B31 spluttering and flashing back through carb

[john11668]

This is about the bike we converted to electric start, which for about a year has run sweetly with no significant problems. http://classicmotorcycleforum.com/index.php?topic=5722.msg26756#msg26756

Recently it started spluttering while out on a run and had to be recovered to the workshop .
We found that the clamp retaining the dynamo and Mag  had worked  loose allowing the dynamo to move,  and we wondered if the timing might have jumped a tooth and set about checking.

It asks for 7/16” before TDC at  full advance .   We found it at about 1.7” so adjusted back to  7/16".  It then would not start at all.  Changed the plug and got it to fire but was still running rough and popping back through the carb.  No matter where we set the advance lever , it still backfired when we eased the choke back to normal running position.
Then tried at 7/16” btdc with full retard,  (on the basis that we could advance it on the lever to find a sweet spot.)   We were unable to find a point where it would run without choke, yet without popping and flashing back through the carb.  Full advance takes it to about the 1.7” btdc  figure , which is where we found it.

Turning to carburettor we  tried all the needle positions of the carb ( Amal 928) and no improvement. Just a bit more smoke at highest needle position
Even tried a new carb which does  exactly  the same .   I know the proper carb should be Amal 276  but the bike had always had this one and it has run well since we recommissioned it .

We have checked compression (115-120 psi  cranking).   We have tried a couple of new plugs too.  Contact breaker points  gap is 0.012” 
Despondency is starting to take hold  of us .
Can anyone point us in the right direction.  If we are going wrong we just cant see where.
Any assistance will be much appreciated 

John

[john.k]

Sounds like you got a blockage in the fuel somewhere.....is there a fine gauze strainer  in the threaded connection to the tank.......might be blocked........anyway ,check the fuel flow to the carby ,with the cap on the tank(sometimes cap breathers cause problems).....also possible the capacitor in the mag has given up ....or even the windings ....but replace the capacitor first.

[R]

Yes, are you sure that a flush of rust particles hasn't come down through the fuel system,
this creates problems like you describe - and it can take a significant amount of cleaning
to ensure they are all gone and all the jets are clear. Particularly on the idle side of things.

Did that 1.7" inches of advance correspond to one tooth out on the timing chain ?
I'd triple check that your spark advance is still where it is quoted it should be, too much spark
advance can't be tuned out and could present as if it was too lean.
So might it be if it was too retarded too ?

Remember, 90% of electrical problems are carburettor, and vice versa !!

[john.k]

Gear train to mag.....if it has "jumped" one tooth ,then the gears will be a mess.......must say Ive been inundated with silly timing claims by  newbies who dont know how to remove backlash  before checking the timing........One even asking about his car,I could clearly hear the clatter of valves hitting pistons ,cambelt timing one tooth out.......he says he cant do it any better......must be the engines fault.............Good story tho....millenial says "can you have a look at my car?".......No ,I  dont know nothin about cars....."What kind of mechanic are you ,doesnt know about cars"......".A bl**dy smart one." sez I.

[john11668]

Tank has a new tap and filter , and we have been testing on a seperate (lawn mower)   tank.   Much easier to to measure 7/16" Btdc with tank off the bike .
Clear plastic pipe and new banjo filter so pretty certain we have got control of  the muck.  Added to new carb which is also spotless and behaves just the same.

Am  now wondering whether to go back to 7/16" on full advance  and another new plug  just in case we had a duff one .
What is the best plug for a B31

[Rex]

By fannying around with the carb settings you've introduced a lot more variables though.
If the problem was  slipped timing then that's the area you should have concentrated on especially if the fuelling wasn't giving concerns before. Don't know the settings for a B31 but 7/16 sounds a lot. Just set my Star Twin at 5/16 as per the book.
I'd recheck the timing personally. BTW have you done the timing successfully in the past, or is this a first time for you?

[john11668]

Gear train to mag.....if it has "jumped" one tooth ,then the gears will be a mess.......must say Ive been inundated with silly timing claims by  newbies who dont know how to remove backlash  before checking the timing........One even asking about his car,I could clearly hear the clatter of valves hitting pistons ,cambelt timing one tooth out.......he says he cant do it any better......must be the engines fault.............Good story tho....millenial says "can you have a look at my car?".......No ,I  dont know nothin about cars....."What kind of mechanic are you ,doesnt know about cars"......".A bl**dy smart one." sez I.

As you say John K ,  B31 has a gear train . but there were no signs of damage.  I dont think the mag pinion would have slipped on the taper seat either as it was pretty tight,  but when the books say 7/16" and we were looking at 1.7 inches something must have moved. I inderstand 7/16 equates to about 38 degrees (which seems a lot) ,  no Idea what 1.7  " equates to but must be close to half the stroke so say 90 degrees ,  so something has to be well out of kilter .
The mag on these is held down on shims by the dynamo clamp and even with all tight there can be lateral movement between the gears which can be  seen if you Jam the gears with a screwdriver blade for final tightening . Under normal  circumstances the backlash is low so wouldnt expect it to give more than a couple of degrees error btdc.  We did however try to ensure that the backlash was "taken up " as we clamped the pinion back on.
We are not really newbies , but would own up to being unfamiliar with B31  mainly because we have had no trouble up to press.

