Engine Won't Start

[Alan @Ncl]

Thanks, and good idea Jeff.  Here is a review and summary of the latest situation, in Q&A form, following your logic:

1   IS THERE FUEL IN THE CARBURETOR AT THE RIGHT LEVEL: Yes, checked with plastic pipe.  Level holds constant too.  Tickling enables flooding so that fuel flows out of the primary air hole.
2   DOES THE FUEL AND AIR GET MIXED IN AN ACCEPTABLE RATIO AND IS THIS MIXTURE DRAWN INTO THE ENGINE:  Not sure.  See below.
3   IS THERE EVIDENCE THAT THE AIR FUEL MIXTURE IS COMPRESSED IN THE CYLINDER: Yes.  My hand held tester suggests that both sides are fairly equal and usually show about 135psi.  It has shown as much as 185psi
4   IS THERE EVIDENCE OF A SPARK FROM THE MAGNETO: Yes.  Both plugs fire consistently with fat blue sparks when resting on the rocker box. New plugs, leads and non suppressed caps have been fitted.
5   DOES THE SPARK OCCUR AT THE RIGHT TIME AND IN THE RIGHT PLACE: Yes.  Timed accurately and checked at 3/8 BTDC on the compression stroke, with correct direction of rotation.  Also checked that leads are not reversed.  Even checked continuity from plug tip back down lead, through pickups and magneto secondary circuit to earth (3.9 kohms, total resistance) when at TDC.
6   IS THERE A SPARK WHEN PLUGS ARE IN THE CYLINDER AND UNDER COMPRESSION?  Not sure.  See below.

I know I have been a bit pedantic here but I am trying to ensure I have missed nothing.  The area of most uncertainty are Q2 and Q6.  I should know definitively the answer to Q6 when the 'live plug' tester arrives.  This leaves Q2 to which the following comments apply.

I am a bit ambiguous about plug wetness.  They do normally seem rather dry to me, especially given all the tickling and choking etc.  The exception was when I sprayed fuel in the pots and in the  carb intake.  My original suspicion was carb related as it did run, after a difficult start, before I stripped the carb.  However, it ran hot and upon trying to open the throttle or the air slide, even slightly, it would stall, even though hot.  After 2 or 3 thorough carb strip and cleans,  the best I get is an occasional weak splutter.  Have checked the pilot primary air flow path and it seems clear.  Both jets also seem clear.  Have experimented with air screw either side of the 1.5 turns setting and different throttle stop positions. Have used clean fresh fuel and even tried spraying easy-start down the intake.  Have cursed and kicked mightily.

I hope people are not getting too bored with this by now and am very grateful for all the interest and attempts to help, that have been offered so far.

[lawnmowerman]

Hi Alan

I am following this with great interest. I had similar problems but at least it started but run hot, backfired and spat back through the carb, Have cleaned out the carb and found a flake of dissoved petseal in the pilot jet acting like a flap valve. There was also a coating of crud inside the jet body. I tried cleaning it out but went for a new jet to be safe. I have not tried starting it since cleaning the carb.
This may have been a contributary factor to my problems as it happened after the winter layup, allowing any residual fuel to evaporate and leave residue.
The underlying problem since I have owned the bike is misfiring, running on one and backfires when starting from hot using the manual retard. I checked the timing and it was just over 1/2" BTDC so needed retiming - also the points were 15 thou so would have contributed to the advanced ignition - strangely enough is has never pinked.
I have just taken the mag off and the pinion was tight on the taper so does not look like it has slipped. I am taking the mag to a friend of mine who rebuilds them as I am sure that is causing the problems. At least when it is done I can rule out ignition problems.
I think that may be your way ahead if the spark indicator shows a problem.


Jim

[Alan @Ncl]

Thanks Jim
I have followed your thread too as there may indeed be overlaps.  I guess you have already tested your mag for a spark when turned by hand (I tried a slightly longer screw, temporarily in place of the safety screw, and it jumped a gap of about 3/16 inch, no bother).  Also, if you have a multitester you could check the resistances of the primary and secondary coils. Could also check that non suppressor caps are fitted and there is continuity of the HT circuit with nothing more than the mag secondary coil resistance.  Sorry if I am repeating what has already been said and tried but mag rebuilding is an expensive last option.  I will be interested to see what your mag man comes back with.  

As you say, it might still turn out that mine is a mag problem too.

I have attached a little schematic that I drew up while trying to suss out how the mag worked (me being a novice to all this).  Don't know if this helps anyone else.  After a couple of minor edits from a kind forum correspondent, I think this is now basically correct.

[MG]

Hello Alan!

Nice work with the schematic drawing, looks great. The condenser is actually fitted on the other side of the armature, but the electrical connections are absolutely correct. The original ones had 200nF (mind you, with a tolerance of probably 10-20%), so a modern 220nF equivalent does the job. I'm using self-healing MKP types, explicitly specified for impulse loading, which is very important. The 400VDC version of those can easily be accomodated in the armature, using the original bracket and some epoxy resin.
There seem to be some suitable ceramic condensers available now as well, but I don't think there are any long term experiences available for these yet?

