spit and kick back - timing or weak mix?

[hoogerbooger]

Noted. Thanks both.

No pinking.

As ethanol increases the oxygen content in the mix, I understand why it will tend to make the mix leaner and why you therefore need to consider tweaks to the carb jets. 

Still a tad unclear if kick-back and spitting through the carb at start up are typical for those who just flood the carb on the tickler?

I'm not so sure about the effect of octane on timing though........but I've been reading !!

What I've read so far says  a higher octane rating decreases the likelihood of auto-ignition and pinking....but doesn't affect the timescale of combustion/burn time........so  ( if I'm understanding) it doesn't directly affect the timing.....but may indirectly affect it, if you have to retard because you are getting pinking (to prevent damage to the engine).   Higher compression engine= more potential need for a higher octane to avoid auto-ignition/pinking.

(Also sounds like ethanol actually reduces the likelihood of pinking, by reducing 'epicyclic combustion').

As the B40 seems to count as low compression, it appears 95RON fine......and based on what you have both said: judge any need to further retard...on symptoms/ or tests to see if performance improves.

( basically I'm writing this down before I forget what I think I've learnt & to see if others have comments !! )



[My main challenge now is to see if  I can unscare myself to progress to more open throttle testing ........ and the chop test. Need a few more rides before that I think. However, the top half of the throttle seems to pull well through 3rd and 4th ...........Seems less happy and tinny/rattly at constant mid rev speeds in 3rd.]

[scifi]

I used to have one of the first B50MX bikes, and the ignition was something like 10deg full retarded to 30 deg fully advanced.  As it kicked back a few times, I altered the 'travel' of the advance unit to give 0 deg to 30deg. 
The 'mod' involved shortening the return spring, and filing a bit away at the fully advance stop.
I think it gave a softer slow running, and still had full power at max revs.   It would pull easily from any hairpin corner, without the need to slip the clutch.

[hoogerbooger]

another short ride ( mainly to test my recalibration of speedo). As yesterday seems to accelerate well from half through 3/4 throttle in 3rd and 4th. Not brave enough to try the chop test (yet), but a picci of plug attached anyway. I think this suggests  ignition mix/combustion is ok? 
(had to go down to 2nd and pootle a bit to get to house but switched off as soon as could)


(Exhaust still not quite sealed at head as getting a few black oil/watery drips below exhaust rose.....so that won't be helping. Next time I have the exhaust of I'll use silicone to reinstall)

[scifi]

That plug is a bit of a contradiction.   The centre porcelain shows to be slightly weak mixture, from your recent sprint around the countryside.   But the outer metal part is thick in carbon which must have accumulated over the last 2000 miles.
Best bet is to use a new plug, and hope that the carbon does not reform.   The gap between the porcelain and the body determines the heat range of the plug.

[V500]

I'd agree with that, but plug colours aren't a very reliable indicator now that we have E5 and E10 fuel.

[hoogerbooger]

Should say I had presumed (guessed) the black sooty part was just because it'd not burnt off after previous start ups on a tickled carb that where I didn't then ride it and shut it down while not warmed up and maybe still somewhat flooded carb. Before the next ride I'll clean it up and see what it looks like then.

Is it the case that ethanol affects spark colour? If so why?

[V500]

No idea if ethanol affects it as such, just that modern fuels don't give accurate plug checks as they did years ago.

[rhyatt]

The black outer body is due to that running cooler due to the heat being dissipated into the alloy air cooled head(water cooling has a slight variation).
The centre ceramic is the part to gauge your mixture adjustments

[Steve.S]

I think you could answer your own question regarding spitting back by fitting the choke slide, and see if it makes any difference.
You are quite correct in saying that a higher Octane Rating decreases the likelihood of Pinking.
It is a common belief that Pinking is caused by Pre-ignition. It's actually caused by Detonation (otherwise known as Knock). They are two different things, although linked.
The mixture is ignited by the spark plug, and as the Flame Front travels across the combustion chamber, the unburnt mixture in front of it spontaneously combusts, producing a high pressure wave, which, when it hits the cylinder wall, produces a 'Pink' sound. This is called Detonation, and is usually caused by over advanced ignition.
Pre-Ignition, on the other hand, is usually caused by a hot spot, and produces no sound. At it's worst, it will reduce engine power, or even cause the engine to stop.
Pre-ignition can cause Detonation, hence the link.
It was found that raising the Octane number of the fuel reduced the likelihood of detonation (as you say), allowing raised Compression Ratios and more timing advance, thus increasing power output.
Auto-ignition is not a word much used in the UK. (Perhaps in the USA?).
Auto-ignition is when the engine continues to run when the ignition is switched off. It's usually referred to as Running On in the UK. I have never seen a British single cylinder motorcycle engine 'run on'. It seemed to be more common with Motor Car engines in the '50's due to carbon build up and insufficient cooling.
Auto Ignition is also linked to Pre-ignition.
If you are interested in the combustion process, you can do no better than investing in a book by Sir Harry Ricardo "The High Speed Internal Combustion Engine", but it is a weighty tome.

Anyhow, enough of all this technical bo****ks, as long the thing starts, get out there and enjoy it while you can, but try not to let it pink.