Very Bad Timing - Embarrassing Mistake.

jan nelder · Aug 17, 2023

Peach said:
Hello.
(Bike : '73 Commando 750)
I had a misfire due to corroded wires in the crimp-on connectors at the (Boyer) pick-ups.
Had to take the plate off to change the connectors - so I bought a shiny new strobe.
I was chatting at the time - probably showing off my new strobe - and I set the spark at exactly 31° (correct)
..... With my son maintaining RPM at 2000. (Very incorrect)
Yes - TWO thousand - I really don't know where my head was.
HERE'S THE WIERD BIT : (My phenomenal stupidity aside)
The bike worked fine. Gentle tickover. Plenty of guts. No pinking. 
Only noticeable symptoms : a bit of kick-back on starting.
Did about 600 miles or so before, while pondering over this kick-back nuisance, I realised what I'd done.
Kicked myself harder than the bike kicked me.
Corrected now - I had the spark at over 45° btdc at full advance (estimated figure - well outside my markings on the alternator)
I'm a bit worried about the hidden damage I may have done.
QUESTIONS :
1) Why did it work okay with the spark so wildly advanced?
2) What damage should I expect?

I did something very similar with an 850 rebuild, and ran it in with a lap of the M25 around London, about 125 miles, before checking the timing properly by strobe: 42 max advance. It seemed to run ‘normally’ , and luckily it was running in time, so no undue stress on engine in other respects.

baz · Aug 17, 2023

I once accidentally fitted a triumph AAU to my 850 commando
It started and idled ok
But when revved it was popping and banging
All was revealed when I put the strobe light on
And it advanced the wrong bloody way!!!!

acotrel · Aug 17, 2023

A lot of road bikes are probably jetted very rich. Advancing the ignition timing has a similar effect to raising the comp. ratio or leaning off the jetting. The three things are in balance. I usually make sure the timing is 'right' for the fuel, then jet to suit it without raising the comp. ratio.
Some guys get improved performance by raising the comp. ratio - they could probably get the same improvement by jetting more carefully, or changing the timing,
Nobody ever seems to discuss carburettor needle shape on this forum. In the 1970s, every new model of Japanese motorcycle had different needles and needle jets in its carburettors - they did not do that without reason.
Most modern Japanese motorcycles probably have fuel injection and programmable advance curves. To get a Commando going as fast would take a lot of work. But in some ways , a Commando might be better. A well-developed 961 might be quick.
With these things, you never know until you try. It is easy to close your mind and give up.

Peach · Aug 17, 2023

Another interesting thing I did was fit the two wires (B/Y and B/W) the wrong way round. I'm not ashamed of this one - it's really easy to do - the White stripe gets creamy yellow with age and oil - tricked me. A bit more contrast would help. I had a nice spark but miles away from where it should be. It didn't run but did some nice pops in the exhaust.

Very Bad Timing - Embarrassing Mistake.

jan nelder

baz

acotrel

Peach

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