Backfiring on downhill descents

[Minto]

Hi all
Its been a few weeks since ive had time to pop in, but been meaning to since my last ride out at the end of August!!
While i was out, i took a plod over to the Ponderosa at the the Horseshoe pass near Llangollen. The bike ran ok most of the way but I hit some trafic at Llangollen and she cut out three or four times. Riding up the hill and round the lovely twisty bits it went nicely, but after a brew and a natter I headed off. Bike started fine, but as soon as i got to the steeper downhill bits, the bike started popping and banging and backfiring, this stopped as soon as i opened the throttle again but would reoccurr every time i shut the throttle.
Ive not had a minute since to go and look at her, but thinking maybe the points have slipped (again) though it does pull ok on the flat or up hills.
Anyone got any thoughts?
Apologies for probably novice questions.
Jase

[Triton Thrasher]

Leaks between the exhaust pipes and the cylinder head.

Or maybe carburettor coming loose.

[Minto]

Cheers TT, ill give those a look at.

[CheeserBeezer]

I agree, exhaust leaks, probably where the pipes fit in the cylinder head. Does it do this on both sides? Also worth turning your tickover down a bit.

[Minto]

Cheers
It feels like its both sides but difficult to ascertain with any certainty as it only does it while decelerating sharply like on steep down hills. Funny you should say about the tick-over, its as low as i can get it but still a little high i think. Id bought some new carb bits to try to cure this (needle and jet, bigger main jets etc) a while ago but not fitted yet.
How would a high tick-over make it behave like this? Just trying to understand the intricacies of these machines.

[CheeserBeezer]

High tickover is allowing fuel into the engine even with the throttle closed. The banging you can hear is fuel igniting in the exhaust system.

[Rex]

And which is usually an indication of a carb with a worn out slide/body.
Time for a carb resleeve or replacement?

[Triton Thrasher]

its as low as i can get it but still a little high i think.

Doe your throttle cable not have enough slack to let the slide right down?

[Triton Thrasher]

Banging in the pipe on over-run is almost always excess air getting into the exhaust.

Often, it’s air leaks at the head.  It can also be caused by a very weak carburettor idle mixture, but not as loudly.

There is always unburnt fuel in the exhaust gas.  There shouldn’t be enough oxygen present in the hot (engine) end of the pipe to allow that fuel to burn in the pipe.

Until the guy with the problem does some checks and reports back, we’ll be lengthening this thread with obscure suggestions, followed by arguments.

[Minto]

its as low as i can get it but still a little high i think.

Doe your throttle cable not have enough slack to let the slide right down?

I changed the cable a while ago, but as an emergency make do measure i used a length of bicycle deralieur cable and a screw on barel end, which required three pairs of hands to do, and initially i did cut this short, but has since been replaced, with a slightly longer cable, on which there is now a bit of slack for adjustment.
But still the tick-over is slightly high.
 I had raised the needle a notch a while back in an attempt to correct the running issues i was having at the time, but maybe the combination of this and maybe a worn needle and needle jet are making things impossible to set up properly.

[Minto]

High tickover is allowing fuel into the engine even with the throttle closed. The banging you can hear is fuel igniting in the exhaust system.

Ahh, i see, thanks that makes absolute sense now.

[Minto]

And which is usually an indication of a carb with a worn out slide/body.
Time for a carb resleeve or replacement?

Yep, im thinking that the carb may need some professional attention or replacing, its been very difficult to set up, air screw makes no difference to idling, etc.
just not able to stretch to a new carb at present.
Any idea how much a resleeving job would hurt? Or who foes such work?

[Triton Thrasher]

Screwing the throttle stop right down, so the slide is completely closed, should stop the engine.

Does that not happen?

The way to test for air leaks into the inlet manifold (which can make it hard to slow the idle down) is to have the engine idling and spray WD40 on the joints.  If the engine note changes, there’s a leak.

[Rex]

Any idea how much a resleeving job would hurt? Or who foes such work?

I know Martin Bratby used to do this, but no idea if he still does.
My A7 carb was shagged in so many ways that a new replacement Amal Premier was purchased. Works well too.

[dave55]

Should anyone need any Amal carbs doing (both Monoblocs and Pre Monos are same prices ) then give him a ring on 02392  580708 after 6pm. His name is Kevin Traill  at Alverstoke Restorations in Gosport Hampshire  he is helpful and likes a chat
and no I am not related in any way and he gave his permission to put his details on here . 
had my pre monobloc sleeved over 2 year ago and still doing well, came back like new for under £50
Regards dave