[aircap]

Hey, true believers:

My rudimentary understanding of internal combustion has me a little confused about something, and I figured probably plenty of you folks could straighten me out.

My '72 Daytona 500 pings at low RPM under load, such as when taking off from a stop. I believe octane is a major contributing factor to this, as the highest octane pump gas in my neck of the woods is 91. (I did run some toluene in my last tank of gas, which did seem to help some, but did not eliminate the problem.)

It's my understanding that "ping" is preignition, which occurs when the air/fuel mixture spontaneously explodes under high compression (as in a diesel) before being ignited by the spark plug. If that is the case, then why is it commonly advised to retard the timing as a solution? I mean, the problem isn't that the spark plug is igniting the mixture too soon; the explosion happens without its help. So why should moving the spark back make a difference?

I'm sorry if it's a stupid question...there's probably something obvious I'm missing here. Can someone explain it to me?

Also, any additional ideas on how to eliminate my ping are most welcome. It's pissing me off.

Thanks!
Aircap

 

[McQueen]

I had this exact same problem on my '71 Daytona. Goose it under load and it would ping like crazy. After consulting here, on other boards, even the local shop, I ended up with more questions than I started with.

I ended up doing a top end job (not because of the pinging) and it seemed to help. Though obviously, I'm not saying to do a top-end job. Only thing I can figure is that the new pistons are free of carbon buildup. Theold ones were caked, and I thought maybe some of it was staying hot and igniting at the wrong time.

Who knows. I never did suss out the reason why. And i still have a "bog spot" right off the line.

My only advice is to throw some 200 mains in it, richen up the mix a bit (via needle and pilot), and keep the revs up while you find the solution to the problem.

You aren't alone and I'm hoping someone will come up with a correct diagnosis/fix...

 

[Snakepit]

The lower the octane and higher the compression, the faster the mixture burns. Therefore combustion occurs too early, trying to push the piston downward before it reaches TDC. (pinging)
Retarding therefore lets combustion occur later and prevents the pinging or higher octane slows the burn

 

[aircap]

The lower the octane and higher the compression, the faster the mixture burns. Therefore combustion occurs too early, trying to push the piston downward before it reaches TDC. (pinging)
Retarding therefore lets combustion occur later and prevents the pinging or higher octane slows the burn

That's assuming that the igniton is caused by the spark plug. My understanding is that the fuel is combusting before the spark goes off due to the increased volatility of low-octane fuel. Is that not the case?

 

[Beemie]

The manual of my '76 requires 97 octane, and the bike demanded that since new.

Just as a test, can you get some racing fuel?
Some gas stations in my area carry it, but you can go to most any racing track and get a gallon.
That runs around 110 octane and is often leaded.
You can run it straight or mix it with your 91 octane, say equal parts should give you about 100 octane.

See if that cures it.

 

[Snakeoil]

You are really mixing up two different conditions. Pre-ignition is as you describe. It is normally due to heavy carbon build up. The carbon increases the compression ratio somewhat, but more importantly tends to hold heat. It is probably still glowing if you could look in there while the engine is running. This, combined with the increased compression ratio (compression creates heat) will ignite the fuel without a spark. You cannot correct this problem with an ignition adjustment. This is also called dieseling and is that embarrassing chugging of your Dad's Buick after you shut off the ignition until is sputters and groans to a stop in front of your friends.

Most pinging however is caused by too low an octane for a given compression ratio. As was stated by others, the lower octane fuel burns faster. Or more correctly, octane slows the burn rate of the fuel. When you hear pinging, the mixture is actually detonating, and detonation is another term used in place of pinging. When the fuel detonates, as the term implies, it explodes and creates massive stresses and very high temperatures. This is what will hole your piston. If you pull a piston out of an engine that has been pinging a lot, the top of the piston will look like somebody went after it with an icepick. In the case of pinging, the fuel is still being ignited by the spark, only too early and the mixture explodes. If you fire the spark plug a bit later, the fuel is more compressed and tends to burn instead of detonate. But, when you open the throttle under heavy load, you add more air and fuel and now the later spark is still not late enough to provide a sufficient compression to maintain a burn and the fuel mixture detonates.

The ping you hear is actually the shock wave of the fuel detonating. This is why combustion chamber shape, lean mixtures, electronic fuel control, cam overlap and spark control are so important today. All those help maintain a fuel burn even with high compression ratios and today's crappy fuel.

If your engine is truly pinging, which I believe it is, I would retard the spark a bit as recommended by others. Or you could put a thick head gasket in place and lower your compression ratio a bit. I'm talking generically here and not Triumph specific. I'll let the Triumph experts comment on thick head gaskets or other ways to lower compression ratio to accomodate today's crap gasolines.

You could always run CAM 2. That's the easiest, but not the cheapest solution.

