Kind of following on from my earlier posts on the subject but as it's over 3 months since I've been able to visit the bike I thought a new post in order.
It's a rebuilt-by-me SS90, starts fine, idles fine, picks up and revs fine in neutral.
Pulling away, it misses badly from about 1/2 throttle in every gear. The problem is only there under load i.e. back on the stand in neutral it revs smoothly.
So far I have:
Converted to 12V and replaced the points with a Boyer KIT00059
Replaced the coil, HT lead, cap and plug
Replaced the original monobloc with a new concentric from Burlen configured by them with 170cc Main Jet, .106" Needle Jet, 19 Pilot Jet and a 3 1/2 Throttle Valve
Checked the valve clearances and valve lifter setup
Checked and rechecked ign timing
Changed the fuel
Checked good flow of fuel into carb
Sacrificed a goat
None of the above has made any difference and I'm now running out of ideas.
Engine was assembled using the RR book, I've stuck with stock valve clearances for now.
I thought that the small end bush showed a very small amount of wear on the gudgeon pin but I haven't replaced it.
Any thoughts on further diagnosis or possible causes please?
Many thanks,
Bernie
Do you ride with the headlight on? I ask because I'm wondering if the battery charging is a bit marginal; I assume that you wouldn't test or start it with the lights on and it would be okay. Then pulling away with the lights on, it's all too much, the voltage drops, and you get a misfire. Even if you don't use the headlight, I suggest you check the voltage.
If the voltage at the battery is okay, check for the same voltage (or maybe very slightly less) across live and neutral as close as you can get to the ignition module; there might be a poor connection. Electronic ignitions often struggle if the voltage isn't up to spec.
(Edited) I've just seen a wiring diagram; of course you have an ammeter. Still do the checks, though.
If you ride it at high revs with small throttle opening, does it misfire?
It could be a carburettor issue.
Hi Bernie
Considering a recent experience I encountered on my B40, I wonder whether this may help ?
Inadvertently, I ran my bike with the ignition switch in the emergency start position and encountered exactly what you describe, misfire under load. This was soon sorted en-route once I realised my mistake. I wonder if during the re-build there may be a wiring / switch issue and the bike is running in the emergency start position ? (If your SS90 has emergency start, that is) Switching on the headlight whist on the road may give a indication if it's an electrical issue by adding more load thus potentially increasing the misfire.
Just a thought.. however, it could be one of many things, Perhaps the carb intake could be blocked with stray Goat's fur !
Good luck anyway.
Cheers
Colin
Rereading your initial post, you mentioned the settings of your Concentric.
If you check a recent post in https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=8381.msg43899#new
you'll find a difference in main jet.
Thanks for all your replies.
I'll wire a voltmeter up for the next test and see if I can get it to misbehave on the stand by engaging a gear and applying a bit or rear brake - will be easier than having to ride it to reproduce the problem. Charging seems OK from the ammeter but as was mentioned - good idea to check everything.
On the carb settings, I took a look at the link to the earlier thread and then found some more info on Drags site, part of which I've reproduced below:
B40 1961-65 376 1 1/16 190 3 3 .105 20
B40 USA 62-63 389 1 1/8 160 3.5 3 .106 30
B40 SS90 62-65 389 1 1/8 200 3.5 3 .106 30
B40E 1965 376 1 1/16 190 3 3 .105 20
B40 WD 1969 626 26 130 3 2 .106 20
This got me wondering whether the main jet size should be the same for a monobloc and a concentric when used for the same application?
If so then maybe I should actually fit a larger (200) jet as per the above....
More testing next week and I'll update further.
Cheers,
Bernie
RR Rule of Thumb (pg 197) says a concentric should need a reduction in the main jet of 20 compared to a monobloc of the same size ....Plus lower the needle 1 groove ( if available)
Main jet size may depend on what kind of air filter you have.
Wouldn't the wrong jet size either cause starvation or too big then bogging down and feel like it's running out of puff rather than misfiring? Sounds electrical to me but as the OP has done so much, at a bit of a loss to think of what exactly.
Spent some time today diagnosing. Connected my timing strobe and noticed missing flashes coinciding with missing firing - so indicating electrics as many of you have suggested.
Last thing to be checked.... the original ignition switch is the culprit. Kicking myself of course for not checking that earlier and kind of feeling sorry for the goat.
Still, a temporary replacement tomorrow and hopefully a nice long run without any hiccups!
Thanks again all.
Cheers,
Bernie
Well done Bernie,
You got there in the end. As you say, poor goat or perhaps it was a new begining and therefore not totally in vain.
Well done; I bet you're pleased. It might be that the goat sacrifice worked, just not in the way you expected.
You've got me thinking, now; there's a rare breeds farm not too far from me, and though they'd notice if a goat went missing, they do have quite a few reindeer...
You could give it a try Rog - I believe there is a league table of sacrificial values maintained by the Gods - goats are I believe only second but I had to settle for that due to availability constraints :)
Cheers,
Bernie