Not GS-specific: ethanol in fuel vs. mixture & jetting

[mr72]

A recent set of issues with my non-GS has sent me hunting for information. Some of you can correct me where I am wrong about this, but this may shed some light on what seem to be mystifying geographic differences in how these bikes run.

With non-ethanol gasoline, the theoretical ideal AFR (mixture) is 14.7:1. This is what kind of gas [most of] our GS500s were tuned for from the factory, and we've all learned that they tend to be lean on the stock pilot jet (idle, light throttle, low rpm). This is a volumetric ratio, which is important. So imagine with a 37.5 pilot jet you might be running a factory Mk1 GS500 at 15:1 AFR.

The pump fuel in the UK (and maybe other parts of Europe) is typically 5% ethanol. The ideal AFR for this fuel should be around 14.4:1. Note that's significantly richer than the non-ethanol fuel's ideal ratio. So your GS will run even more lean on UK pump gas than it would on non-ethanol gas.

In the US we have been seeing "up to 10%" ethanol fuels from the pump for the past few years. Realistically the ethanol in these fuels is hard to estimate. But the important thing to know is that with E10 the ideal AFR is 14.08:1 which is much more rich than it would be for non-ethanol fuel. If you tune for non-ethanol fuel then the bike will be running quite lean if you put in E10. The big problem in the US is that the gas can be as little as 3% and as much as 10% so you really have to tune for E10 and then live with it being a variably extra rich if you run <10% ethanol unknowingly.

Other countries have other amounts of ethanol in the fuel so you should consider this when taking tuning recommendations from others who are not in your country. On my GS I am running 125 main jet and 40 pilot and it runs perfectly like this on non-ethanol fuel which I can buy at exactly one gas station fortunately near my house. However, it runs quite lean on the E10 that I wind up getting from miscellaneous other pumps if I have to refuel away from home. This means your GS in the UK with 125/40 might be lean on your pump gas, and for those of you in Brazil you have a whopping 27% ethanol content in your fuel and that will require very different jetting since it will be quite dangerously lean on 125/40, since it's stoich at about 13.1:1. That's a 10% change so I would think you're going to have to run 42.5 or 45 pilot jets and up to 135 or 137.5 mains just to get it to run close to right, and that's with the stock airbox/filter.

Hope this helps someone out. It definitely helped solve some of my confusion.

[the side note: my Triumph is tuned via flashing ECU and the tuning package I have was built for UK fuel, but I also have a slightly more restrictive air filter than it was designed for. Together this conspired to make my bike very rich when running non-ethanol fuel and nearly perfect on E10]

[Kilted1]

Good info there.  I can only add that the ratios are not volumetric but mass.  For every gram/ounce/pound/stone/whatever mass unit of fuel, you need the 14.7 of the same mass unit of air.  Of course air is difficult to weigh but it does have mass and it also varies in density and constituent gas concentration so there's another uncontrollable variable. 

Of course none of this effects the process of fine tuning the pilot needle to find the spot where it runs best.  Once you have it dialed in, it should run across a range of conditions, a few percent this way or that.  It may not be perfect but should run acceptably at altitude even if you tuned it near sea level.  If the air is less dense, there will be less vacuum on the diaphragm and less fuel sucked through the jet.  So the system is somewhat self-balancing in that respect.

I try to buy fuel at the same place or at least of the same brand so hopefully I'm getting at least some consistency there but gasoline is also going to vary from one batch to another at the refinery.  The best we can hope for is a narrow range and a solid base-line.