Intake backfire puzzle ????

[TimP]

Hi everyone, I’d really appreciate your ideas as to how to cure this persistent problem. Low speed intermittent backfire

200TS
Electrex ignition
Std Amal carb and jetting
Valve clearances correct.

Valve and ignition timing checked - all the marks are spot on and the valve opening and closings are within 5 deg of book values when measured.

The bike starts easily and will run happily at > 2000 rpm but when I try to set the idle to around 1k rpm it will tick over for a minute or so then cough once - slow to a very slow speed - cough again then stop. I’ve seen a short burst of yellow flame shoot from the carb once.

I’ve tried fitting a, known to be good, alternative carb - problem continues.

Idle mixture adjustments makes no difference to this issue

Ignition timing advancing or regarding - different running but the backfire still occurs

It seems to be marginally worse when hot - ie after I’ve been trying to cure it for 30 mins

Your thoughts please
Thanks

[Jordan]

Sometimes an air leak in the exhaust system can cause banging noises.
Check the exhaust pipe joints at the cylinder head and muffler.

[TimP]

Thanks for your quick reply. This backfire is only in the intake rather than the exhaust. I’ll check the joints you suggest for leaks though.


More info

The valves have been recently replaced and the seats recut -however the guides were left as found ; quite worn, as considered to be fairly loose but serviceable.

[Duccout]

Hi Tim,

It sounds like weakness to me, possibly low fuel level or an air leak at the manifold..

Good luck,

Colin

[blethermaskite]

Sounds like weakness to me too......have you checked that the carb flange is perfectly flat? are you trying to use the std Ducati rubber carb/head thick gasket? if you are get rid of it! Cheers George

[IanHood]

Personally I think the standard rubber between the carb and the head is much maligned. They work perfectly well when in good condition and when not over tightened, and also provide some useful vibration damping (particularly on the 450).

Having said that, they are often in poor condition and often squeezed too hard

Ian

[Bevel bob]

I spent a whole week riding round the IOM with a 5 mm hole in the rubber carb spacer on my 250 . the hole was at the very bottom and invisible . Bad starting ,unreliable idle , flat spots , horrible. Once found a stick jammed in provided instant wonderfull running . The real problem was that i was the only mechanic for three missbehaving bikes that were our sole transport. How about 10 breakdowns in one day , but we kept riding.

[mrkprsn]

Is there any way to try another carb? My 200GT uses a dellorto ubf24bs. Not saying its a great carb by any means but you can at least eliminate the carb as the issue. I just rode my 200GT with the original dellorto carb in a motogiro and we did 195 miles the first day with no backfires. A fellow rider rode a Mach 1 with the SS1 carb and of course that would back fire sometimes when slowing down.

[mrkprsn]

Just after I wrote my last reply, a friend who is on this forum (Blaat) fixed his backfiring problem. When he bought his Mach1, a dunstall muffler came with it. He just switched to a regular silentium and it fixed his problem. Curious, what muffler do you have?

[Bevel bob]

I had a new silentium and it was too restrictive as its suitable for a range of capacities. the orriginal Mach 1 unit had a larger outlet end cone and possibly other differences. Now running on a Norton cigar silencer from a 1960 Dominator that fitted with the std Norton bracket !!. looks similar enough to the silentium to not notice.