Went out on the M20 to enjoy the late afternoon sun and noticed a gurgle from the crankcase breather on the first few kicks. The first 1/2 mile saw a light smoke trail which I'm guessing points to wet sumping.

Running well and soon clearing to it's normal self it then backfired a few miles in - something it's never done before. It then did the same on slight over run a few miles after that.

Is the wet sumping and backfiring likely to be connected or purely coincidental? It it helps, my motor has done 400 miles since being re-built.

email (option): lee@twowheelstested.co.uk

When is it backfiring Lee ? If it's on the over-run then it's probably nothing more than an air leak.

On a rising throttle, assuming timing's correct then either a lean mix or perhaps a fouled plug clearing itself.

Is it a colossal explosion or a bit of popping and banging ?

Hi Rik

It's on over run, not a massive downhill after full throttle type of over run, just a bloody big bang shortly after closing the throttle.
Timing must be slightly advanced as I heard it pink and retarded it slightly a couple of times to suit.

email (option): lee@twowheelstested.co.uk

Lee,

I'd check first for air leaks at inlet and exhaust but if it's a really big bang then it could be unburnt mixture igniting in the silencer which would hint more at extreme richness. It's always difficult diagnosing at a distance. An air leak has to be favourite.

..Of course it could just be that your bike is a real attention seeker and it hates it if people don't look up when you pass.

It's probably an air leak at the exhaust junction on the cylinder, the unburnt fuel re-ignites because of the extra air drawn in, try re sealing the joint with silicone sealant (yes it will withstand he temperature) this may cure it.

email (option): davmax@ntlworld.com

Lee,My M20 has always backfired when decending some of these steep hills here in Devon.I really should get it sorted,i'm sure Dave is right about the leak at the barrel/exhaust joint,mine has certainly got a leak at that point.Thing is i've got used to it now and think i would miss it. Mick.

i think i read on here that the back firing could be cured by using super unleaded though whether this was to do with harleys i am not dead sure but an easy cure to try

email (option): roger.beck@node6.com

Hi Lee, I had experienced this back fire but couldn't keep a happy tick over as well, all pointed to excessive fuelling and as per my earlier thread it wasn't sorted until a new carb fitted. I had the old one reconditioned but that didnt help. Replacement carb is expensive so hopefully its a simple air leak.
Pete

email (option): cruaser@aol.com

My bike used to pop and bang on a closed throttle. It was like being on a rifle rang. A new carburettor completly cured it- and gave me a reliable tickover

Thanks for the tips chaps.

The carb issue is a worry as I've got a reconditioned one and read the earlier posts/threads with interest - does this problem usually present itself straight away or take a while to bed-in?
It's ticked over & run well for 400 miles since being rebuilt and its only backfired twice, but that was on the same run and bloody hell, it was loud! I didnt know if the wet sumping had left some fumes in the exhaust which were ignited on overrun. I'll get the silicon out and make sure I'm wearing earplugs on the test run.

email (option): lee@twowheelstested.co.uk

After all the hassle with my bike before, during & after the trip to France, I decided that I needed to go through all in the top end of my motor, inc mag, clutch & box

Today after going through things methodically over the last few weekends, I had completed 120 miles before 12.30 with a correctly & accurately timed maneto, gapped points, newly seated valves with correct clearances & a tank of texaco 98, the bike in my opinion has never felt better, compression could be better, new rings should sort.

Popping & banging I had at times on over run or the occasional rifle like backfire - all gone

My gearbox & rebuilt to std spec clutch gave me gear shifts like never before

All in all I was delighted with my excursion through Berkshire, Buckingham shire & Oxfordshire

Lesson learned - having taken my time & double checked all, the little green bike delivered

Well pleased after a good spin out today

:-)

Job

email (option): Jonnyob1@googlemail.com

Hi Lee,Yesterday we clocked up 136 miles on a round trip to Cobberton Combat Museum, with both M20s (Shane rode the 2nd one)before leaving I put some fresh exhaust paste round the top of down pipe on Ram-Jam (Ron knows which bike that is) the outward trip was fine, on the return some of the paste blew out making the bike sound like the 1812th overture on overrun, so it maybe that simple for your bike, anyway we had a great day, good to sit and chat with Ian, (whose milage must have been more than ours) and Rob and chew the fat about 2014

Hi Lee, my M21 was the same did 1200 miles then backfires at random when shuting the throttle,I cured this by resealing the exhaust at the head and where the silencer joins the down pipe with a little exhaust paste. This helped but what mainly cured it was droping the throttle slide right down and getting the slow run realy slow with the pilot air jet.

Cheers Pat

Ah bugger, I was planning on a run to Cobbaton - well, I was going to cheat a bit with half of it done by van. I'd better write the 2013 date in the diary!

Has anyone used a Colourtune to set the carb? I know very little about carb set-up and have just read the Amal page on the Technical Info side of this site and wondered if one of these kits would make the job easier? In the mean time, I'll change the oil, check tappet clearances, sort the timing and seal both bits of the exhaust.

Thanks for all the help.

email (option): lee@twowheelstested.co.uk

I used a ColourTune on a Triumph 3TA and it was interesting to see the flame inside the cylinder change while playing with the settings! It was a real help.

email (option): viaconsu [at] planet [dot] nl