 

[stewdog68]

I'm with snakeoil on this, well because he typed it all

also wrong spark plugs(heat range) and hot intake air speed up the problem

retard a little and see.

thicker head gasket if needed(they sell them

i just copy paste this::
Ignition timing is another cheap and easy area to work on. If your engine detonates at low engine speeds at part throttle, consider retarding the initial timing by 2 or 3 degrees and then adding that amount back into the total by increasing the mechanical-advance curve. For example, let’s say you have 18 degrees initial timing with a total of 36 degrees and your engine rattles a little at part throttle, especially right off idle. You could cut the initial back to 15 degrees and add 3 degrees to the mechanical advance. The total remains at 36, but now the engine doesn't’t death rattle every time you let the clutch out.

problem is hard to do without knowing what you are doing. need springs or E.I. that allows you to do this

 

[aircap]

Ah, thanks, Snakeoil... So it's not an issue of the mixture spontaneously combusting, but rather that the explosion caused by the spark plug burns too fast. Raising the octane retards the rate at which the explosion occurs, allowing it to come to fruition at or about top dead center rather than beforehand, no?

What the hell is CAM 2? A special gasoline? I can't find anything at all higher than 91 around here...there used to be a Conoco in my area that sold 93, but they've merged with Phillips and now 91 is the highest they offer. There are a couple race tracks around...suppose I'll start making some phone calls.

A couple other questions, then:

1) How do I retard the timing? I have the Haynes manual, which has kind of muddled instructions in this department.

2) The old-timer I got the bike from said he remembered guys actually putting a shim-like gasket under the jugs to shorten the stroke, thereby lowering the compression. I'm not interested in such madness myself, but has anybody else heard of that practice? Kind of the inverse of using a fatter head gasket up top.

Thanks again, everybody. I appreciate the input a lot.

 

[stewdog68]

don't do that barrel trick!!! easy way to f some stuff if it comes loose. thats what i call hillbilly or back yard mechanics.

cam 2 is racing fuel and its not cheap!!!

you have points?

 

[aircap]

Hey, stewdog...thanks for the warning, but that jug-shimming thing is the last kind of nonsense I'm interested in doing. Just wanted to see if anybody else had heard of such.

Yes, the bike still has the factory ignition points setup.

 

[rousseau]

Aircap
Your "ping" goes by other names. Some call it "spark knock", or "pre ignition", can be called "detonation" some even say "dieseling" The cause is all of the above posts. In other words,the cause could be low octane, or too much compression,or the ignition timing, carbon build up on pistons is also a cause. Plus one that wasn't mentioned is heat. Air cooled engines are the worst. I'd bet it does great if the ambient temp is in the 50s.

I have found that people tend to complicate things that sometimes have simple solutions. Don't make a "science project" out of this.

Please try racing fuel before you go screwing around with the tune-up. Believe me when I say this is all you need to do. Like one of the others said , you can mix it 50/50 with your normal preimun gas.

 

[aircap]

Thanks for your two cents, rousseau. I just now found a local dealer who carries Sunoco race fuels in 104, 110, 112, 114 & 116 octane...believe me, I always try to go the cheapest/easiest route first, so a fuel upgrade is my first avenue of exploration.

When I get a chance to try it, I'll let you all know the result.

 

[Beemie]

Thanks for your two cents, rousseau. I just now found a local dealer who carries Sunoco race fuels in 104, 110, 112, 114 & 116 octane...believe me, I always try to go the cheapest/easiest route first, so a fuel upgrade is my first avenue of exploration.

When I get a chance to try it, I'll let you all know the result.

I use Sunoco 110 (called "Standard") in mine. I suggested mixing 50/50 just to make a stong mix, but mine does fine on 2 parts premium 93 and one part 110 Sunoco, giving about 98 octane. Sunoco runs about $7.50/gallon around here but I have the New England distributor 6 miles away so we may be benefitting from that. Thursday and Friday afternoons are interesting times around there.

Like I previously said, mine always demanded high octance since new. It was even particular about brand. Gulf No-Nox was a no-no. It was Sunoco 260 all the way; Mobil in a pinch.

Be aware that the fuel runs hot, so the bike will act tempermental in the heat of summer with respect to idling and restarts, but I'd bet that it will not ping.

 

[Snakeoil]

Stroke shortening gasket

So you don't walk away with a misconception, raising the jugs with a thicker gasket does not "shorten the stroke of an engine. The stroke is the distance the piston travels from BDC to TDC and you can only change that by changing the throw of the crank, which is done by moving the crank pin away (lengthen) or towards (shorten) the center of the crank.

I believe the idea to shim the jugs instead of the heads was thought up because there the gasket does not have to deal with the temps and pressures of the head.
regards,
Rob