Inexplicable top end rattle

[daveyf]

Refitted my DR130 cylinder and the buzzing rattling sound is back. I have rebuilt and replaced EVERYTHING in the engine. I convinced myself the noise was from loose rivets in the primary drive (it wasn’t). So before I replace the top end why the f@#* would something as simple as cylinder / piston be responsible for this rattling noise? I couldn’t see anything amiss when I replaced it. Oh well that was a wasted day….

[oldscooterbloke]

If it has only appeared with the top end changed it is a very good chance its top end related.
Could be piston slap if its under size.
Could be compression height to small and just touching.
wrist pin too loose in the piston.
very difficult if your not near it or listening to trace it.
a piece of rubber pipe is a good way to locate the noise, holding it to your ear and the other end placed on the engine in various location with motor running, you will be amazed how accurate this method is to pin piont it.

[daveyf]

Thanks. Seemed nice snug fit with the small end bearing / gudgeon pin. Ran fine for about 1000 miles, thinking I’ll just swap out the top end

[jonnysnatchsniffer]

the buzzy rallte you hear is probably the number plate or the back of the head cowl or some loose bit somwhere when in resonance, if you have straight cut primaries the can clank a bit too
id not worry about it, if you want something to listen to buy a fucking stereo, its a 30 year old air cooled 2 stroke and it will make noises, its deto that you wanna avoid
and if its like a bagg of marbles in the top end then it could be that
does it do it al all revs?

[daveyf]

99% sure it’s the top end. Does it from tickover and at all revs. Swapped around everything on the scooter, after I took the DR top end off it was running great, no rattling noise (but slow) on a standard 125. Like a fool I put the DR kit back on for a bit more oomph and the rattle was back, exactly like the sound of a rattle can aerosol. It’s not the straight cut primaries have a very distinct whine which I really like.
I’m not generally that fussed about noise as my Lambretta makes a right racket but this is doing my swede in. Also trying to understand why? The kit only had a few hundred miles on when it started

[jonnysnatchsniffer]

does the rattle get faster with the revs? are you using the right ignition timing for the kit? piston right way round?
if it increases with the revs well then it could be too small a squish so piston kissing the head
it could also be nothing, just re measure the whole top end, especially the piston bore clearance tho piston slap is unlikely on such a fresh top end.
what bearings do you use on the crank?

[daveyf]

Thanks will try all that. It does get faster with revs, piston right way round. Strange thing is this has only happened when I change out this top end. So when it started rebuilt engine, still happened so changed top end for standard one which cured it. Put the DR130 on and the rattle is back. Will update on results

[oldscooterbloke]

Hi me again, it's simple to do a squish test with some lead solder, before you dissgard it as crap.easy to adjust with a thicker lower gasket

[daveyf]

Squish was 1.5 mm. Decided to put the old cylinder back in and it was running lovely until I put wrong spark plug in by mistake (long plug from my Lammy). So locked up the engine which means I have to put the original V90 engine back in……I have been slowly rebuilding this one with a Polini kit.

So will check the piston to bore clearance on the DR130 but in the meantime putting the scoot back together which means there are more opportunities for idiotic mistakes and cock ups

[jonnysnatchsniffer]

Get yr squish down a bit, 1mm will be fine, it's not like yr revving to 14000rpm, more likely more like 7500 on the dr, the less squish the better, mine is at 0.8 with a 54.5mm stroke so you should be safe at 1, and sort the timing out18 degrees should be fine on static ignition
Could be piston slap but you would probably see cracks in the skirt