Retarding ignition to prevent knocking.

[Stevex]

The bike in question is a LM2 with a standard ignition system.
Last summer I noticed that grabbing a handful of throttle at around mid rpm would induce knocking, especially in higher gears.
Since then the bike's been fitted with a lightened flywheel and I've reset the points gaps and set the static ignition to 8°.
Full advance on the 850 Le Mans is 34°, but would it be advisable to maybe set this to 32° in the hope of eliminating the knocking?
Would I actually notice any difference in power delivery by retarding by 2°, not that it matters because I have to back off the throttle every time I hear the engine knocking?
I had a thought that maybe I was running regular fuel (95 octane in UK) instead of premium (97 octane); if so, would the almost 2% increase in octane rating make that much difference?
The bike was also running with a standard exhaust, not sure if this would have any effect, but I'm in the process of replacing the end cans with some slightly modified Lafranconi Competitione items.

[twhitaker]

would the almost 2% increase in octane rating make that much difference?

Yes.

[Antietam Classic Cycle]

Have you ever replaced the advance weight springs? Could be they're stretched and letting too much advance, too soon.

[Vasco DG]

My first thought too. Also has the distributor been swapped at any point? If it's got one out of a small valver in it then it will have different springs that give less overall advance but it is achieved earlier.

Pete

[Daniel Kalal]

I found with my Cal2 that a Guzzi centrifugal ignition advance mechanism can go years--perhaps even decades--without new lubrication; but, eventually...

[Aaron D.]

Yes, by all means check springs, use premium, and avoid retarding the timing, that way lies burned valves, aye.

[jabberwocky]

Before I had the crowns of my pistons cut down (they were higher than stock) on my T3, it knocked badly. I retarded the timing a bit, and yes it did help reduce pinging, but it was VERY hard to start, hot or cold. Definitely less power too. Then I put the timing back where it belonged, and the starting improved, power came back, and I used octane booster until the next winter when I'd have time to pull it all apart and remedy my too-high compression. I don't think you'll be happy with retarded timing, even if it does reduce your knocking/pinging.

[jacksonracingcomau]

Before I had the crowns of my pistons cut down (they were higher than stock) on my T3, it knocked badly. I retarded the timing a bit, and yes it did help reduce pinging, but it was VERY hard to start, hot or cold. Definitely less power too. Then I put the timing back where it belonged, and the starting improved, power came back, and I used octane booster until the next winter when I'd have time to pull it all apart and remedy my too-high compression. I don't think you'll be happy with retarded timing, even if it does reduce your knocking/pinging.

apples and pears
T3 unit has initial at 2 BTDC, retarding that will put it At or after TDC, very hard to start
LM has initial at 8 can be 6,4 or even 2 without causing starting problems.
Not just springs the T unit has more throw , but can be trimmed easily with spacer around shaft

When I first came to Oz (perm) you could only get 95octane, I'd tuned mine to run UK 97, it didn't like 95, pinked it's head off (high comp 992)
I've run that engine at 6 initial (32 max) for 23 years, even though we can get 98 now, not at every station, certainly not in woop woop.
Of course, running 98 octane with 34 will give it a tiny bit more go, negligible in real world.
I've built another engine since, low comp twin sparker will run happily on 91, horses for courses

So Steve, advantage of running 97 is true but retarding to run 95 will cost you very little in performance for the gain of absolutely certain to not pink and potentially destroy engine.
Note that old stale 97 is often worse than 95, to run peak performance fresh fuel is crucial, never buy from slow turnover stations and chuck if unused for a few weeks
Running 2 deg less WILL NOT hurt anything, detonating will.

[pehayes]

How many miles on this bike?  Have the heads ever been reconditioned?  The valve stems do not have seals.  If the valve guides get ovalized (after 40K miles?) then it can easily suck oil on the intake stroke and contaminate the air/fuel mixture.  If other specifications and fuel are correct, then pinging is a clear symptom of oil contamination.  Redo the valve guides.

Patrick Hayes
Fremont CA

[Stevex]

Thanks for all your replies...a couple of answers to questions; I guess I should have included this in my first post.
I had the dizzy off the engine about 400 miles ago and I stripped it; no appreciable wear and I renewed the advance springs; there is nothing in the comprehensive paperwork that came with the bike to suggest it's not the original.
The bike has just over 34K on the clock with no reason to doubt it's accuracy, I have loads of previous MoT certificates to authenticate mileage.
The heads haven't been touched since new, I've no doubt reconditioning them will make a big difference and I'm already planning this as a job to do next winter.
The engine isn't burning oil but when rotating the engine with the headers off I could feel some exhaust valve blow-by when approaching TDC; when I bought the bike (about 18 months ago), it still had the original grey plastic bellmouths fitted and I don't think the carbs have ever used filtered air.
JCR, interesting what you say about retarding the ignition, it'll be something to try if the high octane route doesn't help but I've found that Shell's V Power Nitro+ is 99 octane so expecting that to be the answer. Am surprised though that the super unleaded goes stale so quickly.

