Background:
I've been experimenting trying to use ethanol free gas, which hasn't been easy. I even found an app for my iphone called Pure Gas that locates the gas stations that have ethanol free available, mostly boat marinas in my area. I've been trying several different brands of ethanol free gas over the last few weeks with mixed results. My bike seems to run best with a lower octane rating of 86 or 87. It runs terrible at 92 octane, backfiring and fouling plugs; unfortunately only 1 out of 10 gas stations that carry ethanol free gas within 25 miles of me sells 86 octane none ethanol, so I'm limited to buying from just one place.
Question:
I've heard the horror stories about gas with ethanol gelling up after it sits for a while and clogging carburetor jets, but is it really a fatal problem using 86 octane regular gas if I use it up and refill with fresh gas a couple of time a week?
Thanks!
Is your bike the K0 in your avatar? Those are a little unique, compared to the later 750. They had very richly-jetted carbs, if the whole setup is stock and you have HM300 pipes. A note about the pipes: if you have a sandcast or VERY early 1970 K0 with the HM300 original pipes, the mainjet may be #120 and the needles may be in the center notch in the carbs. But, if the pipes are NOT original, and are still HM300 pipes, the mainjet should be #115 and the needles should have their clips in the 2nd slot from the top (blunt end) or you will have plug fouling issues galore. The later HM300 pipes are more restrictive than the earliest ones (and even those had fouling issues).
If you have HM341 pipes, the jetting must be changed. This number depends on your airbox: if you have one of the few remaining OEM K0 airboxes with the huge inlet slots and these pipes, the mainjet should be #110 and the needles should be in the 2nd slot, which is the 4th groove from the blunt end. If you have a K1 airbox, the needles should be in the center slot with the #110 mainjet. If you have a K2-K4 airbox, the mainjet should be #105 and the slot should be #4 for the needles.
All of these tunings are for running premium gas.
If you'd like to run on less octane, you should richen things up slightly: for most folks this consists of just raising the needles one notch from the above ratings, or else increasing the mainjet by either a 2.5 or 5 size from the above ratings. With today's ethanol-laced, slow-burning, self-cleaning, unleaded, nitrogen-enhanced, EPA-approved (and tinkered) fuels, these new Keihin "half-step" jets are proving to be very helpful... :/
The shorthand version: today's ethanol-laced, midgrade gas burns at the same rate as 1960s premium (i.e., slowly). This is why the midgrade gas is here today. Today's premium burns VERY slowly, suitable for engines with compression ratios in the 11:1 ranges. That's not your SOHC4 engine. Yours was designed to use leaded, 95 octane GASOLINE, which lubricated the valve stems (instead of stripping them of lube, like ethanol solvents do) and upper piston rings. So, if you have too much octane, it results in leftover unburned fuel in the chamber during overlap (when both valves are momentarily open at the same time, between intake and exhaust strokes), which ignites from the waste spark of the Ignition system and causes a burn-back into the intake tract at low speeds. This disrupts the incoming flow (like blowing into a straw) for the next cycle, and the engine stumbles. Then, when you pass about 1500 RPM it smoothes back out. (Sound familiar?)
