and pops .
Then did likewise for 7/16" before TDC  and did likewise   (See pic 1)

Then timed it as per instructions above at 7/16 before  and checked on a couple of forward rotations  before locking it up . Then fitted a brand new plug , and started it , (did not start easily)  but got it running  at a fastish idle, and checked with a timing light.
Quite happy that it is near as dammit .  See pic 2 showing the TDC mark under the strobe at ful retard.

Advancing the lever brings the 7/16  mark onto the reference  with a strobe reading of about 35 degrees advance and the idle picks up (sorry but I did not have a tacho with me )

All then sounds pretty reasonable  until you try to reduce choke  and all the popping spluttering and backfiring starts again. Will not run at all without full choke.
Does this  whether you have the old carb on , or the new one which was supplied by "draganfly" and said to be set up for this bike.  of course we have adjusted slow running and idle screws and checked that needle is in the same (mid)  groove in each case.
See pic 3 . Bike has run happily for months on the old one  until it started to play up. New one is no better.

Of course carburation is now the suspect but I am not sure  :(

Any other suggestions please?   Can valve timing be known to move on these.
What else could it be ?
 

Offline Rex

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Re: B31 spluttering and flashing back through carb
« Reply #23 on: October 26, 2019, 07:40:12 PM »
Full retard has no role in the timing process. In fact, on some bikes full retard is ATDC. It's the fully advanced figure you need to concentrate on here.
I don't get the bit about marks and pops on the sprag wheel. I prefer the time-honoured method of a pencil/lolly stick/cocktail stick down the plug 'ole, then there's no need to account for lash etc.
If there was a fault with the carb(s) like a blocked pilot jet, then no amount of timing resets etc would cure it, but it seems a bit coincidental that both carbs show the same symptoms.

Offline chaterlea25

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Re: B31 spluttering and flashing back through carb
« Reply #24 on: October 26, 2019, 09:33:42 PM »
Hi All,
Quote
Can valve timing be known to move on these.

Not unless something jammed the timing gears and sheared the key in the crank pinion
Remove the tappet cover, wind the engine to TDC, if both tappets are free (the pushrods should be free to rotate at TDC on compression stroke) then turn the crank one more complete turn
As the piston rocks over TDC you should be able to see the exhaust tappet dropping and inlet rising this should happen equal amounts either side of TDC

I now see you have a a concentric, The pilot passage bush within the body gets blocked,  a 16 thou guitar string or a no. 78 drill can be used to poke through the pressed in pilot bush
It has happened that new concentrics are not set up properly or have swarf and so on inside

John

Offline R

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Re: B31 spluttering and flashing back through carb
« Reply #25 on: October 26, 2019, 09:52:06 PM »
Few engines even fully retarded would get right back to a tdc spark, the range of movement is usually limited to prevent that.
And as mentioned previously, many magdynos won't even spark if fully retarded.

If the engine will run with lots of choke on, then mechanically there can't be too much wrong with it. ?
Its either got a massive air leak, or perhaps the ignition source got shoogled up and isn't in the best of health.
Or the jetting for these concentrics is way out, but somehow it contrived to run earlier ??

Note that there have been reports of new concentrics where the drillings weren't entirely completed,
or had swarf in the passageways - so a new carb isn't entirely above suspicion ?

Offline john11668

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Re: B31 spluttering and flashing back through carb
« Reply #26 on: October 26, 2019, 10:11:31 PM »
I see your point about full retard Rex  but similarly I am presuming that full advance only comes into play once you get motoring.
We timed it in the "time honoured fashion" with bamboo coffee stirrer and turned a couple of revs to ensure we had consistencey at the 7/16" point before starting.
Only used the timing light and pop marks so we could observe that mothing was really changing on fire up, and we see the advance lever alter the timing from around TDC at full retard,  to the 7/16" BTDC at full advance, so timing would seem to be spot on.
As you say, carburation is maybe  unlikely to be suspect when a proven carburettor exhibits the same symptoms as a new one.

So what else could be affecting this ?  Is there anything in the gear train which can go wrong which might (for example) knock valve timing out of kilter ?  We havent yet  looked at the cam gears  but maybe have to go there next .  Would help though if we could find out if there are any known problems in this area. 

We intend to look at Valve timing next Chaterlea. I understand the crankshaft gear is on a woodruff key so will look at that first  then follow your instructions on the valve positions .  The new concentric  now has a removable pilot jet  which we have checked and seems clean.
The fact that both behave in the same manner seems more than a coincidence  .  Would  a duff pilot jet cause the bike to splutter and die  with a small reduction from full choke?
We are not aware that gears ever jammed and from first sight there is no sign of damage . but will look further

R,  The AR lever shifts the cam plate quite a long way, so I was not surprised to see full retard taking it back to TDC. Full advance speeds the engine up quite a lot  on full choke but only brings firing point to our 7/16" mark and no further so we dont think timing is moving as we start to run.  Bike got quite well warmed up  but reducing choke even a little  caused spluttering and  dieing  so well puzzled really. Seems to run best at about 1/3 advance  but still dies if choke is reduced

Offline R

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Re: B31 spluttering and flashing back through carb
« Reply #27 on: October 27, 2019, 01:13:12 AM »
If you give it a big handfull of throttle while reducing the choke, what happens. ?
Even if the idle circuitry is all blocked up, and it runs happily on a big handfull, its a blocked up
carb on the low speed side of things (VERY common in amals left unused for a good while)
It sounds like the mixture is woefully weak - or there is a massive air leak someplace.

Broken valve spring(s) can produce the same symptoms - but with double (or triple ?) springs,
shouldn't be so drastic.  Cracked cylinderhead is also possible = big air leak - seems unlikely in
combination with the magdyno coming loose.

You still haven't said ? if its got a magdyno (or electronic ignition ?), but ailing ignition where the
plug can't deal with sparking under pressure isn't unknown.
Magneto can do this to you, but they generally fail slowly and give you a bit of warning.
Dirty points in a magdyno also will do this, you inspected/cleaned the points lately ?

Its there someplace, you just need to find it.
« Last Edit: October 27, 2019, 01:17:17 AM by R »

Offline john11668

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Re: B31 spluttering and flashing back through carb
« Reply #28 on: October 27, 2019, 09:34:00 AM »
Sorry I missed the question but it is Magdyno . Points have been throughly  cleaned and adjusted  (0.012")
Had been running  regularly and sweetly for months( rather than left unused) and just suddenly went  pear shaped on a short local run  at which point it was noticed that the Dynamo was loose.  Spark seems strong but cant tell what is going on when it is in the cylinder. Have tried several plugs   two of which were new . If  spark was week would the strobe operate reliably?

I Dont think that increasing revs  improves the situation regarding   the choke . We dont rev the guts out of this old lady.  it seldom excedds 45 mph on a run , Maybe 50 downhill  but will try the process just to eliminate that possibility.

Will  start  looking closely this week for air leaks,  valve timing ptoblems ,  etc

Offline chaterlea25

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Re: B31 spluttering and flashing back through carb
« Reply #29 on: October 28, 2019, 12:01:50 AM »
Hi Again,
Try squiring some petrol with a spray bottle into the intake with the engine running as you open the throttle
If the engine revs up as you do this then its a carb problem
A quick check of the valve springs can be done by levering up the tappets through the inspection cover
Both should have equal pressure
A sticky inlet valve is another possibility

I had an A10 in for repair that showed great compression but would only run on one cylinder when started
one cylinder's inlet valve springs were soft as putty  ::)

John