Is the figure of 7/16" btdc correct ?   and should this be a full advance figure?  If so with the range of movement on the plate cam this would take us well into the retard  range if AR lever is at full retard .

I think we are back to original carb settings Rex but I tend to agree with the fannying around remark. Being honest it was other "experts" who introduced that element.  Timing (on this bike)  was a first for us cos it gave no problems previously and I have to say I was a bit rusty, but we made a proper puller for the pinion and the fag paper is released at the 7/16" point. .
Will go back to that figure at full advance , and try again with a new plug.
Were working to the following docs , but started to wonder whem the bike would not start after this procedure

[Rex]

The backlash in the timing gears for this single cylinder mag (MO1L?) has always been it's problem, and I'm sure there used to be shims available to put under the mag base to reduce it.
That said, as long as you wind the engine backwards far enough, then forwards to the correct timing point (always full advance) the backlash shouldn't matter. It's not a track Goldie after all..

[chaterlea25]

Hi All,
Check for any end play or vertical movement of the magneto armature in its housing, running with a loose mag will damage the bearing insulators
If the armature has movement timing is impossible without first getting this sorted
B31 has usually a tight wire for full advance magdyno, so the timing must be set with the cable pulled all the way
then set points opening to 7/16in BTDC

If the mag has been left run loose it will damage the fixing dowel location slots in the crankcase
then tightening the strap will pull the unit towards the engine causing too tight a mesh and a gear "whine" when running
A dodge is to wedge the mag body back so the shaft is central in its oil seal housing by fitting an alloy strip  of suitable thickness between the mag and strap at the base of the mag
run a strip of paper through between the mag and idler gear while turning the engine, it should be deformed but not cut into tiny strips
Then if necessary place shims as required to correct the gear mesh between crankcase and mag body

£1 to  penny the mag is in trouble
John

[john11668]

I think that extra shims would increase the backlash Rex, but backlash seems fine here so will try again at 7/16" , then turn it onward (manually) a  few revs to check that it is releasing the rizla at the correct  point each turn. That should make sure the backlash is taken up properly. I have a "C" spanner which allows me to turn it directly (on the crank) and fairly precisely . Then will try starting with new plug  and report back. May even add a degree wheel onto the attached stub shaft so we can see what is going on.

Thanks for all the input . Am sure we will get there in the end .
I think one of the great things about a forum discussion is that it really does help you to get your own thoughts in order

Thanks for your recent contribution chaterlea. While we do the above will take the mag pinnion right out and have a close look. The backlash seems reasonable  and there is no discernable whine  but will  have a close look at the mesh with the strip of paper.  As I said before there appears to be a small amount of flex apparent  when you jam the gears to tighten the pinion onto its taper.  As you say it is tight wire . 
After timing is it reasonable to start with full retard, and then advance a bit once running?
If it turns out that mag is damaged are parts available for DIY repair

[R]

Not knowing what sort of ignition you actually have, as a general rule, a lot of Lucas type magnetos and magdynos
won't give a decent spark if you give it FULL retard.

Best try half retarded, unless that causes some kickback.
In which case you need to kick it harder !!, after decompressing of course.

[john11668]

We dont do kicking R.
Bike has electric start .   It usually starts with no problems  just  coughs and splutters back.

Are we generally of the opinion that running with more than the 7/16" BTDC  advance  will cause the spluttering and popping back which the thread is all about

[john.k]

mention of jamming a screwdriver between gear teeth to tighten/loosen the nut,then mention of "flexing" of the magneto shaft............hopefully .being aware that the mag shaft is a stub  press fitted into a brass armature end,and quite easy to bend /make loose in the brass piece.......the catch being the brass piece is a matched assy with the rest of the armature,and wont be amenable to individual parts swapping........moral of the story being you cant fix cuckoo clocks with a pipe wrench,not very well anyway.

[john11668]

Valid remark john K but instructions do say that you should  not  hold the CB rotor with a spanner  due to weaknesses in the shaft .

 So how else do you  stop the mag shaft from rotating while you nip the pinnion onto the taper . 
We are engineers! We are not talking about swinging on a Stilson wrench here, and  the  clamp holding the mag dow in is hardly the most positive fixing  so a bit of flexing was not a surprise!

Please tell us the correct way so that we avoid  future mistakes  as this pinion may have to be pulled off a time or two yet.

[Rex]

Personally I loosely fit the timing pinion, and with the engine at it's correct firing point BTDC and the lever at full advance (plus points just opening obviously) use a suitably large socket to give the timing pinion (not the shaft!) a light tap to lock it onto the shaft, recheck the settings and if all OK then tighten the mag pinion.