Hitchcocks Forum

My Pal has a 2008 500 Classic iron barrel.
He recently rebuilt the bottom end on account of main bearing failure.
At the same time he fitted a Boyer Branson ignition system (I know, he should have done one thing at a time but the A/R was knackered and it's almost as cheap to fit the BB)
Anyway, the engine will start cold and will restart hot (even if it's not running particularly sweetly) Trouble is, if you leave it for 15 minutes it refuses to start until stone cold again.
I'm fairly sure we have the timing more or less right and the carburation can't be far out either as it starts OK from cold (or hot as long as you don't wait around)
I know nothing about BB systems - can anyone shed any light?
Our next move is to strip and clean the carb (again) unless anyone has a better idea.
It has a Monobloc fitted but that was on and OK before the rebuild. It also seems to need a bit of throttle to start from cold and looks like it's running a touch rich but by no means sooty.

which boyer bransden is it MK4, digital?



is the air valve still in place on the monobloc?

Yes, mk4 I believe. Yes, air valve fitted.

Perhaps try a non resister type spark plug along with copper cored HT lead and standard champion type cap.

BB are not keen on old carbon type HT's and maybe the plug cap resistance is increasing with heat from the plug..

I have fitted Mk3 and Mk4 BBs without any problems, I always use copper cored HT leads as I have had problems with carbon leads in the past. The symptoms you describe seem (to me) to point towards carburation, I would definitely spend some time with the carb, something has not gone back in the same way it came out, if the spark is in the right place when cold and Hot, why would it not be when warm? Obviously new mains is a major intrusive job, have you checked elsewhere, valve timing, tappet clearances etc. Unlikely but as well to check them.

I agree with Lennie, In my experience Boyer box either work or they don't. Given the symptoms you describe I would be looking at the Carbaration route a little closer. What does it do if you run (from stone cold) and stop the engine after a few (10?) seconds. (before the carb gets warmed up by the engine heat)

Thanks chaps. I was hoping for a "I know what that is" reply but I don't get that lucky.
I'll check the cable and cap but there seems to be a fat blue spark all the time. Nice to know they generally either work or they don't.
I can't see that it's anything wrong on the rebuild as it runs fairly well when it does start.
I suspect carburation too and this was always a bit of a long shot. Trouble is, we've stripped the carb once and found nothing amiss.
Time to try again - Oh well....

Shane, if you start cold it will stop and start OK even when hot. It's just if you leave it 15 minutes it turns awkward. There is an insulator between the engine and carb.

Hi Pete, This really does sound like heat soak. Am I right in guessing it all works ok until the carb gets warm to the touch then the problems begin?

I've not actually tested for that Shane but it sounds like a possibility. As I say, there is an insulator (tufnol ?) Between the engine and carb but it's something to look at.