Ignition Advancing

[gavinfdavies]

I currently run my bike on Shell V Power 99 octane fuel. Seems to keep the fueling smooth despite running though all the winter filth. Is it worth advancing the ignition a couple of degree's to take advantage of the high octane fuel? If so, could this be done by turning the rotor by the relevant angle, or would i be better off moving the pickup? bike is an L5 with end can and buggar all else. Any ideas? Most other post seem to focus on either SP cdi's or race bikes etc.

[AFM327]

Unless you have milled the head for more compression you don't need the 99 octane which helps prevent detonation. You'll make more power with 92 octane, and save money at the same time. Higher octane fuels make less power.

[gavinfdavies]

Yes, but the additives also help with the fuel system. Did i mention my gpz? I run it with open bell mouths, and yet the inside of the carbs never get dirty, the jets never gum up etc.

If running low-octane fuel, modern engines retard the ignition to reduce nocking, but this reduces power. Also, some older engines (such as Novas etc) use to come with high or low compression versions (normally 10:1 and 8:1 for example) for those people who might want to run cheap fuel. In russia you can still buy 85 or was it 70 octane fuel, at about 15p a litre. Tends to destroy anything more highly tuned than a trabant.

In short, to take advantage of high-octane fuels you can raise compression (skim head, new pistons, remove base gasket etc) or advance the ignition timing.

[Tirpitz]

Can I ask - if you're not going to do anything to take advantage of the higher octane fuel, am I rigt in thinking that you would then be wasting your money? I ask because I was considering using premium unleaded, but if I'm going to get less power with a standard engine I won't bother. I can stick some Redex in once in a while to clean the carbs etc.

[gavinfdavies]

there is a threade somewhere on here, one of the guys (MOS i think) ran his bike on shell v power for a while, then switched to 99 octane tesco fuel. within a few tanks the throttle response was noticeably worse, but one tank of v power sorted it out. Good premium has extra additives too, not just higher octane. The big boon for me is no need to clean the carbs. Percentage wise, it also doesn't cost much more now, was £1.36 compared to £1.29?

The most usual mods to make better use of it are removing the base gasket (this drops the whole cylinder head and barrels down relative to the crank and pistons, boosting the compression a little) and advancing the ignition a little, maybe 4-6 degrees. I'm interested if any one has simply tried rotating the timing wheel by a few degrees to achieve this?

[Tirpitz]

Cheers Gavin, I think I'll stick to premium as I was intending then.

[Mori Man]

Jamz done the thread on using different fuels

I am sure Zimm said he notched the pulsar and advanced one of his motors with no ill effects.

[zimm]

Jamz done the thread on using different fuels

I am sure Zimm said he notched the pulsar and advanced one of his motors with no ill effects.

yeh,.. there's a thread somewhere going into detail..

with it advanced as far as it would go the topend power went a bit flat.. just a case of dialling it back till it feels right ..

that engine was later fitted with programmable ignition and the rev limiter moved to 15,500rm ... after several months of testing different advance curves and bouncing off the redline on track it was stripped (to port the head and fit new rings) .. no damage from detonation was evident anywhere.. (3 bent valve stems.. but that was unrelated, and the bike still ran fine and put out 68bhp @ the wheel about an hour before being stripped) so i'd say its a pretty safe mod. I ran the bike on whatever brand of "super" i could get

however it wont gain you much either.. crisper throttle response is about it.. and it probably makes hot starting trickier... still... i've slotted and moved the pickup on 3 bikes now.. the throttle response is worth it for me, although programmable ignition is way better as your arent just moving the whole curve up

[gavinfdavies]

cheers for the info. how far did you end up advancing it then (by shifting rotors i mean)? I'd probably try 6deg to start with. I've seen that dynopro do adjustable cdi units, for about £80, and it's tempting. i've a horrible suspicion i may have a slightly blown head gasket (piston to coolant if it's possible on these), due to overheating and overpressurising the coolant. exahust is a little steamy too. no leaks to be seen, but she's heating up alot faster than usual. I've given the rad a good wash to clear it out, and i know it's long overdue shims and carb balancing, but if those don't improve it i'm gonna strip the head off and check. still runs fine and makes power. just did about 300 miles on her... if it is blown then while it's off i'll send it off to be ported. can you get bigger vavles on these? or just more aggressive cams? might then get that cdi.