Fuel Of Choice

Which is the best fuel to use in my 850 Mk 3? I have easy access to ethanol free gas and 100 octane Low Lead aviation fuel. Given the bike was born in the days of non-ethanol leaded fuel is Avgas a viable option?

I have recently found a supplier for 100LL. I love it. In the past, I have mixed 50 50 110 racing fuel and 93 premium. This 100ll is much easier, not too expensive, and stores easily up to a year and some say 2 years.

Avgas is not a good choice for several reasons; First, the price, second, availability and third, it has additives that keep it from freezing at high altitudes that may not be healthy for engines not designed for it.

Many postulate you don't need lead, but we all know lack of lead means premature valve and seat wear. These problems can be mitigated with modern valves and seats designed for unleaded. The second issue is octane. Stick with fuel in the 90s and don't advance the timing out to the gnat's ass of detonation and it'll be ok. Lastly, ethanol. Not sure if there are soft parts available for old fuel systems that can withstand the onslaught of alcohol. But there are bottled additives that can help prevent phase separation and water accumulation.

I have been blending my own fuel, but if I get it out on the road, I will have to be prepared to add my goodies to the pump gas I find. 1 oz. of Lead Substitute, 1 oz. of Octane Booster and 1 oz. of Startron Ethanol additive per gallon of 90+ octane unleaded 10% ethanol gasoline. You can also buy race fuel, but like Avgas, the price per gallon is not cheap. Plus, you can only pump it into a can since normally it's not taxed like other fuels. A recent exception I noticed was a pump in the pits at Barber with Sunoco leaded race fuel. Again, not cheap and rarely available.

The only time I change the ignition timing from the manufacturers' spec. is when I use methanol fuel for racing when I add 4 degrees advance. Every other time I use fixed timing and jet to it. If the fuel has a higher octane rating, you can probably jet slightly leaner without getting detonation.

I ran my stock MKIII on normal (unleaded) 91 octane premium pump gas here in SoCal for years with no pinging. That's with ignition set at stock 28° full advance. It has such a low CR that I could probably have got by with the mid-grade, or even regular, but I never tried them. The cost difference isn't very significant for my sort of riding habits, and I didn't see any reason to take the risk of pinging on those 100°F+ days we get here.

I'm not sure the lack of lead is a real issue with the Nortons. They already have pretty hard valve seat inserts. Might be a problem with high mileage, but I'm not a high mileage sort of rider. Somebody like comnoz, who does some serious mileage, might have more insight into that issue.

Ken

737captain said:

Which is the best fuel to use in my 850 Mk 3? I have easy access to ethanol free gas and 100 octane Low Lead aviation fuel. Given the bike was born in the days of non-ethanol leaded fuel is Avgas a viable option?

Click to expand...

If you plan to go farther than one tank of gas per trip the choice is limited to what you find on the road. Your 850 (stock) is low compression compared to the 750s, so less concern. Current pump gas should work fine for the engine, but watch the carb float bowls and fuel lines for degradation.

I have not had any problems with valve seat wear using unleaded fuel on stock seats.

The head I took off my bike this summer was on it's second set of exhaust seats -in 200,000 miles.

Would leaded fuel make the seats last longer? -probably.

If your running a cam with severe ramps -then lead might help.

the low lead is a misnomer when you look at the lead levels in the old leaded gas we use to have. it has a very high lead contend and if you have been around aircraft engines you will see what I am talking about. as jim posted it is not a probe blem on most of these heads for the normal user. so tony just run pump gas and forget the av gas.

comnoz said:

I have not had any problems with valve seat wear using unleaded fuel on stock seats.

The head I took off my bike this summer was on it's second set of exhaust seats -in 200,000 miles.

Would leaded fuel make the seats last longer? -probably.

If your running a cam with severe ramps -then lead might help.

Click to expand...

Jim are you running higher-than-stock quality valves and seats and how much does the precise fueling of EFI have to do with valve and seat longevity?

Danno said:

Jim are you running higher-than-stock quality valves and seats and how much does the precise fueling of EFI have to do with valve and seat longevity?

Click to expand...

I am using Kibblewhite Black Diamond valves and have as long as I can remember. I average 30 K on a set of valves. I think I ran a set 50 K once. I have never had a valve failure but I replace them when I re-cut the seats.

If anything the EFI is a little harder on valves as I keep it tuned about as lean as I can and the egt is on the high side. Jim

On my Mk 111 I'm usually searching for a Shell Station or ESSO Fill-up option. Premium has no ethanol , lower grades do , and eventually this hurts floats and their tip needles.