Scooterotica

sean brady scooters wrote:you may well be right...........but i beg to differ..........surely the closer to TDC the spark occurs the more chance it has of running backwards.........
if for eg your timing was at say 30 degrees BTC..........it may kick back......but not actually run backwards cos in that direction the spark would occur at 30 deg after TDC..........
on the other hand..........if your timing was say 5 degrees BTDC it would then fire at 5 degrees after TDC and then stand more chance of running.................

Sorry only just read your other post.
I see what your saying but the engine needs something to kick it off in the wrong direction first and a late spark could never do that as the piston would already have gone past TDC.
Only if the timing is too advanced can the piston be pushed down the bore BEFORE it has reached TDC, therefore turning the crank in the opposite direction. The crank being forced back in the other direction is what causes a kick back and in some cases the engine will continue to run.

i also think that a high comp ratio can sometimes be enough to send the piston back in reverse ..........
interesting topic this though.......

anyway ........we will soon see whats amiss,as the scoot in question is coming here later today............
i will of course post up what we find.........

sean brady scooters wrote:i also think that a high comp ratio can sometimes be enough to send the piston back in reverse ..........
interesting topic this though.......

Ah, but would it then run?

Ive had this happen too me

Merrily scootering along towards Stacton hill and scooter back fired and missed.
Came to a stand still. So kicked it off and it back fired and started to run but sounded sluggish
Put her into gear and low and behold i was driving backwards.

I pissed my self laughing and put scoot on stand & started to swap out the stator.
Swapped out stator, kicked her off and away we went no problems.

So what caused it a faulty Pickup

This pickup had been on for about a year & a half

firekdp wrote:

sean brady scooters wrote:i also think that a high comp ratio can sometimes be enough to send the piston back in reverse ..........
interesting topic this though.......

Ah, but would it then run?

Yes it would run but like a sack of puds.

RinB wrote:

firekdp wrote:

sean brady scooters wrote:i also think that a high comp ratio can sometimes be enough to send the piston back in reverse ..........
interesting topic this though.......

Ah, but would it then run?

Yes it would run but like a sack of puds.

Yes it will run like a sack of spuds because,in effect, the spark is now happening after TDC. Your faulty pick up must have caused advanced timing. My quote you've used is "will it run if too high compression forced the piston back down before TDC?" I don't think it can.

just had a quick look and check on this scoot............
timing was set at approx 9 degrees BTDC...................
and to complicate matters more........theres also a fault showing from pick up /red wire.......
...............

had this happen to me once on a P2 (twenty odd years ago). started ok, was a bit unresponsive on the throttle, but what the hell, clicked it into 1st and set off, imagine my surprise when I nearly cracked my chin on the headset, I looked behind in bewilderment imagining someone was standing behind. I switched it off then kicked it again and everything seemed fine.
I guess if I hadn't been starting off from my parents gravel drive I may well have a set of false front teeth !

just to recap on this now that dan and i have finished it
.....................finally ,after much chew.......
when it first came in it was running quite sickley........missfiring at higher revs and at one point had even run backwards......
so basically ignition and timing was sorted first........
its a 225 classic rapido by the way.......with 28mm delorto carb/nk pipe..............
and so with ignition done ,it was time for a dyno run..........
7.9 BHP...............
obviosly there was other probs.........so carb was stripped down to clean and check out
.......it was then apparent that the carb was very old and worn out........and that the jetting would need reworking anyway.........
so we fitted a new 30mm.........and after a few more dyno runs got the jetting sorted.............

the carb on this is run open,but there is no hole in the panel which is very close and restrictive..............
but now the results.......15 bhp.........a very good improvement from 7.9......

Wouln't like to open a branch of your business down here Sean would you, I'd staff it for free?