Cheers, Markus

[Alan @Ncl]

Thanks Marcus
Glad you liked it and even more glad it seems basically ok.  Have now modified diagram to include your refinements.  I think I might just be brave enough to attempt changing a condenser, if it is suspect.  I know know who to ask if it comes to that.

Alan

[lawnmowerman]

Hi Alan

Just been out to the workshop and tested my Mag. I only have a cheapo digital meter but I am getting 0.4 ohms on the primary. The slipring is also in a bit of a state with a load of grooves and black deposits - it is almost like someone has sprayed lacquer over it as I really had to dig the meter probe in and make a scratch to get a reading and that varied between 40 and 95k.
Certainly not what I expected to see - I wonder if the PO has sprayed some sort of damp proofer in it and it has solidified.
Sounds like it is time for a rebuild! I will drop it in tomorrow morning.

Jim

[Alan @Ncl]

Hello Jim
Sounds high does'nt it?  But at least its something!  My old mag was showing open circuit but it still managed a weak spark.  But apparently this can happen if it is jumping an internal break.  Good luck tomorrow.  Keep us posted on progress.

Alan 

[groily]

Reckon you should be brave Alan, as they're really not that hard to replace if you can get the bearing inner race off the contact breaker end. Given that capacitors are cheap, and the most likely point of failure probably, there's nothing to lose and maybe a lot to gain.
I did a couple in the summer, using ceramic conden sers which came from Independent Ignition Supplies; one's on my A as a replacement for a premature failure on an SRM rebuild (and it has never run better than over the several months since), t'other is on the shelf in my spare mag for when anything else dies.

[Alan @Ncl]

Thanks G
Sounds encouraging.  Ceramic certainly sounds a bit more substantial than squashed up paper.  I think I saw those caps (£15.45 + VAT). 

I still need to establish first though, if there is a problem with the mag as currently installed, as at surface level it seems fine.  The old one was definitely knackered as it is showing open circuit reading on secondary and also has a gouged HT slipring.

If I do end up going in there, on my basically 'good maggy', I was hoping it might be possible to change a condenser without pulling the bearing and slipring.  But from what you say, this might not be the case.

Alan

[Alan @Ncl]

Thanks Marcus.  My mess up; certainly not yours.  What was I thinking of?  Getting too hung up over presentation rather than functionality.  I think it is correct now and tidied up a bit more so please let me know if I have done anything else foolish or ambiguous. Then maybe we can remove any older diagrams to leave only the latest version to avoid confusion.

I now understand more fully about the need to dismantle thanks to the comments from yourself and Groily.

Alan

[MG]

Perfect now Alan! I've removed my previous post with the scribbled drawing.

I found the old thread where I wrote about THE tool for removing mag bearings:
http://www.a7a10.net/forum/index.php/topic,2811.msg18714.html

[Alan @Ncl]

Thanks again MG for your help and for pointing me to this reference.  I now await the arrival of the plug tester (also allowing some time for my swollen right kicking ankle to recover).

Alan

[groily]

Yup, a bit pricier than rolled paper caps at that price, Alan, but mine seem good - so far (a few thousand miles only). Apparently there are 1000s in service, a lot of them on car mags. But no markings on them, so can't give you a spec.
And yup, you do have to pull the mag armature apart to get there BUT - with Markus' great chuck puller (I still want one!) or some other (cheaper!) device - once that bearing race is pulled the rest is simple enough. With your pukka schematic and maybe a decent exploded view (eg from the BSA Service Sheets), there should be no surprises.
Bit fiddly detaching the drive end to unsolder the old cap. and solder a new one in, and then it can be a fiddle to true up the armature on reassembly. There are dowels/pins, situated at 10 o'clock and 2 o'clock on the CB end (looking at the thing end-on the way it goes on the bike), and 180 degrees apart on the drive end although it isn't symmetrical. Plus the 2 through-screws accessible when the slip ring is off. Had the things been built for regular disassembly I think there would have been more attention paid to the longitudinal alignment, but as Uncle Joe was probably screwed into making the things for a £ a pop in those days . . .
If you haven't seen it, do have a look at Wilfried's workshop at www.brufnut.de - it's uplifting!.

[Alan @Ncl]

More good info here Groily, if it turns out I do find myself down this route.  I did see Wilifrieds document and it was fascinating but well beyond anything that I would dare to attempt.

Must admit that if it turns out that this mag is duff, I may start looking more closely at the Thorspark alternative option.  Think I may open/reactivate a thread on this while waiting.

Alan

[kiwipom]

hi guys/groily, what a great trip through Wilfrieds Workshop a special man with special talents, thanks for the website note it was certainly worth a visit,cheers
Bob