Moto Grand Prix
Main Motorcycle: 1973 Triumph TR7RV
Join Date: Oct 2013
Location: Pleasant Hill, CA USA
Other Motorcycle: 1964 Triumph Mountain Cub
Hi Stopngo, This is not so simple. The combustion chamber shape on our old bikes is not the best for low octane. It allows very large valves & can make good power, but really demands a high octane fuel to work properly.
Modifications can be done with valve timing, piston squish, dual plugs etc. to make bike work on lower octane. Triumph just recommended low compression pistons. Getting into 1.1-7.4 compression ration really helps. I have experience with both these. I will go with 7.4 on my Tiger 750 when pistons are needed.
I have had to use 87 on occasion. Trust me, old Triumphs in stock condition do not work well with it. You need to go at least 6deg retard to reduce pinging. On hot days 100f+ I don't think you can get away with 87 even then. When I try -6 deg on both 650 & 750 twins the bike just doesn't seem to feel peppy at all. Just runs sluggish. I can't explain this, but tends to start harder cold also. Many will ping on 91 also. We have no 93 here. I almost always set timing back 2deg. on both 650 & 750 twins. Takes some of the ping out anyway. 3 deg back on my bike I can feel the change in motor performance. With points full advance is 2000rpm.
I don't have experience on other types, but Boyer ignition has a very desirable advance curve with full advance coming in at 3500 in a mostly linier fashion. This can substantially reduce ping at lower RPM up to 3500. Many of the motors that I've seen with holed pistons this happened while riding above 3500. I set Boyers at -2 also so it's 36b @ 4000 checking RPM.
Using too low of octane is a recipe for holed pistons. I've seen many holed pistons here in California. In the old days of 98-101 octane they never pinged. Holed pistons were quite rare.
You are most correct E85 is up to 85% ethanol. As was stated 51-83 or 85%. On cars the oxygen sensor can determine E85 by the readings it gives for E85/flex fuel cars. It changes to E85 map in computer. E85 on earlier computer cars will flag mixture out of range & set check engine light. Not so easy to figure out. We had a few customers use E85 on flex cars. The mileage is very bad. Not quite half. The power feels normal to me.
Back in the 70s on the race bikes some of them I was sort of involved with used 100% alcohol fuel. We started jetting at doubling all jets sizes. Then go from there. The needle taper would be different for alcohol also. Just like on the Indy cars they made more power from alcohol & ran cooler. I'm not versed on exactly why. I think because of the oxygen level. There is less BTU per gallon so you need more fuel.
In California 10% ethanol is required. We still have precious few race gas stations left. They sell 100-101 octane California approved fuel. Bikes love it. However it's not the same as old 70s 100 in ping or performance. We can buy off road only leaded 110 octane. It works perfectly just like old 70s 100. However it's not at all the same. It coats plugs differently, smells different & evaporates instantly off carb after a tickle. Leaves gray residue on carb though.
I experimented with every type fuel I could find & every octane booster I could find. The only one I found that actually worked was Torco fuel accelerator. It's the same ingredient in Torco race gas which works good. I don't try VP race additive. Wasn't available to me.
I find in California going up 5% on all jet sizes tends to work better with our fuel. Slide may or may not need to be richened.
So if you really want to use 87 you must get LF Harris 7.1 pisotns, set timing back as far as you can stand it & richen mixture. Your '68 has 9.0 from new. A year & half ago put 7.1 in '69 Bonnie. Fully cured ping & it runs really, really good. No mixture change needed as the mixture was already richened for California 91. Have not & will not try 87 in this bike unless forced to. We can only get 87 & diesel in very remote areas. But haven't had this bike there yet. Doing road tests the bike didn't loose power in real life, even up steep roads against strong winds. Higher elevation doesn't seem to effect power more than normal. Only been up to 8000' feet though. I can't feel difference from sea level to 5000' in power in real life riding.
If you what to use E85 I expect you'd need to set up carb like the alcohol versions. Parts are listed on Amal site. But you can't swap between E85 & normal gasoline.
Just like race gas E85 is hard to come by. I was done with the 100 leash of race gas & would not recommend the leash of E85.
Unless you have a very special reason not to, my recommendation is use the highest octane you can get. Then go from there with timing & mixture. Experimenting with low octane is very risky, be careful. Gear down & spin motor higher.