Hi folks,

Petrol additive - the magic bottle that turns unleaded into leaded fuel - is getting harder to find round here, but I wondered if its actually necessary?

My bike is an '84 34L, so was "designed" to run on 4-star, but I wonder what this actually means. As I understand it, lead was originally added to fuel to be kind to valves in iron block engines, where the valve seats are not hardened. This doesn't seem to be an issue in the aluminium XT engine, since the valve seats are already hardened (alu valve seats wouldn't last very long).

Which leaves its other two roles: upper cylinder lubricant and octane booster. The XT seemed happy to run on any-old-octane fuel in Africa, so I've mentally ruled that one out.

So, should I bother continuing to find the additive, or is it all smoke-and-mirrors?

Bump.

So, either nobody else puts additive in their old XTs, or you all don't know why you do it?

I cant speak with any authority here but a mate has an old 81' XT550. He doesn't use additives and never has and his motor still pulls like a tractor with a compression ratio that is spot on. I would guess it is not necessary.

that is a good question. My friend is a machinist and engine builder, He told me that older motors that at one time ran on leaded fuel are now safe to run on unleaded fuel. due to the fact that lead has penetrated and embeded the valves as well as the old seats, He said it could be a problem if the motor was driven for long highway miles non stop I meen realy pushed hard. I have a 86 xt and I run unleaded with no additive so far no problems.

Now that doesn't make sence. Lead additive was used to cool the ignition fires down and run higher Octane. Without lead the fires burn hotter leading to pre detonation, pinking and valve seat impaction. The first two is not a major drama as these bikes are notoriously rich which is cooler in itself and have huge tolerances compared to cars and higher performance motors so can withstand a bit more "abuse", especially in the temperature ranges. As for the valve seal impaction issue, any lead used to "lubricate" the seal burns off pretty quickly, but impaction is credited as taking may tens of thousand miles with plenty of warning when damage starts to occur in terms of compression loss. Just reseat them.....

Doesn't make any sense hey? well first of all lead doesn't lubricate the valves! it's more of a cushion between valve and seat, and a pregnation of lead into seat and valve does not wear off just like that. If you want to act like you know everything then answer this guys question, and cut all the gibberish......

A lot of sorcery there, while physics as usual work differently in the US and in the old East ... As a lot of other things too.

Lead:
Function one: increase octane number - something like the resistance of a fuel to ignite spontaneously with heat (caused by pressure and hot engine parts in the vicinity). This function has entirely been taken over by refinery techniques and other additives. No worries about that. A fuel with a lower octane number does not burn hotter than one with a higher number. The only thing the increased octane number decreases is premature ignition.
Function two: lubricating / cushioning the valves on the seats. As you will notice, the tappets act slightly eccentrically on the valve stem, so the valve turns during operation - the function is to evenly wear valve and seat, remove dirt etc. But, softer materials need to be lubricated otherwise they would wear too fast. In addition, indeed the lead cushions the valve returning to it's closed position.
But, think about this, how can a material that has been impregnated into another material cushion something. For that it should be between the two materials bumping into each other. Truth is, the 'film' of lead wears fast if you do not maintain it by using leaded fuel.

Our 55W ('86) runs perfectly fine on unleaded fuel, already 50.000 km, no execssive valve clearance increase etc.
Conclusion 1: no problem to use unleaded fuel without lead replacing additives.
Conclusion 2: see function one: octane booster not needed.

Auke

__________________
Auke
http://aukemarieke.blogspot.com

Thanks for the info chaps.

Just been out for a play with a tank of un-additivified unleaded, and I notice that the bike seems to run a bit leaner - more popping on the overrun than before. Just a matter of a quick twiddle of the pilot screw and I shall be free from the tyranny of carrying around my little red bottle forever more.

A picture from today's "test", on my favourite wiggly pass - sorry about the quality - its really hard to squish images into the 39kB file limit for this forum ...

Attached Images

 

Jap bikes have been lead free since the seventies, as have American cars. All to do with Californian pollution laws from that era.
Aukeboss is right on the money with his explanation of what lead was for.
So, two months down the line, I take it all is well with the bike?