T120 pinking

[toz]

Hi,
     Just aquired a lovely 1972 T120 Bonni. It rides well, starts first or second kick and seems smooth enough.

However, when it gets hot, and only when hot, it pinks considerably at mid range revs or when under load. The pinking goes all together if I open her up a bit but its annoying me and I dont want to do any damage.
Started of on 95 octane unleaded but went on to 97 to see if it made any difference, nope, just the same.

The only other thing I can think of is to retard the timing slightly, should I ? or is there something else I should be considering?

Thank you in anticipation.

[Rex]

Yep, retard it just enough (maybe take a few tries) so that the pinking just goes away on a real hot-and-heavy load condition.
Pick a nice hill on a nice day, and keep the timing plate spanner handy.....

[toz]

Thanks Rex, I will.

[wetdog]

is this not carburation , why should it go away at high revs under load ?

[Revband]

"why should it go away at high revs under load ?"

Quite simply because as the revs increase the the time period between firing and TDC becomes shorter and the piston is passed TDC before the burn really gets into its stride.

On these old engines the advance retard mechanism is a little crude, and by now it is likely that the springs are weak allowing full advance at too low a revs.

This type of ignition timing has always be a bit hit and miss and a compromise is usually accepted.

[wetdog]

Quite simply because as the revs increase the the time period between firing and TDC becomes shorter
i thought this period was greater as the timming advanced , i understand the flame rate bit also the springs on the bob weights , does the bike tick over nice and slow and even ? has someone fitted elec ignition ?

[Rex]

is this not carburation , why should it go away at high revs under load ?

Never said it would. I'm advocating a typical situation where pinking (caused by advanced ign timing) is most likely to happen, and so can be rectified in a "real" situation rather than on test.
 Never mentioned high revs either.

[toz]

No elec ignition fitted and ticks over fine and slow. Total rebuild by previous owner about 1000 miles ago.

[wetdog]

"The pinking goes all together if I open her up a bit " if the bike is on standard comp , and the timming is correct , and the carb balance pipe is not leaking , there should  be no pinking on modern fuel , what sort of rev range is this happening in ?

[Revband]

I said

"Quite simply because as the revs increase the the time period between firing and TDC becomes shorter"

Wetdog says.

"i thought this period was greater as the timming advanced , i understand the flame rate bit also the springs on the bob weights , does the bike tick over nice and slow and even ? has someone fitted elec ignition ?"

It seems you misunderstood my post Wetdog, let me clarify, lets say that the bike is fully advanced at 2000 revs and is pinking as the revs increase it cant advance any more and the pinking stops due to the higher revs.

[wetdog]

and only when hot? why only when hot

[Revband]

"why only when hot"

Because when the engine is hot it gives a faster burn.

Rex I think you have two threads mixed up

[Rex]

You're right.....lemme get back out to the work-shop.....bloody PCs ... grrr..

[toz]

I would say 2500 to 3500 ish Wetdog.

[toz]

Knocked the timing back a couple of degrees today, seems a bit smoother and pinking gone all together. Got her hot, long steep hilll in top, fine.

Thanks.