Knocking and backfiring

[cmtstu]

hi guys
I have recently gone to work on an MZ thats been stood in a shed for 10 years.
I replaced the PVL ignition pickup which burned out twice on me and re wired the bike.
Then i moved onto the carb, cleaned it down and adjusted it up.

The bike now starts and runs quite well but it seams to knock quite badly off the throttle especialy when riding i know these engines werent grweat but it is bad enough to make me cringe at the moment, i have moved the timing around for hours and hours and the point where the pickup is on its stopper point and the bike runs and starts best but it still knocks quite baddly, if i move it back the the other way it backfires and stutters and is dam near impossible to start.
I took it for a run out round the block yesterday and it seemed to run ok but every now and then it would bog down and backfire and miss with a little choke this would stop and i could continue as normal nocking the choke of again and it would be fine for another half a mile and then repeat. also off the throttle it knocked really badly.

can any one sugest what could be causing this? Ive taken out the bad fuel, changed to a 8 plug as it had a 6 in cleaned the carb to the point i can now strip it and put it back togethor in about 60 second flat. ive tweaked mixtures till im blue in the face and have adjusted the timing backwards and forwards and have found that i cant get any further than around 3mm BTDC and it only runs half decent when its on the stoppers for the Pickup. if i move it back which would be advancing it, it bangs like hell or it backfires and stutters.

[den]

big end bearings comes to mind here,,have you checked,plug out, top dead center, screwdriver in hole to feel piston head,now rotate stator a little and feel for ANY play.ie , piston not moving but stator does a tad..
someone may knwo a better way to check without stripping engine

[philtrophysport]

The only surefire way of seeing if the bigend has gone is to remove the head and barrel and see if theres any play in the connecting rod,if you can move the piston up and down on the crank by any amount then you will need a new big end bearing,dont confuse this with lateral (sideways) play on the crank which should be there.Hope this helps.

Phil.

[MichaelTorre]

Although the op didn't say he was beating on the crank,
would a crank that has had its end hammered to install
a clutch or bearing exhibit this behaivior ?
M-

[MichaelTorre]

The reason I mention the hammered crank is somewhere I read
a crank that rotates in an elipse from being hammered looks a
bit splayed or pigeon toe.
Point being , the points may not work consistently.
M