BSA Gold Star misfiring

[bigjos]

Hi, looking for help once again....after spending a year refurbishing an old 350 gold star everything is now up together and looking good. It starts and runs fine in the workshop but when I take it out for a test ride it misfires as I try to accelerate, so that I have to ease the throttle back then just open gently to get some revs going - and even then it won’t rev very high.
In the workshop it seems to rev up ok......
I’m thinking it’s a carb. / fuel issue but not sure how to sort it?
Any tips appreciated.

[iansoady]

I'd be looking at blocked main / needle jet to start with, but could easily be a magneto issue. Could also be poor fuel flow - blocked tap etc.

[Rex]

The same symptoms as my recently-completed A7. Turned out it was a failing capacitor in the (rebuilt) mag.

[john.k]

Assume you have checked that you have no supressed/high impedance ignition components?......also will assume you are aware they double magneto voltage and cause failure even in rebuilt mags?......but symptoms sound a bit like a faulty spark plug.

[antoni]

The same symptoms as my recently-completed A7. Turned out it was a failing capacitor in the (rebuilt) mag.

Can't usually offer any advice on here, but this time...

I can say that condensers, which are capacitors, are often unused OLD stock. That doesn't help when a common failure-mode of the condensers is simply their age and not how much use they've had. Don't give your full trust in a NOS condenser.

[Rex]

This one wasn't, it was  small plastic component.
Only DiYers would ever use a NOS capacitor; pro rebuilders wouldn't want the risk of early failure, which is bound to happen with a NOS capacitor 50+ years old.

[bigjos]

Sorry should have added that I have fitted electronic ignition so no issues with magnetos etc.
Carb has been stopped down and cleaned so pretty sure there are no blocked jets.
Could it be a timing issue?

[Rex]

Why don't you check the timing then?

[antoni]

This one wasn't, it was  small plastic component. Only DiYers would ever use a NOS capacitor.....

And an electronics engineer would not use a small capacitor. A physically large capacitor with a polypropylene dielectric and a high voltage and current rating would be selected. The large physical size would mitigate against the high ambient working temperature and make sure the relatively small power (therefore heat) generated through its low ESR (effective series resistance) would be easily dissipated.

[iansoady]

The Brightspark external magneto capacitor seems to work very well and it's tiny.......

[antoni]

Brightspark costs 13 pounds . This:  https://uk.rs-online.com/web/p/polypropylene-film-capacitors/8829187/    costs 94 pence for a pack of 10, and I think less likely to fail.

[Rex]

And an electronics engineer would not use a small capacitor.

Well, as I say to the missus, "small is relative" and it's the rating that needs to be the prime consideration, but that said, my point was that it wasn't NOS but a relatively recent fitting.
To be fair, it wasn't me who had the mag rebuilt, and nor did I rebuild it, but I wouldn't have used someone who termed themselves an "electronics engineer" to do it anyway.

[bigjos]

Well at the end of the day it seems the problem is probably lack of choke on the carburettor. I put my hand over the inlet and it revs up fine, then once it has warmed up properly its fine! Thanks for all your suggestions.
Incidentally the timing was out just a little too