[jacksonracingcomau]

Fuel
Since lead has gone, modern additives are brilliant anti-knock, we can actually get pump 100 in odd stations, what we called 5 star when I pumped petrol as a schoolboy.
But it's shelf life is terrible. People blame ethanol but our ethanol free is just as bad, forms globs in carbs if you don't use it
Note this is Aus and UK that I know, US fuel may be different.

Valve guides
Inlet guide wear wear WILL add oil to mixture but is also the most easily diagnosed problem there is
Smoke on overrun, puff on gear changes.
If not noticeable, it's insignificant but peering down inlet port would show burnt oil on valve stem
Unfiltered carbs far more likely to wear rings, smoke on power, noticeable oil consumption

Exhaust valve leak
This is an issue, prove it with leakdown tester or compression tester now (pay a mechanic for test before ripping to bits if you can't borrow one)
Leaking valve will eventually burn out completely, to reseat refaced valve will cost sixpence, a few quid for a valve if you need it, can get exy after damage is done, esp if head falls off
That could already be your pinking problem, if one cylinder is only half working, of course it will pink

[sib]

Yes.

Different bike, but I agree.  My V7 Stone never knocks on 89 octane (US), but knocks when lugged on 87.

[Stevex]

Exhaust valve leak
This is an issue, prove it with leakdown tester or compression tester now (pay a mechanic for test before ripping to bits if you can't borrow one)
Leaking valve will eventually burn out completely, to reseat refaced valve will cost sixpence, a few quid for a valve if you need it, can get exy after damage is done, esp if head falls off
That could already be your pinking problem, if one cylinder is only half working, of course it will pink

If the valves are blowing by will a lap in be sufficient or will the seats need re cutting?
Of course it depends on their state when removed, but I'm thinking best case here.
What sort of wear should I expect since the engine has been run on unleaded?

[jacksonracingcomau]

If the valves are blowing by will a lap in be sufficient or will the seats need re cutting?
Of course it depends on their state when removed, but I'm thinking best case here.
What sort of wear should I expect since the engine has been run on unleaded?

Can't know first (I went to buy Psychic News this morning, shop had sold out, I should've known really !!)
I'd prob reface and reseat at least, even if you can lap it in, seat will be really wide, won't cost much to do it right. Valve may be U/S, depends how long it's been blowing

Wear from unleaded--none. Your valve probably a neglect thing, heads never retorqued, clearances not set,
Do retorque the other side even if no leakdown.
Leakdown really tells whole story, do it hot. If you have to pay a score for test it will save you loads with knowledge of where leak(s) are and how bad
As you say it isn't using oil, I'd fix the problem , ride it, don't fix what ain't broke

34K miles is almost new, only thing that I'd expect to be worn out is timing chain, get to that next, sooner if really noisy.

[Furbo]

Good advice here, my take:

40K miles on a LeMans with unfiltered air, yeah, new guides and a valve job are prolly a good bet. A compression check will tell you about the rings.

Of note - I've always found it beneficial to set the timing dynamicaly - at full advance - and not worry too much about the setting at idle. 

[Cheese]

I've done it. When I toured on my Eldo years ago, the quality of fuel, temperature, humidity, etc. would change the way the bike ran. Specifically, if it got poor fuel and began to detonate, I'd loosen the distributor clamp and retard the ignition so the motor would tolerate the conditions. worked well as a roadside fix.

Peter

[Two Checks]

Actually oil would contribute to detonation.
And AAIK using AVGAS in a road vehicle is a legal no-no.

[rocker59]

Just run premium fuel and call it good...   

[twhitaker]

Could have something to do with lead in it maybe?

Has more to do with tax.

[Two Checks]

Exactly. No road use tax.

[Rough Edge racing]

 The ingredients of Marvel Mystery oil is no secret according to the internet....

The product is composed of 74 percent mineral oil, 25 percent stoddard solvent, and 1 percent lard. Ingredients may also include Benzene and Naphtha depending on the source.  Some feel adding a little mineral oil to certain blends of gasoline is beneficial...Others feel any kind of oil to gasoline invites detonation....Some believe placing a battery on a concrete floor will discharge it....