I had to take a trip to a nearby town (Bodmin) to pick up a parcel. On the previous trip there had been a bit of popping in the exhaust on the overrun which I had read is indicitive of a leak in the exhaust system. So I took off the exhaust and sealed all the joints with bathrrom sealant (weird I thought, but this is the recommended way).
On this run the popping was a lot worse. I found I could reduce it by running with the choke (actually its an enrichener) engaged. So an air leak then!
So, with the engine idling I tried wiggling the carb up and down (see later). The revs went up and down. I deduced that the rubber manifold was leaking.
It was not the standard Bullet manifold.
And I could see cracks in it. So I took it off to have a look.
The rubber looked in a sorry state
When I got the bike, the previous owner also gave me his box of spares which contained the “original” (maybe) manifold and a couple of coupling rubber tubes, both leaking when tested. Why had he changed the manifold?
A search on ebay found these items for sale for £9.48 from India. Dearer than an Indian coupling, but cheaper than a UK made coupling from Hitchcocks.
I decided to buy the Hitchcocks one and after removing two gaskets from the metal part of the manifold, and another two from the cylinder head, it all fitted.
I now find that the idle screw position is quite critical in removing “popping” in the exhaust. The method in the book is to turn the idle screw fully in, then adjust the throttle stop until it is idling slowly. Then screw out the idle screw for maximum revs, adjust the trottle stop and repeat until the idle is “right”. I find that setting the idle so that screw is further IN than the optimum idle speed (i.e. it’s idling a bit slower) gives less exhaust popping.
Seems OK now.
I confess I was a bit worried about the bike running lean and possible burning something out (like a valve or a piston). The blued exhaust was a bit worrying. So with some apprehension, after a run, I took out the spark plug for a look.
It was fine – grey colour, and strangely the blued exhaust seemed a bit less blued then when I got the bike!! I don’t know how that is possible.