B31 - should I be using Lead additive with the petrol?

[Fraz]

My B31, I've just been told whilst on a run, that when it stutters, a puff of black smoke comes from the exhaust.

I've done 40 miles today without issue and then suddenly it all started again

When it cuts out and won't re-start, there's a huge backfire in the exhaust and then it will start up.

My pals suggest the exhaust valve is sticking and fuel is dumping.

They also say I should be using lead additive with my Super Unleaded.

Does this all seem feasible?

Could it be the answer to my random running issues?

[KernowPete]

Do you notice a loss of compression?

[Fraz]

I'm not sure how I would tell?

It coughs and splutter, once it's started misbehaving.
I can make it do it more by winding on the throttle a little (and wind it off to recover the engine slightly)

Just done a compression test and got 75psi

[Steve.S]

Well, is it much easier to kick over when it won't start compared to normal? If it is, then it could be the exhaust valve sticking. Make sure the valve lifter is not holding the valve open. 75lbs sounds a bit low for 6.5:1 compression ratio. Is it gutless? I would expect nearer 100lbs or more. You  should add a little oil through the plug 'ole and see if the pressure increases. This will seal the rings temporarily (and to a degree the valves). Ideally you should carry out a cylinder leakage test, but the kits cost about £100.00. However I think this is separate to the problem you have.
So from what you have said, I would suggest possible magneto failure (which probably means it's the carburettor!)
It may help if you took a neon with you to fit when it won't start to see if you have a spark.
Regarding Lead Replacement Additives, it could be argued that the valve seats would have work hardened by now, so being unnecessary, but if you select one with an octane improver, you may be able to run more ignition advance before pinking sets in, giving you a little more power. I would recommend using the Castrol product.

[KernowPete]

Sorry - separate to fuel additive. Have you taken the plug out when it’s playing up and looked at it? If so is it wet and what colour is the insulator? Is this the first time you’ve used your compression tester? I’ve had iffy ones in the past. When my B31 is cold I can put all my weight on the kickstart lever (near the top of its stroke) and it’ll hold me up for a few seconds. I’m 75kg. Mine pulls well.

[Roy]

Just to add if you are not allready doing so ,the throttle must be wide open when doing compression test,regards Roy