Help - bike's gone bang!

[Worty]

Returning home today after a 100m ride when bike started popping, banging (misfiring) then packed up.  Tried to kick it a couple of times but just popped and banged.  Checked plugs - fine with good spark.  Checked points - all good.  There is a bit of oil in the left hand silencer - right hand silencer is nice and dry.  What do you reckon - could it be a valve problem (broken spring, for example)?  Any help to diagnose would be much appreciated so I can get back out on the road asap.

Cheers in advance!!

[Billybream]

Could it be stale fuel, on my first ride out after winter lay up never seems to run very well until the tank is refreshed.

[a101960]

Checked points - all good.

Sorry to hear about your problem. I had a similar breakdown a few years back and in my case it was caused by one of the points faces falling off. I am not sure how they are held in place (welded) I suspect. Anyway at first glance everything looked to be O.K. I could see nothing wrong. The points even appeared to be opening and closing, and it was not until I started to remove the points plate that I noticed the point face lying on the cam ring. I replaced the points and the engine started right up. The symptoms I had though, exactly matched yours. Not saying that is your problem, but it s worth checking. The sliver of metal that makes up the point face is very thin and small which is why I probably did not notice anything being amiss, because everything looked O.K.
John

[Worty]

Thanks for your speedy replies!!

Fuel - I had half a tank of old fuel but it was sealed (taps off and tape put over fuel cap breather) to try to keep it as fresh as possible.  Bike was a bit sluggish but I burnt off 50 miles of it before refreshing with super unleaded - then went the other 50 miles really nicely.

Points - this is a good point (pun not really intended!?).  However, would the plugs not spark if the points face had fallen off?  Spark did seem quite healthy on both pots.  Will look at them a bit more closely though as the checks were done roadside.

Cheers again guys.

[a101960]

Will look at them a bit more closely though as the checks were done roadside.

Yes that's where I checked mine at the side of the road. It was not until I got the bike back home that I noticed what had happened. The facing is very small so would not be easily noticed. It is always possible I suppose that the timing has slipped, not very likely I know, but it is possible.

[Worty]

Could you tell me how the timing could slip.  I've never had this on any vehicle I've ever owned, so would need some pointers (another bad pun, sorry)

[duTch]

 

Help - bike's gone bang!

 That sounded like the piston was coming up to say hello...

 

Could you tell me how the timing could slip.  I've never had this on any vehicle I've ever owned, so would need some pointers (another bad pun, sorry)

   It could slip if the nut was not done up on the pinion. (They don't need to be super tight though)

 Maybe check the carby for loose jets or blockages....I think a loose or dropped main jet could have those symptoms. The other week after I'd had my Gutzzi apart, I discovered the needle clip had somehow dislodged and the needle was stuck protruding up out of the slide - that made it run uneven...... *conf2

[a101960]

Could you tell me how the timing could slip.

Not a common occurance, but it has been known to happen. The pinion relies  on a tapered shaft to keep everything tight and it is clamped into position when its all tightened up. I am sure you have heard or read about the pinion slipping during the tightening up process when the ignition is being timed. Occasionally the pinion will slip spontaneously for no apparent reason under normal running conditions. Somehow the pinion manages to become loose. As Dutch has said maybe the nut was loose or perhaps it could have backed off a bit. Something else has just crossed my mind. It could be that a push rod has jumped ship. It does not happen often, but once again it is not unknown.

[berger]

I have not been to the pub, don't know about the oil bit but when mine had symptoms like that it was condenser breaking down under load. off to pub tmow

[Worty]

Thanks for all the suggestions fellas!

Tried to kick it over this evening when it was nice and cool.  One big pop/bang/ then nothing at all.  Compression seems fine.

Would the pushrod try and make its way out of the engine?  If so, non have appeared.

Carb was brand new, but will check it anyway.

Timing is an interesting one.  Can totally see how this would screw things up but not so sure how its happened.

I guess its time to start taking things apart and having a look.  Just a git how it's on its first run of the 'summer'.

Let me know if you have any more thoughts.

Will keep you updated!

[morris]

Carbon traces from the brushes on the slipring can cause similar

[Worty]

Cheers morris, but would that stop the bike starting altogether?

[morris]

Cheers morris, but would that stop the bike starting altogether?

Yep. Mine did. It’ll arc round the slipring and the spark then goes to both plugs at the same time.
At the plug it will show a good spark though.
I remember when trying to start it popped and banged but wouldn’t run.
When checking the spark at the plug, I accidentally forgot to put one of the caps back on and miraculously, on the next attempt it started to run on one cylinder.
But as soon as I put the cap back on it died and refused to start again.

[Worty]

Thanks morris.  Presumably giving the slip ring a really good clean then applying a light touch of oil again will be the way forward.  Might even try removing one of the plugs and see if it fires.

[duTch]

 

Presumably giving the slip ring a really good clean then applying a light touch of oil ...........