B31 Poor Performance

[Steve.S]

I forgot to mention...I think a spark jumping a gap of 3/16" indicates 8,000 volts. The magneto should produce more than that. A good magneto will jump a gap of 5/16" or 15,000 volts, but don't go any wider than that or you may damage the windings. I believe you can now buy Spark Gap Testers. If 3/16" is the maximum gap it will jump, maybe that's your problem. Over the years I have had many unsuccessful rewinds done by professionals, who blithely say, just send it back and we'll do it again.
I hope you get it sorted.

[militaryron]

Steve I'm following this thread out of interest as I'm restoring a rare WD B30 which is essentially the predecessor to the B31.

You mention the timing is set at full retard?  If so I guess this must be at TDC? (I haven't got a B31 spec sheet) The B30 timing is 1/2" BTDC on full advance. (All my WD bikes are set at full advance) I think the only other difference being that all mine are tight wire advance, by post war they are slack wire advance.

Could you please post up the B31 spec to allay my confusion.

If indeed the timing is erroneously set at retard instead of advance, then it will be gutless and probably run extremely hot.   Ron 

[yellow B31]

by post war they are slack wire advance.

My 1949 B31 is tight wire advance.

[militaryron]

Yes it works just as well both ways, although slack wire makes more sense because if your cable brakes, it defaults to full advance.

For the wartime bikes, it was just Royal Enfield who fitted slack wire mags. I don't know exactly when the complete change to slack wire commenced.  Ron

[Steve.S]

Ron,
Sorry to have caused confusion. It was BSA who said that timing should be set on full retard at TDC with a 0.002" points gap (or something like that).
Bit I completely agree with you, and set mine on full advance. I think 1/2" BTDC would be a good place to start and if it doesn't pink, maybe increase it to 9/16" or even 5/8" depending on what fuel you are using. I believe this equates to about 35 to 40 degrees of advance. However, each to his/her own, and whatever works best for you. I've never checked the timing on full retard, but I suppose if you're suffering from kickbacks it might be interesting.
Sorry, I don't have any specs list for the B31, but I'm sure someone does.

[idie]

B31 ignition timing on full advance should be 7/16 BTDC

[martinh10]

I'm happy that my timing is spot on at 7/16 fully advanced. I've tried settings either side of that.

[neil1964]

Have you actually checked the valve timing with a degree disc (I guess you have.....) ?
Set tappet at 18thou for this
IO 25 degrees BTDC and closes 65 degrees after BDC
EO 65 degrees BBDC and closes 25 degrees after
Lift should be 0.3"
Might be fun giving it a spin on an open pipe for a bit if your neighbours are understanding!

[martinh10]

I've not actually done this yet - I'll make it my next job.