mag sparks

[Guy Wilson]

I have flat pistons in the cylinders. I think there is enough clearance ...( I hope)
Guy

[Bonnieboy]

Hi guy,
               I had exactly the same problem  after a complete rebuild it would not start I was useing a bth electronic mag again the spark was ok even changed the mag for
my spare bth again nothing put the bike on my home made electric roller start nothing kept tickleling the carb spraying  down the carb nothing.I was begiinning to think
maybe the new conrods are wrong. checked the valve timing twice checked the ignition  3 times nothing, a friend said try my k2f i already had a auto advancd gear so
replaced the bth with the k2f placed the bike on my rollers tickled the carb sprayed down the carb started the rollers it was coughing banking fireing banking poping
and eventually it was running,
         The spark plugs never appeared to be wet almost certain the carburettor is at fauilt will order a new one,the thing now is do I stick with bth although never had a problem
or go with k2f. your thoughts

                 Bonnieboy

[Guy Wilson]

I've a had a hiatus since my last post...(and thank you BonnieBoy.. you seem to have at the same experience...)

I'm a bit further along but still with no luck and doubting I ever had any ability with old bikes... it still won't start.. I'm building the bike with my son who is beginning to lose patience and interest..which would be a disaster!

I've checked the static timing, swapped over a carb that I know ran my other A10 and even though it felt wrong to do so, took the mag of my running bike to try on the non running bike.. still nothing... compression is even in both cylinders and compatible with my other Gold Flash.

the only 'clue' I have is in desperation, I started timing the mag in 5 degrees increments and pretty soon got a vicious kick back..so the mag seems to be igniting the fuel...

The valve timing appears correct and I've tuned the engine over slowing following the order of the valves..

I'm going to check the drive of the crank hasn't slipped or lost the wudruff key although that seems like a stretch and I'm pretty sure I can see the key with the oil pump off...

Any other suggestions welcome!

thank you!
Guy

[Rex]

So clearly the juice is getting to the cylinders, the sparks are sparking and there's enough compression to make things work.
I have a Velo Venom which (after a long-term rebuild from a pile of parts) behaved much the same. After going through it all several times I put a degree disc on the crankshaft and checked both the valve and ign timing to the degree.
Turns out the ign timing was close enough to run but the valve timing was out despite it being correct to the cam wheel markings. The engine hadn't been apart for years so wasn't a mismatch of parts, but how long had the various owners put up with difficult starting and just put it down to Velo recalcitrance?
Anyway, I'd check the valve timing if I were you, then you can cross that off the list of potential problems.

[Guy Wilson]

thanks Rex.. I'm moving back to the mechanical side if things and the valves are next .. it seems logical.. it's easy to go down a rabbit when you're on your own... !
Guy

[Swarfcut]

 Earlier posts mention a "bag of bits"....

 Reading back, a place to start would be the basics of the timing gears, crank, cam, idler gear and their static relationship. Firstly, are they all the correct gears as regards timing marks? Longstroke gears are identical in in form, ratio, etc, but the timing marks differ. More info in the Longstroke section, where this problem has been encountered, and the gears are illustrated so the difference is obvious.

 Next, are all keyways/keys as they should be? All assembled correctly timing marks to spec, and finally a working mag set to fire at whatever BTDC? If the valve clearances are set, valve gear doing its job, oiled bore and sparks in the right time and place, a slug of gas and it should at least fire. If it doesn't, well that's me leaving stage left, as an aside, leave the mag end cap off to avoid any unintended kill switch fault.


 Swarfy.

[limeyrob]

Remind me, how is the mag?  I ask becuse I wasted hours trying to get my bike to start after the rebuild and it was a blocked idle jet and a duff mag.  I'd stripped and cleaned the mag and it gave a decent (or so it seemed) spark when hand turning so I didn't suspect it.  Frustrated I built another out of the parts box and it looked to give a better spark so I got them both on the bench with some plugs and a socket hand brace and turned (using the drive nut and a socket) each at 90-100 rpm ie about 1 1/2 turns a second.  The mag that I thought was good would only spark on one cylinder about 50%, the parts box special gave a decent spark on both about 95% of the time at under 100 rpm.
Fitted it and it fired 2nd kick.
I think the problem with hand flicking the mag is that its actually nearer 150 rpm instantaneous whereas starting needs consistent sparks at as steady 80-90 rpm.  I certainly fooled myself into thinking it was OK.

[Guy Wilson]

as its now fitted with a carb and a mag that I know run my other bike, I'm inclined to explore the mechanical and static timing.. the odd thing is that it run once a month or so ago after a lot of kicking over and it refused after that.. we tired bump starting, fuel in the plug holes, spraying quick start in the inlet etc...
Question: if the crank is 180 degrees out on the static timing..what does that do to the valve timing? is it 90 degrees out on the cam timing?
Guy

[limeyrob]

When you build up the crank both cylinders are the same, which stroke becomes which is set when you time up the camshaft.  The camshaft sets which cylinder is on compression and which on exhaust, that then sets which way the slip ring on the mag faces for the timing.  Since its near impossible to see where the sip ring is with the mag on the bike you can get an idea from where the points are.
You say it ran that stopped and wont restart.  That suggests to me that something moved:
The key on the crank pinion (oil pump off, LH thread)
The key on the cam.
The timing of the mag
The mag points (the little key on the taper)
Is is auto A/R? If so are you wedging to full advance OK and is it moving properly.
Check the tappet clearances in case a push-rod has moved.

[Guy Wilson]

Earlier posts mention a "bag of bits"....

 Reading back, a place to start would be the basics of the timing gears, crank, cam, idler gear and their static relationship. Firstly, are they all the correct gears as regards timing marks? Longstroke gears are identical in in form, ratio, etc, but the timing marks differ. More info in the Longstroke section, where this problem has been encountered, and the gears are illustrated so the difference is obvious.

 Next, are all keyways/keys as they should be? All assembled correctly timing marks to spec, and finally a working mag set to fire at whatever BTDC? If the valve clearances are set, valve gear doing its job, oiled bore and sparks in the right time and place, a slug of gas and it should at least fire. If it doesn't, well that's me leaving stage left, as an aside, leave the mag end cap off to avoid any unintended kill switch fault.


 Swarfy.

Swarfy's ideas are interesting and that's where I'm going next..
the mag is correctly timed with the advance fully advanced BTDC.. I suspect it could be something to do with the timing marks on the crank to the valve timing being off...to be continued...
thank you !
Guy

[Guy Wilson]

So again..after a brief break to go and do other things financial.. I came back to the A10 and the refusal to start.
I'd switched the mag from my running A10 and also a carb that the other A10 runs on.. The refusal to start is still there..
I went through the valve timing with a degree plate on the crank and it all appears to be doing what it should, when it should.. so I have assumed (I know I shouldn't make assumptions) that the valve and static timing is correct. I set the Ignition timing at 32 degrees BTC with the auto advance fully advanced - its roughly 8.7mm before TDC if I backed the piston back from TDC.. still nothing..so I began retarding the mag timing in increments of about 5 degrees on the Mag.. it kicked back pretty strongly and after a few more adjustments, it fired up and ran well for a minute or so.. it started a few times after that and then refused again.. I rather stupidly didn't check the Mag timing to see where it was when it was running.. Nothing is loose.. and nothing appears to have shifted or moved...
The compression reading is the same in both cylinders and is almost identical to my other A10.. the engine that is proving hard to start does have non-standard pistons in - I think from a A series car - not 100% sure where the are from..
Anyone else had this sort of conundrum?
Guy

[Guy Wilson]

these are the pistons its running on ... the tops are not a bad as they look in the picture now..