Octane & Ignition advance

76 kz 900 with 1,015cc block bored to 1,024cc 10:1 comp ratio, B8ES's and Dyna S.

Running real well at stock advance on 87 octane but I JUST WANT TO SEE if it's got any more with 89?

Drained all the 87 out this evening and fresh filled with 89. Off the bat, NOT running as well with 89.

Should I advance the spark and if so, how much? 0.25" (5 degrees ahead of stock?) before the "F" mark?

Any advice. Thanks.

Different fuels have different specific gravities, in other words they can be more or less dense. You may need to alter your jetting to get the best from another fuel, otherwise your engine might run rich or weak (most likely weak on a higher octane fuel). 5 degrees is the most I would go to. Ideally get the a/f checked on a dyno.

Lorcan,

Thanks for the input. I'm going to advance it 3mm (4 degrees ish) ahead of the "F" marks, on the advance mechanism, when I get home. I'll take it out for a run and maybe do a plug chop and see what we've got.

I'll report back when I have something.

So should I be running 87 on my bike?

pidaster wrote:

So should I be running 87 on my bike?

Depends.
If your engine is stock, then you have about 8.5:1 compression ratio, and you should be running with 87 octane (unleaded regular).
If your motor's been worked on, and the compression is boosted (10:1 on BARNEYHYPHEN'S ride), then higher octane might work better.

Update...

Put the light on it when I got home and advanced the spark @ 1,3400rpm (on the bike tach) to about 3mm before the "F" mark for both 1/4 & 2/3.

Pretty simple because both puckups had been right on the "F" for the 87 octane so just involved moving the back plate round a "tad" (technical term for 3mm).

Running extremely well. No hesitation off the line 7 no apparent adverse results.

I've only had her up to about 80mph (5,000rpm) so far due to the new rings on #4 (see prior "Heavy smoke...." post) but at 5k she was purring and felt like there was lots more left to tap in to.

Thanks again everyone's input on this.

The point of higher octane fuels is to avoid detonation, or "knock". They do not usually on their own provide any extra power (often quite the opposite)as their energy content is normally low, but what they do is allow you to run more compression, or boost, or ignition advance safely and allow you to make more power and get better throttle response without knock.

Increase of octane slows burn time. The piston rolls over the top and the pressure drops. By the time the gases start rapid expansion, its too late too make any power since the piston is moving down the cylinder ahead of the game. What im wondering is how well your motor is running with 87 and stock timing. If u feel vibration in 5th gear when u gas the motor it may be too advanced, and more power and smoother running would be a result of retarding the timing slightly past the fire mark. Just something to add, good luck !

The reason I decided to "convert" to 89 octane was my nieve attempt to "clean-up" the burn process. My thinking was that highter octane gas might be a better match for a 10:1 compression ratio? So I advanced the spark 3mm ahead of the "F" marks, to allow for the slower burn.

The bike is good off the mark and seems to have plenty of get up and go. The only MINOR problem I seem to be suffering is around 3,500+rpm, the engine feels as though its got a "miss" on one pot? I can gas it past 3,500 and whilst it's pulling it feels fine but whenever I cut back to a steady cruise, whether it be 3,500rpm, 4,000 or anywhere upto 5,500rpm, I get the "miss" back? I haven't taken it past 5,500 yet since the re-ring so I can't comment past 5,500.

It "feels" like an electrical miss rather than a gas/mixture miss, if that makes any sense? like a plug is "failing" at the highter revs?

Just a thought.... has anyone ever had a "problem" with the relay, in the WG coil rewire, "keeping up" at the open/close rate necessary at highter revs? Is it possible to have two (2) relays in the rewire system, one for 1/4 and the other for 2/3?

Any thoughts would, as ever, be most appreciated. Thanks.

Barney:
The coil rewire only supplies a constant voltage to the coils and does not trigger them. This is accomplished by the pickup coils and ignitor.

HWMS, thanks for the clarification. This would at least seem to rule-out the relay in the "miss" question. Back to plugs & leads this evening. Thanks again.

Well... discovered I had inadvertantly set my plugs to 45 thou! Two feelers, 23 & 22 = 35, no, not even using new math! That must be my problem.

Installed fresh new plugs New plugs, correctly set at 35 thou. Pulled all wires, checked and cleaned all ends and refitted securely, used small zipties to secure the boots tightly on the coil towers.

Started very well, as usual, and reved freely in neutral. Took her out for a test.

Accelerated very well, no bogging off the line, good through 5,500rpm BUT, at 3,500rpm (1/8 - 1/4 throttle) at CRUISE, some fairly heavy hesitation. Accelerates up out of hesitation "zone" very cleanly, no bogging.

Me thinks I do NOT have an electrical problem, I think I have a TOO small PILOT jet problem?

Any thoughts? Thanks.