Triumph T140V cuts out

[jonnierw]

Hi,

I have a Triumph T140V 1977 that has had a Boyer ignition unit fitted. It had been sitting around for ages and the carbs and fueling lines and filters are clean. There doesn't seem to be any fueling problem and I've tested continuity of all the wires which are fine.
The bike runs fine when started but after riding it for a while (15+ mins) when I slow down for lights and the revs drop the engine just cuts out. The bike then won't start for about 20mins (i'm assuming till the engine cools down).

This is my theory:
I don't think the the Boyer unit under the seat has any problems and when I put a multimeter across the contacts it seems fine. But I think the Boyer unit electronics near the base of the R/H cylinder is overheating and when the revs drop a safety feature kicks in on the Boyer unit and cuts power to the plugs till it cools down sufficiently.

Any advice would be greatly appreciated (Lonodn drivers aren't very patient when you have to paddle your bike across 2 lanes of traffic at a roundabout!)

Thanks.

[L.A.B.]

The bike runs fine when started but after riding it for a while (15+ mins) when I slow down for lights and the revs drop the engine just cuts out. The bike then won't start for about 20mins (i'm assuming till the engine cools down).

If it's a Boyer problem, then it sounds more like a low voltage problem, either due to a defective battery, or the charging circuit may not be working properly?  20 minutes "rest" would give the battery enough time to recover some voltage-so the ignition system then begins to work again.  Do you normally ride with your lights on in daytime? How old is the battery?

I don't think the the Boyer unit under the seat has any problems and when I put a multimeter across the contacts it seems fine.

I doubt that is a conclusive test.

When the engine cut out, did you pull the plugs out and check they were not sparking?

But I think the Boyer unit electronics near the base of the R/H cylinder is overheating and when the revs drop a safety feature kicks in on the Boyer unit and cuts power to the plugs till it cools down sufficiently.

There's no built-in "safety cut-out" in any Boyer Bransden system-as far as I'm aware? Either some part of the system is overheating and going open circuit as a result-maybe a bad wire or earth connection or a kill switch fault, or a faulty coil primary circuit? Or it's simply a low voltage problem, which is something Boyer Bransden ignitions are known for (the latest B-B Micro MkIV version is less sensitive to low voltage than the old MkIII type.)   

[Gromit]

Dear Jonnie

Don't think it is a Boyer problem mine has a Boyer about 10 years old now and has worked fine except when the battery is knackered, then it is a bugger to start and will do the same as yours and cut out at low revs.

Also as previous posts there is nothing in the "ponts" replacement plate ...bottom right hand corner other than 2 coils and a bit of iron to react with the whizzing round magnets that trigger it all to happen.

Could be that you have 2 x 12Volt original coils fitted too that doesn't help as the Boyer has both coils connected in series and fires them both together .....so called wasted spark principle as both plugs spark together with one on compression and one at top of exhaust stroke. So if you have low volts there isn't enough oomph to fire effectively 24 volts worth of coils satisfactorily. I bought two 6 V far eastern ones and they gave a hugely improved spark ...
The original system has two separate points, coil and plug sets in the same points plate which gets replaced with the Boyer and the coils get rewired at the same time in series.

Some things to think about ...

Regards Gromit