Hi everyone ,im after some help .My bully 199b as a really bad top end rattle on tickover (sounds like piston slap to me )then clears when revved .Its just had a new pattern piston,little end bearing and a rebore (not the best rebore i've had done )it had a bit of play ,but guy measured it and said nothing to worry about so i put the top end on.Anybody else had a problem before ....Thanks .....Also should have said it only started to rattle after about 2-3 litres of petrol ,running it around my yard .

Edited June 8, 200818 yr by hewson

Without testing the bike is not easy to say, but sounds the carburation maybe wrong. Maybe even you have some dirt in the petrol valve or in the carb and it does not flow properly.

What carb are you using??

Hi Greeves ,its on standard bing carb ,its had a good clean - blow out ,the slide is quite worn , could this be the problem?.A friend of mine heard it today and he said it sounds like the piston hitting the head .But i had the barrel and head off a couple of weeks ago to check things ,and everthing looked ok apart from the bit of piston play . But the noise to me does sound mechanical.

Almost impossible to diagnose a rattle really from a description but I had a new liner/piston put in my Yam and it rattled badly right from first start up. The rattle wasn't really noticeable when revving the engine but as the engine slowed down with no load it rattled and it rattled on tickover spped and just gentle blipping. Took the barrel and piston back, new liner fitted with the same piston (which was brand new) with instructions to peform a tighter bore. No rattle this time - yet anyway....

Hi again woody ,yeah when its revved up its ok ,just on very low revs and tickover it sounds like its going to self destruct (scary) .Do 325cc top ends fit straight on ....

Don't see why not, the 340 is only an overbored 325 and 325 motors can be bored out to take the 340 piston (apart from the early 325 as the liner is too thin)

Sure someone said once (probably Dabster) that Vesty used to run a 325 barrel on his 340 as the 325 had a bit sharper power delivery than the 340. Mr Renham will have all the answers on that

Hi hewson,

best to measure the piston/bore clearence in the area above the ex port and directly opposite to see what clearence you have. Should be around 2.5 to 3 thou. If you are short of a 325 piston, the Gas Gas 321 piston kit may fit - I haven't tried this yet - but the original Mahle pistons were from the same casting, only machined to a slightly different diameter, gudgeon pin is the same diameter (From memory, 83.2 for the Bult and 83.4 for GG). Could be a problem with the inlet port height as the GG pistons may have a cut away. It's worth a look in case you are unable to locate another Bultaco piston. Also, GG now supply S3 pistons for the 321's, need further investigation to see if these can be used on the Bult.

From the descriptions, it does seem as though you may have too much piston/bore clearence.

Bye, PeterB.